Publications

AGO

2021

New insights into bisphenol A removal from water applying experimental and theoretical studies

R. A. Konzen, P. R. Batista, L. C. Ducati, T. E. A. Souza, L. C. Cavalcante, C. E. Santos, F. J. Bassetti, P. C. Rodrigues, L. A. Coral

Bisphenol A (BPA) is used worldwide as a monomer in the production of polycarbonates and epoxy resins. It has been receiving growing attention over the years, as exposure to even low concentrations of this endocrine disruptor is being linked to serious health problems. This study aims to investigate the efficiency of powdered activated carbon (PAC) in removing BPA from water, as well as to apply a computational simulation to understand the behavior of BPA in solution and its interaction with a generic carbonaceous surface. In the adsorption studies, the mass of adsorbent (0.01–0.04 g) and the solution pH (2–12) were varied in order to understand their influence over the adsorption capacity (qe) of PAC. Pseudo-first-order, pseudo-second-order, and intraparticle diffusion were employed to evalu- ate kinetic data. Lagmuir, Freundlich, Dubinin–Radushkevich, and Redlich–Peterson isotherm mod- els were applied. A theoretical study using density functional theory (DFT) showed that adsorption is mainly caused by C–H...O, C–H...π, lone-pair...π and π‧‧‧π interactions. Equilibrium was reached after 120 min, with PAC removing a total of 96.68% of BPA. The best condition was achieved using 0.01 g at pH 9 and 298 K (246.20 mg/g). Good fittings to Dubinin–Raduschkevich (R2 = 0.985) and to Langmuir (R2 = 0.925) models were achieved, with a calculated maximum monolayer adsorption capacity (qmax) of 367.88 mg/g

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JUN

2021

Solvent effect on the 195Pt NMR properties in pyridonate-bridged PtIII dinuclear complex derivatives investigated by ab initio molecular dynamics and localized orbital analysis

Patrick R. Batista, Lucas C. Ducati and Jochen Autschbach

An ab initio molecular dynamics investigation of the solvent effect (water) on the structural parameters, 195Pt NMR spin–spin coupling constants (SSCCs) and chemical shifts of a series of pyridonate-bridged PtIII dinuclear complexes is performed using Kohn–Sham (KS) Car–Parrinello molecular dynamics (CPMD) and relativistic hybrid KS NMR calculations. The indirect solvent effect (via structural changes) has a dramatic effect on the 1JPtPt SSCCs. The complexes exhibit a strong trans influence in solution, where the Pt–Pt bond lengthens with increasing axial ligand σ-donor strength. In the diaqua complex, where the solvent effect is more pronounced, the SSCCs averaged for CPMD configurations with explicit plus implicit solvation agree much better with the experimental data, while the calculations for static geometry and CPMD unsolvated configurations show large deviations with respect to experiment. The combination of CPMD with hybrid KS NMR calculations provides a much more realistic computational model that reproduces the large magnitudes of 1JPtPt and 195 chemical shifts. An analysis of 1JPtPt in terms of localized and canonical orbitals shows that the SSCCs are driven by changes in the s-character of the natural atomic orbitals of Pt atoms, which affect the 'Fermi contact' mechanism

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FEV

2021

Conformational analysis and electronic interactions of some 2-[2'-(4'-sustituted-phenylsulfanyl)-acetyl]-5-substituted furans and 2- [2'-(phenylselanyl)-acetyl]-5-methylfuran

J. Valençaa, Paulo R.Olivato, Daniel N. S. Rodrigues, Patrick R. Batista, Lucas C. Ducati and Maurizio Dal Colle

The conformational equilibrium of 2-[2'-(phenylselanyl)-acetyl]-5-methylfuran (1) and 2-[2'-(4'-sustituted-phenylsulfanyl)-acetyl]-5-substituted furans (2–7) was determined through the infrared (IR) analysis of the carbonyl stretching band (νCO) supported by M06–2X/aug-cc-pVDZ level of theory. Three stable conformations [sc(anti), ac(anti) and sc(syn)] were obtained in vacuum, with the sc(anti) the most stable for compound 1–6 and the ac(anti) for compound 7. The IR spectrain solution of n-C6H14, CCl4, CHCl3, CH2Cl2 and CH3CN show in general νCO doublets for compounds 2–6, with the exception of triplets in n-C6H14 for 2–4 and a symmetrical band in CHCl3 for 1, 3–6 and in CH2Cl2 and CH3CN for 1. The p-nitrophenyl compound 7 is insoluble in n-C6H14 and CCl4 and displays a doublet in all the other polar solvents. The PCM data allow to ascribe the sc(anti) conformer to the lowest frequency more intense νCO IR component and the sc(syn) one to the other doublet component for compounds 1–6, while the intermediary νCO frequency ac(anti) conformer, with negligible population, is assignedto the third triplet component predicted in n-C6H14 for compounds 2–4. Conversely, for compound 7, the more intense and lowest frequency νCOIR component was ascribed to the ac(anti), whereas the highest frequency one to the sum of the sc(anti) and sc(syn) populations. The conformational preferences of compounds 1–7 are governed by a balance between the orbital and the coulombic interactions estimated by means of natural bond orbitals (NBO), quantum theory of atoms in molecules (QTAIM), non covalent interaction (NCI) and short contacts analysis. While NBO delocalization energies indicate the ac(anti) conformer as the most stable for all compounds, NCI analysis reveals in the sc(anti) and sc(syn) conformers of compounds 1–6 an additional intramolecular stabilizing π‧‧‧π stacking interaction between the furyl and phenyl ring, which is counterbalanced in the sc(syn) conformer by the repulsive coulombic short contact between the carbonyl and furyl oxygen atoms. For compound 7, the ac(anti) conformer turns to be the most stable one as the electron withdrawing effect of the nitro substituent on the phenyl ring decreasesthe stabilizing π‧‧‧π stacking on the sc(anti) conformer

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DEC

2020

Flow Synthesis of 2-[Methyl(pyridin-2-yl)amino]ethanol: An Experimental and Computational Study

Paulo Victor Cuesta Calvo, Patrick Rodrigues Batista, Renan Rodrigues de Oliveira Silva, Attilio Converti, Saleh Al Arni, Carlo Solisio, Lucas C. Ducati, Mauri Sergio Alves Palma

Microreactor technology is increasingly applied in the chemical-pharmaceutical industry for safer and more efficient drug production as compared to the traditional batch process. This technology is employed for the first time to study the production of 2-[methyl(pyridin-2-yl)amino]ethanol, the first intermediate in rosiglitazone synthesis. Under the optimum operating conditions, a single microreactor chip at 160 °C allowed to produce the equivalent of more than five batch reactors at 120 °C. The kinetic study indicated that the reaction is second order. Thermodynamic parameters were determined by the Eyring equation and density functional theory (DFT) calculations with good agreement. DFT results suggested a concerted nucleophilic aromatic substitution reaction mechanism

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JUL

2020

Analyzing the N–H+…π interactions of protonated tryptophan and phenylalkylamines using QTAIM, NCI, and NBO

André F. Rodrigues-Oliveira, Patrick R. Batista, Lucas C. Ducati, and Thiago C. Correra

