Density functional theory investigation of hydrogen bonding effects on the oxygen, nitrogen and hydrogen electric field gradient and chemical shielding tensors of anhydrous chitosan crystalline structure

A systematic computational investigation was carried out to characterize the 17O, 14N and 2H electric field gradient, EFG, as well as 17O, 15N, 13C and 1H chemical shielding tensors in the anhydrous chitosan crystalline structure. To include the hydrogen-bonding effects in the calculations, the most probable interacting molecules with the target molecule in the crystalline phase were considered through a hexameric cluster. The computations were performed with the B3LYP method and 6-311++G(d,p) and 6-31++G(d,p) standard basis sets using the Gaussian 98 suite of programs. Calculated EFG and chemical shielding tensors were used to evaluate the 17O, 14N and 2H nuclear quadrupole resonance, NQR, and 17O, 15N, 13C and 1H nuclear magnetic resonance, NMR, parameters in the hexameric cluster, which are in good agreement with the available experimental data. The difference between the calculated NQR and NMR parameters of the monomer and hexamer cluster shows how much hydrogen bonding interactions affect the EFG and chemical shielding tensors of each nucleus. These results indicate that both O(3)-H(33)...O(5-3) and N-H(22)...O(6-4) hydrogen bonding have a major influence on NQR and NMR parameters. Also, the quantum chemical calculations indicate that the intra- and intermolecular hydrogen bonding interactions play an essential role in determining the relative orientation of EFG and chemical shielding principal components in the molecular frame axes.     

DFT calculations of <Superscript>14</Superscript>N, <Superscript>17</Superscript>O,and <Superscript>2</Superscript>H NQR parameters: Investigation of hydrogen bond interactions in sulfapyridine crystalline structure

DFT calculations of electric field gradient (EFG) tensors at the sites of ¹⁴N, ¹⁷O, and ²H nuclei are carried out to characterize the hydrogen bond (HB) interactions in the sulfapyridine crystal structure. One-molecule (monomer) and hydrogen-bonded hexameric cluster models of sulfapyridine are constructed according to available X-ray coordinates where the proton positions are optimized. Then, EFG tensors are calculated for both monomer and target molecule in the hexameric cluster of sulfapyridine to show the effect of HB interactions on the tensors. The calculated EFG tensors are converted to the experimentally measurable nuclear quadrupole resonance (NQR) parameters: quadrupole coupling constant (C _Q ) and asymmetry parameter (η _Q ). The results reveal different contribution of various nuclei to N-H⋯N and N-H⋯O HB interactions in the cluster where the N2 and O1 have major contributions. The computations are performed with B3LYP and B3PW91 functionals DFT method and 6-311+G* and 6-311++G** standard basis sets using the Gaussian 98 package.     

Hydrogen bond interactions in sulfamerazine: DFT study of the O-17, N-14, and H-2 electric field gradient tensors

Hydrogen bond (HB) interactions are studied in the real crystalline structure of sulfamerazine by density functional theory (DFT) calculations of the electric field gradient (EFG) tensors at the sites of O-17, N-14, and H-2 nuclei. One-molecule (single) and four-molecule (cluster) models of sulfamerazine are created by available crystal coordinates and the EFG tensors are calculated in both models to indicate the influence of HB interactions on the tensors. Directly relate to the experiments, the calculated EFG tensors are converted to the experimentally measurable nuclear quadrupole resonance (NQR) parameters, quadrupole coupling constant (qcc) and asymmetry parameter (η_Q). The evaluated NQR parameters reveal that due to contribution of the target molecule to N–HN and N–HO types of HB interactions, the EFG tensors at the sites of various nuclei are influenced from single model to the target molecule in cluster. Additionally, O2, N4, and H2 nuclei of the target molecule are significantly influenced by HB interactions, consequently, they have the major contributions to HB interactions in cluster model of sulfamerazine. The calculations are performed employing B3LYP method and 6-311++G** basis set using GAUSSIAN 98 suite of program.     

An investigation of hydrogen-bonding effects on the nitrogen and hydrogen electric field gradient and chemical shielding tensors in the 9-methyladenine real crystalline structure: a density functional theory study

Hydrogen-bonding effects in the real crystalline structure of 9-methyladenine, 9-MA, were studied using calculated electric field gradient, EFG, and chemical shielding, CS, tensors for nitrogen and hydrogen nuclei via density functional theory. The calculations were carried out at the B3LYP and B3PW91 levels with the 6-311++G basis set