Tryptophan and phenylakylamines (PAAs) are important biomolecules, which are involved in a myriad of processes. These molecules have been experimentally reported to exhibit N–H+…π interactions in their protonated form; however, this has not been theoretically investigated in detail. Generally, such interactions are observed in diverse biological systems, and their evaluation would be useful for understanding protein folding and functioning. Therefore, in this study, we have described the N–H+…π interactions of the abovementioned classes of molecules using the Quantum Theory of Atoms In Molecules (QTAIM), Natural Bond Orbital (NBO), and Non-Covalent Interaction (NCI) analyses. The results of our N–H+…π interaction energy calculations were consistent with the experimental energies derived from the redshift of the vibrational stretches. The energy values obtained using the QTAIM-based Espinosa’s approach provided a relatively better result than similar approaches previously reported in the literature. Furthermore, we observed that the N–H+…π interaction energy in tryptophan is weaker than the resonance-assisted hydrogen bond energy of the N–H+…O=C; interactions of its three most stable conformers accessible at room temperature. In contrast, the strength of the N–H+…π interaction in PAAs was observed to increase with increasing alkyl lateral chain length. The increased flexibility of longer chains increases the distance between nitrogen and the phenyl ring without disturbing the N–H+…π interaction

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SEP

2019

Dealing with Hydrogen Bonding on the Conformational Preference of 1,3-Aminopropanols: Experimental and Molecular Dynamics Approaches

Patrick R. Batista, Lucas J. Karas, Renan V. Viesser, Cynthia C. de Oliveira, Marcos B. Gonçalves, Cláudio F. Tormena, Roberto Rittner, Lucas C. Ducati, and Paulo R. de Oliveira

This study expands the knowledge on the conformational preference of 1,3-amino alcohols in the gas phase and in solution. By employing Fourier transform infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT) calculations, quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) analysis, and molecular dynamics (MD), the compounds 3-aminopropan-1-ol (1), 3-methylaminopropan-1-ol (2), and 3-dimethylaminopropan-1-ol (3) are evaluated. The results show that the most stable conformation of each compound in the gas phase and in nonpolar solvents exhibited an O–H···N intramolecular hydrogen bond (IHB). Based on the experimental and theoretical OH-stretching frequencies, the IHB becomes stronger from 1 to 3. In addition, from the experimental NMR J-couplings, the IHB conformers are predominant in nonbasic solvents, representing 70–80% of the conformational equilibrium, while in basic solvents, such conformers only represent 10%. DFT calculations and QTAIM analysis in the gas phase support the occurrence of IHBs in these compounds. The MD simulation indicates that the non-hydrogen-bonded conformers are the lowest energy conformations in the solution because of molecular interactions with the solvent, while they are absent in the implicit solvation model based on density. NBO analysis suggests that methyl groups attached on the nitrogen atom affect the charge transfer energy involved in the IHB. This effect occurs mostly because of a decrease in the s-character of the LPN orbital along with weakening of the charge transfer from LPN to σ*OH, which is caused by an increase in the C–C–N bond angle.

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FEB

2018

Thermochromic Uranyl Isothiocyanates: Influencing Charge Transfer Bands with Supramolecular Structure

Robert G. Surbella III, Lucas C. Ducati, Jochen Autschbach, Nicholas P. Deifel, and Christopher L. Cahill

The synthesis and structural characterization of seven new [UO2(NCS)5]3–- and [UO2(NCS)4Cl]3–-containing materials charge balanced by 4-phenylpyridinium or 4,4′-bipyridinium cations are reported. Assembly of these materials occurs via a diverse set of noncovalent interactions, with the most prevalent involving the terminal sulfur atoms, which can both accept hydrogen bonds and/or form S···S and S···Oyl interactions. The electrostatic potential of the [UO2(NCS)5]3– and [UO2(NCS)4Cl]3– anions was calculated and mapped on the 0.001 au isodensity surface to rationalize the observed assembly modes and to provide an electrostatic basis to elucidate the role of the S atoms as both donors and acceptors of noncovalent interactions. Compounds 1–7 display a range of colors (red to yellow) as well as pronounced thermochromism. A computational treatment (time-dependent density functional theory, TDDFT) of the absorbance properties supports the temperature dependence on the ratio of inter- to intramolecular ligand to metal charge transfer (LMCT) bands as obtained from UV–vis diffuse reflectance analysis. Finally, the luminescence profiles of these materials feature additional peaks atypical for most uranyl-containing materials, and a combined spectroscopic (Raman, IR, and fluorescence) and computational (harmonic frequency calculations) effort assigns these as originating from vibronic coupling between the ν1 U═O symmetric stretch and bending modes of the isothiocyanate ligands.

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MAR

2018

The halogen effect on the 13C NMR chemical shift in substituted benzenes

Renan V. Viesser, Lucas C. Ducati, Cláudio F. Tormena, and Jochen Autschbach

Recent research [Chem. Sci., 2017, 8, 6570–6576] showed for R-substituted benzenes with R = NH2, NO2 that the substitution effects on the 13C NMR chemical shifts are correlated with changes in the σ-bonding framework and do not follow directly the electron-donating or -withdrawing effects on the π orbitals. In the present work we extend the study to halogen (X = F, Cl, Br or I) substituted R-benzenes. The effect of X and R groups on 13C NMR chemical shifts in X–R-benzenes are investigated by density functional calculations and localized molecular orbital analyses. Deshielding effects caused by the X atom on the directly bonded carbon nucleus are observed for F and Cl derivatives due to a paramagnetic coupling between occupied π orbitals and unoccupied σ*C-X antibonding orbitals. The SO coupling plays an important role in the carbon magnetic shielding of Br and I derivatives, as is well known, and the nature of X also modulates the 13C paramagnetic shielding contributions. Overall, the X and R substituent effects are approximately additive.

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SEP

2018

Plutonium chlorido nitrato complexes: ligand competition and computational metrics for assembly and bonding

Robert G. Surbella III, Lucas C. Ducati, Jochen Autschbach, Kristi L. Pellegrini, Bruce K. McNamara, Jon M. Schwantesa, and Christopher L. Cahill

Four new [Pu(IV)Cln(NO3)6−n]2−(n = 0, 2, 3) and [Pu(VI)O2Cl3(NO3)]2− containing materials were crystallized from acidic, aqueous media and structurally characterized. The anions are assembled via hydrogen and halogen bonding motifs, which are rationalized computationally. The Pu–NO3 and –Cl bonds were probed using QTAIM and NLMO analyses and found to be polar and largely ionic.

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OCT

2018

Spin–spin coupling constants in linear substituted HCN clusters

Puspitapallab Chaudhuri, Lucas C. Ducati, and Angsula Ghosh

The indirect nuclear spin–spin coupling constants of homogeneous hydrogen-bonded HCN clusters are compared with those of inhomogeneous HCN clusters where one of the terminal HCN molecules is substituted by its isomer HNC and by LiCN. Both the intra- and intermolecular (across the hydrogen bond) coupling constants are calculated for the linear form of the clusters containing up to three molecular monomers using different hybrid DFT functionals. The geometry of the monomers and clusters is optimised at the B3LYP/6-311++G(d,p) level. The effect of substitution by the ionic compound LiCN on the coupling constants of HCN is found to be more pronounced than that by HNC. The Ramsey parameters that form the total spin–spin coupling constants are also analysed individually. Among the four Ramsey parameters, the Fermi Contact term is found to be the dominant contributor to the total coupling constants in most cases. The presence of LiCN in the cluster tends to decrease the intramolecular Fermi Contact values, while HNC increases the same in all dimers and trimers. The contributions of localised molecular orbitals have been analysed for the HCN–HNC cluster to obtain some additional insight about the SSCC transmission mechanism along the coupling pathway.

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NOV

2018

Quadrupolar 14N NMR Relaxation from Force-Field and Ab Initio Molecular Dynamics in Different Solvents

Adam Philips, Alex Marchenko, Lucas C. Ducati, and Jochen Autschbach

Quadrupolar NMR spin relaxation rates and corresponding line widths were computed for the quadrupolar nucleus 14N for neat acetonitrile as well as for 1-methyl-1,3-imidazole and 1-methyl-1,3,4-triazole in different solvents. Molecular dynamics (MD) was performed with forces from the Kohn–Sham (KS) theory (ab initio MD) and force-field molecular mechanics (classical MD), followed by KS electric field gradient (EFG) calculations. For acetonitrile the agreement of the 14N line width with experiment is very good. Relative line widths for the azole nitrogens are improved over simpler approximations used previously in conjunction with single-point calculations at the multiconfigurational self-consistent field level. Overall, the NMR line widths are computed within a factor of 2 of the experimental values, giving access to reasonable estimates both of the dynamic EFG variance in the solvated systems as well as the associated correlation times that determine the relaxation rates.

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JUL

2017

The unexpected roles of σ and π orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

Renan V. Viesser, Lucas C. Ducati, Cláudio F. Tormena, and Jochen Autschbach

Effects of electron-donating (R = NH2) and electron-withdrawing (R = NO2) groups on 13C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The 13C shift substituent effect in ortho, meta, and para position is determined by the σ bonding orbitals in the aryl ring. The π orbitals do not explain the substituent effects in the NMR spectrum as conventionally suggested in textbooks. The familiar electron donating and withdrawing effects on the π system by NH2 and NO2 substituents induce changes in the σ orbital framework, and the 13C chemical shifts follow the trends induced in the σ orbitals. There is an implicit dependence of the σ orbital NMR shift contributions on the π framework, via unoccupied π* orbitals, due to the fact that the nuclear shielding is a response property.

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JUL

2017

Transuranic Hybrid Materials: Crystallographic and Computational Metrics of Supramolecular Assembly

Robert G. Surbella III, Lucas C. Ducati, Kristi L. Pellegrini, Bruce K. McNamara, Jochen Autschbach, Jon M. Schwantes, and Christopher L. Cahill

Assembly of a family of 12 supramolecular compounds containing [AnO2Cl4]2– (An = U, Np, Pu), via hydrogen and halogen bonds donated by substituted 4-X-pyridinium cations (X = H, Cl, Br, I), is reported. These materials were prepared from a room-temperature synthesis wherein crystallization of unhydrolyzed and valence-pure [AnO2Cl4]2– (An = U, Np, Pu) tectons is the norm. We present a hierarchy of assembly criteria based on crystallographic observations and subsequently quantify the strengths of the non-covalent interactions using Kohn–Sham density functional calculations. We provide, for the first time, a detailed description of the electrostatic potentials of the actinyl tetrahalide dianions and reconcile crystallographically observed structural motifs and non-covalent interaction acceptor–donor pairings. Our findings indicate that the average electrostatic potential across the halogen ligands (the acceptors) changes by only ∼2 kJ mol–1 across the AnO22+ series, indicating that the magnitude of the potential is independent of the metal center. The role of the cation is therefore critical in directing structural motifs and dictating the resulting hydrogen and halogen bond strengths, the former being stronger due to the positive charge centralized on the pyridyl nitrogen, N–H+. Subsequent analyses using the quantum theory of atoms in molecules and natural bond orbital approaches support this conclusion and highlight the structure-directing role of the cations. Whereas one can infer that Columbic attraction is the driver for assembly, the contribution of the non-covalent interaction is to direct the molecular-level arrangement (or disposition) of the tectons.

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JUL

2017

Probing the geometry reorganization from solution to gas-phase in putrescine derivatives by IRMPD, 1H-NMR and theoretical calculations

Thiago C. Correra, André S. Fernandes, Marcelo M. Reginato, Lucas C. Ducati, Giel Berden and Jos Oomens

Single and double protonated (E)-1,4-diamine-2-butenes were evaluated as a model system to probe isomerization during the ESI processes employing infrared multiple-photon dissociation (IRMPD) spectroscopy and density function theory (DFT) calculations, including implicit and explicit solvation models. Our results show that the preferential protonation takes place at the amines for singly protonated species and that the double bond is not protonated even under double protonation, as expected from known pKa values. This behavior was shown to reflect the (E)–(Z) interconversion rate, as no interconversion was observed nor predicted by implicit solvation model based on density (SMD) calculations even by the olefin protonation pathway. Explicit solvent calculations show that the singly protonated (E) configuration observed in the gas phase is also the most stable configuration in solution due to molecular interactions with the solvent that are absent for the (Z) configuration. The explicit solvation calculation reverts the supposed gas-phase stability of the (Z) configuration in comparison to (E) from 9 kcal mol-1 (in relative Gibbs free energy) in the gas phase to +89 kcal mol-1 (in total potential energy) as depicted by explicit Monte Carlo (MC) simulations. Together with previous results for the saturated 1,4-diamines from Morton and coworkers that show the (Z) configuration related conformation to be the most stable geometry in the gas-phase due to intermolecular hydrogen bonding, our experiments clearly show that conformational reorganizations can take place during the ESI process. These results suggest that gas- phase experiments and vacuum calculations may not be valid as evidence for conformations in solution without prior testing to check for isomerization during the ESI process.

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JUL

2017

The unexpected roles of s and p orbitals in electron donor and acceptor group effects on the 13C NMR chemical shifts in substituted benzenes

Renan V. Viesser, Lucas C. Ducati, Cláudio F. Tormena and Jochen Autschbach

Effects of electron-donating (R=NH2) and electron-withdrawing (R=NO2) groups on 13C NMR chemical shifts in R-substituted benzene are investigated by molecular orbital analyses. The 13C shift substituent effect in ortho, meta, and para position is determined by the s bonding orbitals in the aryl ring. The p orbitals do not explain the substituent effects in the NMR spectrum as conventionally suggested in textbooks. The familiar electron donating and withdrawing effects on the p system by NH2 and NO2 substituents induce changes in the s orbital framework, and the 13C chemical shifts follow the trends induced in the s orbitals. There is an implicit dependence of the s orbital NMR shift contributions on the p framework, via unoccupied p* orbitals, due to the fact that the nuclear shielding is a response property.

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NOV

2016

NMR J-Coupling Constants of Tl−Pt Bonded Metal Complexes in Aqueous Solution: Ab Initio Molecular Dynamics and Localized Orbital Analysis

Lucas C. Ducati, Alex Marchenko and Jochen Autschbach

The influence of solvent (water) coordination and dynamics on the electronic structure and nuclear magnetic resonance (NMR) indirect spin−spin coupling (J-coupling) constants in a series of Tl−Pt bonded complexes is investigated using Kohn−Sham (KS) Car−Parrinello molecular dynamics (CPMD) and relativistic hybrid KS NMR calculations with and without coordination to water. Coordination of the Tl center by water molecules has a dramatic impact on 1J(Tl−Pt) and other J-coupling constants. It is shown that a previous computational study of the same complexes using static optimized structures and nonhybrid functionals was correct about the important role of the solvent but obtained reasonable agreement with experimental NMR data because of a cancellation of substantial errors. For example, the CPMD trajectories show that on average the inner coordination shell of Tl is not saturated, as previously assumed, which leads to poor agreement with experiment when the J-coupling constants are averaged over the CPMD trajectories using NMR calculations with nonhybrid functionals. The combination of CPMD with hybrid KS NMR calculations provides a much more realistic computational model that reproduces the large magnitudes of 1J(Tl−Pt) and the correct trends for other coupling constants. An analysis of 1J(Tl−Pt) in terms of localized orbitals shows that the presence of coordinating water molecules increases the capacity for covalent interactions between Tl and Pt. There is pronounced multicenter bonding along the metal−metal axis of the complexes.

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AUG

2016

NMR Spin-Spin Coupling Constants: Bond angle dependence of the sign and magnitude of the vicinal 3JHF coupling

Renan V. Viesser, Lucas C. Ducati, Jochen Autschbachc and Cláudio F. Tormena

The dependence of the magnitude and sign of 3JHFF on the bond angle in fluoro-cycloalkene compounds is evaluated by electronic structure calculations using different levels of theory, viz. DFT, SOPPA(CCSD) and SOPPA(CC2). Localized molecular orbital contributions to 3JHFF are analyzed to assess which orbitals are responsible for the 3JHFF and which are the most important coupling transmission mechanisms for each compound. Fluoro-ethylene is used as a model system to evaluate the dependence of the 3JHFF coupling constant on the angle between the σCα-F and σCα'-HF vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between 3JHFF and bond angle, and for the negative signs obtained for two molecules, respectively. One of the fluorine lone pairs, σCα'-HF, σCα-F, σCα'-Cβ' bonding orbitals and the σ*Cα-F antibonding orbital are involved in the J-coupling pathways, according to analyses of pairwise-steric and hyperconjugative energies.

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JUN

2015

Effects of stereoelectronic interactions on the relativistic spin–orbit and paramagnetic components of the 13C NMR shielding tensors of dihaloethenes

Renan V. Viesser, Lucas C. Ducati, Jochen Autschbachc and Cláudio F. Tormena

In this study, stereoelectronic interactions were considered to explain the experimental difference in the magnitude of the known heavy-atom effect on the 13C NMR chemical shifts in cis- and trans-1,2-dihaloethene isomers (halo = F, Cl, Br or I). The experimental values were compared to the calculated values with various DFT functionals using both the nonrelativistic approach (NR) and the relativistic approximations SR-ZORA (SR) and SO-ZORA (SO). NBO and NLMO contributions to the 13C NMR shielding tensors were determined to assess which stereoelectronic interactions have a more important effect on the shielding tensor in each principal axis system (PAS) coordinate. These analyses associated with the orbital rotation model and the HOMO–LUMO energy gap enable rationalization of trends between cis and trans isomers from fluorine to iodine derivatives. Both paramagnetic and SO shielding terms were responsible for the observed trends. It was possible to conclude that the steric interactions between the two iodine atoms and the hyperconjugative interactions involving the halogen lone pairs (LP(X)) and π*C=C, σ*C=C and σ*C-X antibonding orbitals are responsible for the lower 13C NMR shielding for the cis isomers of the bromine and the iodine compounds than that of the trans isomers.

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FEV

2015

Experimental and theoretical evaluation on the conformational behavior of L-aspartic acid dimethyl ester and its N-acetylated derivative

Carolyne B. Braga, Lucas C. Ducati, and Roberto Rittner

In this work the conformational preferences of L-aspartic acid dimethyl ester (AspOMe) and its N-acetylated derivative (AcAspOMe) were evaluated through spectroscopic data and theoretical calculations. Unlike amino acids, their corresponding amino ester derivatives do not exhibit a zwitterionic structure and are soluble in most organic solvents, enabling their studies in these media. Thus, the conformers of AspOMe and AcAspOMe were theoretically determined both in isolated phase and in solution (IEF-PCM model) at the ωB97X-D/aug-cc-pVTZ level. A joint analysis of the experimental and theoretical 3JHH coupling constants in several aprotic solvents allowed assigning the most stable conformers, showing excellent agreement between these approaches. Also, IR spectroscopy allowed us to obtain quantitative data on AcAspOMe conformer populations in different solvents. Natural bond orbital (NBO) analysis indicated that both steric and hyperconjugative contributions count in determining the relative conformer stabilities of these compounds. Intramolecular hydrogen bonding, characterized by Quantum Theory of Atoms in Molecules (QTAIM) and Non-Covalent Interactions (NCI) methodologies, represents only a secondary factor to drive the stabilities of AspOMe and AcAspOMe conformers.

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JUL

2014

Revisiting NMR Through-Space JFF Spin−Spin Coupling Constants for Getting Insight into Proximate F‐-‐F Interactions

Rubén H. Contreras, Tomás Llorente, Lucas C. Ducati, and Cláudio F. Tormena

At present times it is usual practice to mark biological compounds replacing an H for an F atom to study, by means of 19F NMR spectroscopy, aspects such as binding sites and molecular folding features. This interesting methodology could nicely be improved if it is known how proximity interactions on the F atom affect its electronic structure as gauged through high-resolution 19F NMR spectroscopy. This is the main aim of the present work and, to this end, differently substituted peri-difluoronaphthalenes are chosen as model systems. In such compounds are rationalized some interesting aspects of the diamagnetic and paramagnetic parts of the 19F nuclear magnetic shielding tensor as well as the transmission mechanisms for the PSO and FC contributions to 4JF1F8 indirect nuclear spin−spin coupling constants.

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JUL

2014

13C NMR: nJCH and 1JCC scalar spin–spin coupling constants (SSCCs) for some 3-monosubstituted 2-methylpropenes

Ivania T A Schuquel, Lucas C Ducati,Claudio F Tormena, Matheus P. de Freitas, Dora G. Kowalewski, and Roberto Rittner

Twelve methallylic derivatives were studied using 13C NMR spectroscopy and theoretical calculations. Theoretical coupling constants were obtained with SOPPA (CCSD). All atoms were described by the EPR-III basis set, except for halogens, which were represented by the cc-pVTZ basis set. Geometries were optimized at the MP2/aug-cc-pVTZ level of theory, confirming previous results that the gauche and s-cis conformers predominate. Experimental nJCH coupling constants were determined from coupled 13C NMR spectra, while the 1JCC couplings were measured through the INADEQUATE technique. The experimental and theoretical values were in good agreement. Correlations with the usual substituent physicochemical parameters indicated that the 1JC3Hc values (where Hc is attached to the C3 carbon bearing the substitu- ent) exhibit good correlations with Taft’s sigma (σI inductive parameter) and the Swain–Lupton Field Effect (F) (R ~ 0.957), while other correlations were not similarly significant (R values < 0.9).

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APR

2014

Phenylalanine and tyrosine methyl ester intramolecular interactions and conformational analysis by 1H NMR and infrared spectroscopies and theoretical calculations

Rodrigo A Cormanich, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

Amino acid conformational analysis in solution are scarce, since these compounds present a bipolar zwitterionic structure (+H3N-CHR-COO-) in these media. Also, intramolecular hydrogen bonds have been classified as the sole interactions governing amino acid conformational behavior in the literature. In the present work we propose phenylalanine and tyrosine methyl ester conformational studies in different solvents by 1H NMR and infrared spectroscopies and theoretical calculations. Both experimental and theoretical results are in agreement and suggest that the conformational behavior of the phenylalanine and tyrosine methyl esters are similar and are dictated by the interplay between steric and hyperconjugative interactions.

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MAR

2014

A study of the rotational barriers for some organic compounds using the G3 and G3CEP theories

Douglas H Pereira, Lucas C Ducati, Roberto Rittner and Rogério Custodio

The G3, G3CEP, MP4, MP4CEP, QCISD(T), and QCISD(T)CEP methods were applied to study 43 internal rotational barriers of different molecules. The calculated G3 and G3CEP barriers were accurate with respect to those obtained experimentally, typically showing deviations of < 0.50 kcal mol−1. The results for the MP4CEP, MP4, QCISD(T), and QCISD(T)CEP calculations were less accurate, and larger deviations of approximately ±1 kcal mol−1 were observed. The accuracy of G3CEP was comparable to that of G3, but a reduction in CPU time of between 5 and 35% was observed when the dependence of the pseudopotentials on the size of the molecule and atom type was taken into account. The behaviors of the energy components show that these corrections depend on the molecular environment and whether the calculations are performed with all electrons or pseudopotentials. Usually, the predominance of a specific effect follows a distinct pattern when the G3 and G3CEP results are compared. For the G3 calculations, the most important component of the corrected MP4/6-31G(d) rotational energy is ΔE2df,p. Among the 43 molecules, 29 were dependent on polarization effects, ΔE2df,p; 19 were dependent on diffuse functions, ΔE+; and 13 depended on the effects of more elaborate basis functions (ΔEG3large). Similar behavior was observed for the G3CEP calculations: polarization effects were more important for 25 molecules, followed closely by the effect of diffuse functions for 23 molecules, and finally the effect of large basis sets (19 molecules). ΔEQCI correction seldom resulted in significant effects on the G3 and G3CEP calculations.

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MAR

2014

Conformational Analysis and Intramolecular Interactions of L‐Proline Methyl Ester and Its N‐Acetylated Derivative through Spectroscopic and Theoretical Studies

Carolyne B Braga, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

This work reports a detailed study regarding the conformational preferences of L-proline methyl ester (ProOMe) and its N-acetylated derivative (AcProOMe) to elucidate the effects that rule their behaviors, through nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies combined with theoretical calculations. These compounds do not present a zwitterionic form in solution, simulating properly amino acid residues in biological media, in a way closer than amino acids in the gas phase. Experimental 3JHH coupling constants and infrared data showed excellent agreement with theoretical calculations, indicating no variations in conformer populations on changing solvents. Natural bond orbital (NBO) results showed that hyperconjugative interactions are responsible for the higher stability of the most populated conformer of ProOMe, whereas for AcProOMe both hyperconjugative and steric effects rule its conformational equilibrium.

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DEC

2013

A critical evaluation of the s-cis–trans isomerism of 2-acetylpyrrole and its N-methyl derivative through infrared and NMR spectroscopies and theoretical calculations

Lucas C Ducati, Carolyne B Braga, Roberto Rittner, and Claudio F Tormena

Literature data are controversial regarding the conformational equilibria of 2-acetylpyrrole (AP) and its N-methyl derivative (AMP). Now, a detailed study through infrared spectroscopy and theoretical calculations has shown that previous data were erroneously interpreted, since only a N,O-cis conformer is present in solution and that it is the stable conformer in the isolated state (ΔEtrans-cis = 5.05 kcal mol−1, for AP; ΔEtrans-cis = 7.14 kcal mol−1, for AMP). Carbonyl and Nsingle bondH absorption data in different solvents, supported by theoretical results taking into account the solvent effects [at IEFPCM-B3LYP/6-311++G(3df,3p) level of theory] clearly demonstrated that only the N,O-cis conformer is present in solution. However, a doublet was observed for AP, in CCl4 which can be attributed to this conformer and the lowest wavenumber component to the cis dimer form, stabilized through intermolecular hydrogen bonds (NH⋯O=C). The overall preference for the N,O-cis conformer, in AP and AMP, as interpreted by the NBO analysis, indicated that the hyperconjugative effect is the main contribution to stabilize this rotamer, overcoming the possible steric repulsion. 13C NMR experiments at low temperature in two different solvents (CS2/CDCl2 and acetone-d6) confirmed the occurrence of a single conformer since no separated signals were observed.

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OCT

2013

1H NMR and theoretical studies on the conformational equilibrium of tryptophan methyl ester

Claudimar J Duarte, Rodrigo A Cormanich, Lucas C Ducati, and Roberto Rittner

Selected 3JHH coupling constants and theoretical calculations were used to explain the conformational equilibrium of L-tryptophan methyl ester (Trp-OMe) in several solvents. The obtained 3JHaHb values did not exhibit any significant variability and thus indicate that there are no conformational population variations for the side chain of the Trp-O-Me depending on the solvent. Moreover, the potential energy surfaces obtained at the B3LYP/cc-pVDZ theoretical level produced eight energy minima that were analysed by QTAIM and NBO methods. It was possible to conclude that the Trp-OMe conformational preferences were due to hyperconjugative effects involving the nonbonding electron pairs of the main chain nitrogen atom and certain antibonding orbitals (σ*C4-C13, σ*C1-C4 and σ*C4-H12) and also to the steric effects from the nonbonding electron pairs of oxygen atoms and the main and side chain of this system.

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JUL

2013

A theoretical and experimental 1H NMR spectroscopy study of the stereoelectronic interactions that rule the conformational energies of alanine and valine methyl ester

Rodrigo A Cormanich, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

Amino acids exhibit a bipolar zwitterionic structure (+H3N-CHR-COO-) in solution; hence, conformational studies of these compounds have been limited to the gas phase. The conformational preferences of amino acids have been widely attributed to intramolecular hydrogen bonding, despite steric and hyperconjugative effects. In this work, we propose the conformational study of alanine and valine methyl esters, which do not show zwitterionic structures in solution, by 1H NMR and theoretical calculations. The 3JHH spin–spin coupling constants and theoretical calculations were found to be in agreement, showing that the interplay between steric hindrance and hyperconjugation is the forces that are responsible for determining the conformational preferences of alanine and valine methyl esters.

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JUN

2013

A theoretical investigation of the dictating forces in small amino acid conformational preferences: The case of glycine, sarcosine and N,N-dimethylglycine

Rodrigo A Cormanich, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

Amino acid conformational analysis is widely studied in the literature. However, information about the intramolecular interactions that govern their conformational preferences is scarce and it is commonly attributed to intramolecular hydrogen bond formation. The present paper utilizes calculations at the B3LYP/aug-cc-pVDZ theoretical level and QTAIM and NBO methods for glycine, sarcosine and N,N-dimethylglycine conformers to emphasize that arbitrary literature interpretations are equivocal. Also, our results show that the interplay between steric and hyperconjugative interactions rules glycine conformer energies/geometries and such results are confirmed by sarcosine and N,N-dimethylglycine conformational preferences.

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02

2013

Studies on the s-cis–trans isomerism for some furan derivatives through IR and NMR spectroscopies and theoretical calculations

Roberto Rittner, Lucas C Ducati, Claudio F Tormena, Rodrigo A Cormanich, Barbara C Fiorin, Carolyne B Braga, and Raymond J Abraham

The s-cis–trans isomerism of two furan derivatives [2-acetyl- (AF) and 2-acetyl-5-methylfuran, (AMF)] was analyzed, using data from the deconvolution of their carbonyl absorption band in two solvents (CH2Cl2 and CH3CN). These infrared data showed that the O,O-trans conformers predominate in the less polar solvent (CH2Cl2), but these equilibria change in a more polar solvent (CH3CN) leading to a slight predom- inance of the O,O-cis conformers, in agreement with the theoretical calculations. The later results were obtained using B3LYP-IEFPCM/6-31++g(3df,3p) level of theory, which taking into account the solvent effects at IEFPCM (Integral Equation Formalism Polarizable Continuum Model). Low temperature 13C NMR spectra in CD2Cl2 (ca. -75 °C) showed pairs of signals for each carbon, due to the known high energy barrier for the cis–trans interconversion leading to a large predominance of the trans isomers, which decreases in acetone-d6. This was confirmed by their 1H NMR spectra at the same temperatures. Moreover, despite the larger hyperconjugative interactions for the O,O-cis isomers, obtained from NBO data, these isomers are destabilized by the their Lewis energy.

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MAY

2012

Unexpected Geometrical Effects on Paramagnetic Spin–Orbit and Spin–Dipolar 2JFF Couplings

Lucas C Ducati, Rubén H Contreras, and Claudio F Tormena

The second-rank tensor character of the paramagnetic spin−orbit and spin−dipolar contributions to nuclear spin−spin coupling constants is usually ignored when NMR measurements are carried out in the isotropic phase. However, in this study it is shown that isotropic 2JFF couplings strongly depend on the relative orientation of the C−F bonds containing the coupling nuclei and the eigenvectors of such tensors. Predictions about such effect are obtained using a qualitative approach based on the polarization propagator formalism at the RPA, and results are corroborated performing high-level ab initio spin−spin coupling calculations at the SOPPA(CCSD)/EPR-III//MP2/EPR-III level in a model system. It is highlighted that no calculations at the RPA level were carried out in this work. The quite promising results reported in this paper suggest that similar properties are expected to hold for the second-rank nuclear magnetic shielding tensor.

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MAY

2012

Conformational and stereoeletronic investigations of muscarinic agonists of acetylcholine by NMR and theoretical calculations

Julio Cesar A da Silva, Lucas C Ducati, and Roberto Rittner

NMR solvent effects and theoretical calculations showed muscarinic agonists present a large stability for their near synclinal conformations, indicating the presence of significant stabilization factors. Analysis of the results clearly indicated that this stability is not determined by the dihedral around the substituted C–C ethane bond, as stated by some authors, but a consequence of the geometry adopted in order to maximize N+/O interactions in this type of molecules. It can be assumed that acetylcholine and its muscarinic agonists exhibit their biologic activity when the positively charged nitrogen and the oxygen atoms are in the same side of the molecule within an interatomic distance ranging from 3.0 to 6.0 Å.

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APR

2012

The lack of intramolecular hydrogen bonding and the side chain effect in alanine conformers

Rodrigo A Cormanich, Lucas C Ducati, and Roberto Rittner

Although amino acids are considered one of the most important classes of compounds found in Nature, the literature has few reports that have been made to understand the intramolecular interactions that govern their conformational energies and geometries. Actually, it has been arbitrarily assigned that possible intramolecular hydrogen bonding is the responsible force that dictates amino acid conformational preferences. In this paper, calculations at B3LYP/aug-cc-pVDZ level of theory within the NBO and QTAIM frameworks have shown that hyperconjugation and steric effects interplay, not H-bonding, are the intra-molecular interactions that govern alanine conformational preferences. It is also shown that the steric interactions between the alanine methyl group side chain and its main chain influences its energies and geometries.

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MAR

2012

Critical analysis of the through-space transmission of NMR JFH spin-spin coupling constants

Rubén H Contreras, Lucas C Ducati, and Claudio F Tormena

known to show several unusual peculiarities. The main three are (a) in many cases their experimental values are reported to be positive and in others, negative; (b) theoretical values show that in some cases they are substantially contributed not only from the Fermi contact (FC) term but also from the paramagnetic spin orbit (PSO) term, and their respective signs could be either like or unlike; (c) in many cases it can hardly be expected that the corresponding F-H proximate interactions could be considered a "hydrogen bond", whereas in other cases it is evident that they are. For discussing points (a) and (b), characteristics of both the FC and PSO terms are discussed performing qualitative analyses based on their expressions given in terms of the polarization propagator formalism. Point (c) is discussed in terms of the well-known Bader’s QTAIM method as well as recalling the known relationship between the transmissions of the Fermi hole and of the FC interaction of spin–spin coupling constants.

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DEC

2011

Stereochemical Dependence of 3JCH Coupling Constants in 2-Substituted 4-t-Butyl-cyclohexanone and Their Alcohol Derivatives

Denize C Favaro, Lucas C Ducati, Francisco P dos Santos, Rubén H Contreras, and Claudio F Tormena

Theoretical and experimental studies on 3JC2H6eq NMR spinspin coupling constants in both the 2-X-4-t-butyl-cyclohexanone (X = H, CH3, F, Cl, and Br) and in their alcohol derivatives series are reported. Results thus found are rationalized in terms of the transmission of the Fermi contact contribution to such couplings. To this end, dependencies of 3JC2H6eq couplings versus the C2-C1-C6 angle are compared in both series for equatorial and axial X orientations. The main trend is described in terms of the rear lobes interaction. Besides, for X=halogen atom in equatorial orientation a rather strong interaction between oxygen and halogen lone pairs is observed, and its influence on 3JC2H6eq couplings is discussed and rationalized in terms of different Fermi contact transmission pathways.

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DEC

2011

Structural investigations of 5-hydroxy-4,5-dihydroisoxazoles

Patrick T Campos, Pablo Machado, Clarissa P Frizzo, Dayse N Moreira, Alexandre R Meyer, Helio G Bonacorso, Nilo Zanatta, Lucas C Ducati, Roberto Rittner, Claudio F Tormena, and Marcos A P Martins

The X-ray diffraction data determined for eight 3-(R3), 4-(R4), 4,4-(R4/R40) and/or 5-(R5) 5-hydroxy-4,5- dihydroisoxazoles [where R3 = Ph, R4/R40 = H/H, R5 = CCl3 (1); R3 = 4-Br-C6H4, R4/R40 = H/H, R5 = CCl3 (2); R3 =thien-2-yl, R4/R4’ =H/H, R5 =CCl3 (3); R3 =Ph, R4 =Ph, R4’ =OH, R5 =Me (4); R3 =Me, R4/R40 =N–OH, R5 = Me (5); R3 = CF3, R4/R4’ = H/H, R5 = CMe2CH2OH (6); R3 = H, R4 = 4-I-C6H4, R4’ = H, R5 = 4-I-C6H4 (7); R3/R4 = –(CH2)3–, R40 = H, R5 = CF2CF2H (8);] are discussed. The crystalline structure of compounds 1–3 is described for the first time and crystalline structure of compounds 4–8 has already been described in literature. It was found that the supramolecular auto-organization of 1–8 is characterized by hydrogen bonds invariably involving the hemiacetal hydroxyl group. Compound 5 is the only exception, where the hydroxyl oxime group is the participant in the hydrogen bond. Compounds 4 and 8 present intermolecular contact between the hydroxyl group of the hemiacetal and the nitrogen atom of the 4,5-dihydroisoxazole ring. Compound 7 presents similar interaction, where the hydroxyl contact is with the oxygen atom of the 4,5-dihydroisoxazole ring. Moreover, the crystal structure of compound 6 was stabilized by O-H...O interaction between the hydroxyl group of hemiacetal and the hydroxyl group of the alcohol function attached at the 5-position of 4,5-dihydroisoxazole. The crystal structure of compounds 1–3, as described here for the first time, was similar to that of compounds 4 and 7, showing a hydrogen bond O(51)-H(51)...N(2) between the hydroxyl group and the nitrogen atom of the isoxazoline ring. This means that the crystal structure of these compounds was governed by hydrogen bonds O–H...N, involving the hydroxyl of the hemiacetal group and the nitrogen atom of the 4,5-dihydroisoxazole ring. This interaction is relatively robust, showing a pattern in the crystal packing. Compounds 1–3 also have their crystal stabilized by more weak interactions of type Cl...Cl, involving the chlorine atom of the trichloromethyl group.

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SEP

2011

Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

Roberto Rittner, Lucas C Ducati, Claudio F Tormena, Barbara C Fiorin, and Carolyne B Braga

The s-cis–trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl4 and CHCl3 ). These infrared data showed that the O,S-cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS2/CD2Cl2 (-90 ºC) and in acetone-d6 (-80 ºC) did not show pairs of signals for each carbon, due to the known low energy barrier (~8 kcal mol−1 ) for the cis–trans interconversion. Data from NBO calculations show that the nO(2) -> σ*S-C5 and nO(2) -> σ*C2-C3 interactions occur only in the O,S-cis isomer and can explain its conformational preference.

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AUG

2011

Are hydrogen bonds responsible for glycine conformational preferences?

Rodrigo A Cormanich, Lucas C Ducati, and Roberto Rittner

Glycine conformational preferences have mostly been explained as due to the formation of intramolec- ular hydrogen bonding, despite other possible relevant intramolecular interactions that may be present in this molecular system. This paper, within the framework of the quantum theory of atoms in molecules and natural bond orbital analysis, at the B3LYP/aug-cc-pVDZ level, shows that hydrogen bonding formally stabilizes just one of the glycine conformers. Indeed, these theoretical calculations suggest that both steric hindrance and hyperconjugative effects rule conformational preferences of this model compound and may not be ignored in discussions of amino acid conformational analyses.

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FEV

2011

Experimental, SOPPA(CCSD), and DFT analysis of substitutent effects on NMR 1JCF coupling constants in fluorobenzene derivatives.

Janaina Dantas Vilcachagua, Lucas C Ducati, Roberto Rittner, Rubén H Contreras, and Claudio F Tormena

Interesting insight into the electronic molecular structure changes associated with substituent effects on the Fermi contact (FC) and paramagnetic spin-orbit (PSO) terms of 1JCF NMR coupling constants (SSCCs) in o-X-, m-X-, and p-X-fluorobenzenes (X = NH2; NO2) is presented. The formulation of this approach is based on the influence of different conjugative and hyperconjugative interactions on a second-order property, which can be qualitatively predicted if it is known how they affect the main virtual excitations entering into that second-order property. A set of consistent approximations are introduced in order to analyze the behavior of occupied and virtual orbitals, which define some experimental trends for 1JCF spin-spin coupling constants. In addition, DFT hybrid functionals were used, and a similar degree of confidence to compute the 1JCF with those observed for the SOPPA(CCSD) method was obtained. The 1JCF SSCCs for ezetimibe, a commercially fluorinated drug used to reduce cholesterol levels, were measured and DFT-calculated, and the qualitative approach quoted above was applied. As a byproduct, a possible method to determine experimentally a significant PSO contribution to 1JCF SSCCs is discussed.

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OCT

2010

Difference between 2JC2H3 and 2JC3H2 spin-spin couplings in heterocyclic five- and six-membered rings as a probe for studying σ-ring currents: a quantum chemical analysis.

Rubén H Contreras, Francisco P dos Santos, Lucas C Ducati, and Claudio F Tormena

Adequate analyses of canonical molecular orbitals (CMOs) can provide rather detailed information on the importance of different σ-Fermi contact (FC) coupling pathways (FC term transmitted through the σ-skeleton). Knowledge of the spatial distribution of CMOs is obtained by expanding them in terms of natural bond orbitals (NBOs). Their relative importance for transmitting the σ-FC contribution to a given spin-spin coupling constants (SSCCs) is estimated by resorting to the expression of the FC term given by the polarisation propagator formalism. In this way, it is possible to classify the effects affecting such couplings in two different ways: delocalisation interactions taking place in the neighbourhood of the coupling nuclei and "round the ring" effects. The latter, associated with σ-ring currents, are observed to yield significant differences between the FC terms of 2JC2H3 and 2JC3H2 SSCCs which, consequently, are taken as probes to gauge the differences in σ-ring currents for the five-membered rings (furan, thiophene, selenophene and pyrrol) and also for the six-membered rings (benzene, pyridine, protonated pyridine and N-oxide pyridine) used in the present study.

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MAR

2010

Exploring the G3 method in the study of rotational barrier of some simple molecules

Lucas C Ducati, Rogério Custodio, and Roberto Rittner

The G3 method was used to determine the internal rotation barriers of some simple molecules (H2O2, H2S2, N2H4, CH3OH, CH3NH2, and C2H6). The barriers were accurate with deviations lower than 0.5 kcal mol-1 with respect to the experimental data and comparable with other methods like CCSD(T)/aug-cc-pVTZ results. The G3 components showed that the MP4/6-31G(d) energy is quantitatively improved according to: ΔEG3Large > ΔE2df,p > ΔE+ > ΔEQCI. However, the relative energies showed that molecules containing lone pairs in neighbor atoms presented high barriers, which were particularly sensitive to polarization and diffuse functions. The G3Large and QCI effects showed no qualitative or quantitative relative contribution. Molecules containing lone pairs in one atom or only bond pairs presented low barriers and were sensitive not only to the polarization and diffuse effects but also depending on the improvement of the basis set through the G3Large correction. The QCI effect showed no qualitative or quantitative relevant contribution in any of the cases studied.

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JAN

2010

Analysis of canonical molecular orbitals to identify fermi contact coupling pathways. 1. Through-space transmission by overlap of 31P lone pairs

Rubén H Contreras, Gustavo Gotelli, Lucas C Ducati, Thais M Barbosa, and Claudio F Tormena

In this work, a new approach to studying coupling pathways for the Fermi contact term of NMR spin-spin coupling constants (SSCCs) is presented. It is based on the known form of propagating the Fermi hole through a canonical molecular orbital (CMO). It requires having an adequate spatial description of the relevant canonical molecular orbitals, which are obtained by expanding CMOs in terms of natural bond orbitals (NBOs). For detecting the relevant contributions of CMOs to a given Fermi contact (FC) pathway, the description of the FC in terms of the triplet polarization propagator (PP) is used. To appreciate the potential of this approach, dubbed FCCP-CMO (Fermi contact coupling pathways-CMO), it is applied to analyze the through-space transmission of the FC term of JPP SSCCs by overlap of the P lone pairs. This method can be applied using well-known quantum chemistry software without any further modification, which makes it appealing for use as a complement to SSCC measurements by NMR spectroscopy.

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OCT

2009

Molecules with all triple bonds: OCBBCO, N2BBN2, and [OBBBBO]2-

Lucas C Ducati, Nozomi Takagi, and Gernot Frenking

DFT calculations at the BP86/TZ2P level have been carried out for the compounds OCBBCO, N2BBN2, and [OBBBBO]2-. The calculations predict very short distances and large bond dissociation energies for the central B-B bonds. The nature of the bonding situation was investigated with an energy decomposition analysis. It shows that the central boron-boron bonds are genuine triple bonds. The π-bonding contributes between 38-40% to the total orbital interactions of the B-B bonds. The compounds can be considered as donor-acceptor complexes L-->BB<--L between the central B2 moiety in the third [(3)1Σg+] excited state and the ligands L = CO, N2, BO-. The π-backdonation L<--BB-->L for L = CO, N2 is very strong, which suggests that the latter bonds should also be considered as triple bonds. The π-bonding in [OB<--BB-->BO]2- is weaker, which makes the latter bonds borderline cases for triple bonds. The triple-bond character explains the very large bond dissociation energies for the LB-BL and L-BB-L bonds.

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JUN

2009

Conformational analysis of 2-halocyclopentanones by NMR and IR spectroscopies and theoretical calculations

Carina R Martins, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

The conformational isomerism of 2-chlorocyclopentanone and 2-bromocyclopentanone has been determined through the solvent dependence of the 1H NMR 3JHH coupling constants, theoretical calculations and infrared data, using the solvation theory for the treatment of NMR data. In 2-chlorocyclopentanone, the energy difference (E(Ψ-e)-E(Ψ-a)), in the isolated molecule at B3LYP level of theory, between the pseudo-equatorial (Ψ-e) and pseudo-axial (Ψ-a) conformers is 0.42 kcal mol-1, which decreases in CCl4 and in acetonitrile solutions, in good agreement with infrared data (νC-O), despite the uncertainties of the latter method. The conformational equilibrium for 2-bromocyclopentanone is also between the Ψ-e and Ψ-a conformations, with an energy difference (E(Ψ-e)-E(Ψ-a)), in the isolated molecule at B3LYP level of theory, is 0.85 kcal mol-1 which decreases in CCl4 and in acetonitrile solutions, also in good agreement with infrared data.

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APR

2009

Influence of tetralkylammonium cations on the formation of silicoaluminophosphates CAL-2

Darlene L Felix, Mathias Strauss, Lucas C Ducati, and Heloise O Pastore

SAPO molecular sieves were synthesized under hydrothermal conditions from gels containing AlPO- kanemite, silica and a series of four tetralkylammonium (TAA+) salts. The influence of each of the tetralkylammonium cations on the molecular sieve syntheses was studied at constant SiO2/Al2O3 and TAA+/ SiO2/Al2O2 molar ratios of 1.0. The obtained materials were characterized by typical physico-chemical tech- niques for solid materials such as powder X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), thermal analysis (thermogravimetry), elemental analysis (CHN), scanning electron microscopy (SEM) and multinuclear 13C- and 29Si-solid state nuclear magnetic resonance. A theoretical evaluation of the n-butylammonium conformers, energy of interconversion and molecular sizes, was useful to understand the behavior of each system containing different SDA’s. Thus different structures were obtained depending on the TAA+ cation used: in the case of tetramethylammonium the sodalite structure was prepared, tetraethylammonium and tetrapropylammonium cations on the other hand, directed the formation of chabazite structures and for the tetrabutylammonium cation the layered structure of the reactant was not changed. The presence of silicon islands is clear and is an important part of this work. 29Si-solid state nuclear magnetic resonance, elemental analysis of C, H and N and energy dispersive spectroscopy (EDS) techniques were all applied to characterize this aspect.

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JAN

2009

Heavy Halogen Atom Effect on C-13 NMR Chemical Shifts in Monohalo Derivatives of Cyclohexane and Pyran. Experimental and Theoretical Study

Alvaro Cunha Neto, Lucas C Ducati, Roberto Rittner, Claudio F Tormena, Ruben H Contreras, and Gernot Frenking

As a first step, a qualitative analysis of the spin-orbit operator was performed to predict the kind of organic compounds, where it could be expected that the SO/FC (spin-orbit/Fermi contact) and SO/SD (spin-orbit/spin dipolar) yield unusually small contributions to the "heavy atom effect" on C-13 SCSs (substituent chemical shifts). This analysis led to the conclusion that compounds presenting strong hyperconjugative interactions involving the σ*C-X orbital (X = halogen) are good examples where such effects can be expected to take place. On the basis of such results, the following set of model compounds was chosen: 2-eq-halocyclohexane (2-eq), 2-ax-halocyclohexane (2-ax), and 2-ax-halopyran (3), to measure C-13 SCSs. Such experimental values, as well as those of methane and halomethanes taken from the literature, were compared to calculated values at a nonrelativistic approach using B3LYP, and at a relativistic approach with BP86 using scalar ZORA, spin-orbit ZORA, scalar PAULI, and spin-orbit PAULI. Results from relativistic calculations are in agreement with the trends predicted by the qualitative model discussed in this work.

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FEV

2008

The case of intramolecular hydrogen bonding, hyperconjugation and classical effects on the conformational isomerism of substituted carbonyl and thiocarbonyl compounds

Lucas C Ducati, Matheus P Freitas, Cláudio F Tormena, and Roberto Rittner

The conformational isomerism of substituted (substituents = OR and SR, R=H and Me) acetaldehydes and thioacetaldehydes is described in terms of intramolecular interactions, namely hydrogen bonding (when R=H), hyperconjugation involving the carbonyl or the thiocarbonyl system, and classical effects (steric and electrostatic interactions). 3D potential energy surfaces were obtained by scan- ning both X-C-C-Y (α) and R-X-C-C (φ) dihedral angles (X/Y=O and S) and used to identify local and global minima. Geometry optimization and NBO calculations, including determination of NLMO steric energies and deletion of hyperconjugative interactions, were then performed in order to find the governing factors for these conformational equilibria. Hyperconjugative contribution for hydrogen bonding showed to be more important for thioaldehydes, while O-H showed to be a better proton donor than S-H; however, hydrogen bonding also appeared to be as of electrostatic nature. Overall, orbital interactions, particularly those involving the π* system, and classical factors (steric and electrostatic effects) drive the conformational isomerism of the title compounds.

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DEC

2007

An investigation of the electronic structure of some 3-monosubstituted-2-methylpropenes through computational chemistry and photoelectron spectroscopy

Ivania T A Schuquel, Lucas C Ducati, Rogerio Custodio, Roberto Rittner, and Dieter Klapstein

The photoelectron (PE) spectra of some 3-monosubstituted 2-methylpropenes H2C=C(CH3)CH2X [X = Cl, Br, I, OH, OMe, OEt, SH, SMe, SEt, N(Me)2 and N(Et)2] have been recorded. A preliminary analysis is presented indicating some trends in the ionization potentials associated with application of OVGF method and NBO analysis from MP2/6-31G(d,p) and cc-pVDZ level of theory indicating that the more effective hyperconjugation effect leads to the most stable conformers. The sensitivity of the outermost ionization energies of selected molecules with respect to the level of theory was analyzed. Application of the CASPT2 method with ANO basis set and geometries from MP2 calculations provided results in excellent agreement with the experimental data.

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DEC

2006

Effects of hyperconjugative interactions on 1JCH coupling constant for hexamethylenetetramine and adamantane: Theoretical and experimental study

Francisco P dos Santos, Lucas C Ducati, Claudio F Tormena, and Roberto Rittner

The objective of this work was to determine the influence of hyperconjugative interactions on the 1JCH coupling constant for hexamethylenetetramine (1) and adamantane (2). For this end, theoretical and experimental 1JCH were obtained and hyperconjugative interactions were investigated using NBO. It was observed, theoretically and experimentally, that 1JCH in 1 is 20 Hz larger than in 2, mainly due to the nN -> σC-H* hyperconjugative interaction. This interaction occurs only in 1, with an energy of 9.30 kcal mol-1. It increases the s-character of the carbon atom in the C-H bond and the occupancy of the nN -> σC-H* orbital in (1).

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DEC

2006

Conformational and stereoelectronic investigation of chloromethyl methyl sulfide and its sulfinyl and sulfonyl analogs

Lucas C Ducati, Matheus P Freitas, Cláudio F Tormena, and Roberto Rittner

The rotational equilibria of some sulfur-containing model compounds were theoretically determined, and a rationalization of the effects responsible for the results obtained is given. Experimental approaches, namely NMR and infrared, were also used to show the gauche (Cl-C-S-CH3 fragment) prevalence for these compounds and allowed us to elucidate the orbital interactions involved in such systems. The gauche rotamer of the sulfide is about 3.2 kcal mol-1 more stable than anti in the gas phase. For the sulfinyl derivative the relative energies for the most stable rotamers are 0.0, 1.0 and 1.7 kcal mol-1, corresponding to gauche-1 (C-Cl antiperiplanar to S=O), gauche-2 (C-Cl antiperiplanar to lone pair) and anti. The calculated energy difference (anti-gauche) between the sulfonyl rotamers is 2.6 kcal mol-1. Natural bond orbital analysis indicated that the anomeric effect (nS -> σ*C-Cl) plays the main role in stabilization of gauche in the sulfide compound, and that other important hyperconjugative interactions also occur for the remaining compounds.

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AUG

2006

Conformational isomerism and electronic interactions in some α-aminoketones

Lucas C Ducati, Roberto Rittner, and Rogério Custodio

The theoretically calculated potential energy surfaces (PES), at MP2/6-31G (d,p) level of theory, for the α-aminoacetone and α-aminocyclohexanone showed the occurrence of two conformers for the former, being the synperiplanar (Sp) 3.00 kcal mol-1 much more stable than the synclinal (Sc) conformer, while three axial [two Sc and one Ap (antiperiplanar)] and one equatorial (Ap) conformers were observed for the later, with the Ap–eq as the most stable conformation (2.07 kcal mol-1 from the lower energy axial conformer). For the more complex system, the N,N-dimethyl-a-aminoacetone, a 3D potential energy surface was obtained, which led to two stable conformers (Sc–Sc and Sp–Ap; ΔESp–Ap–Sc–Sc = 0.81 kcal mol-1) and two transition states (Sc–ScT and Ap–SpT), when the equivalent forms bringing an specular relationship are discarded. The stability of the different conformers for the three studied compounds were explained by the orbital interactions obtained through NBO calculations.

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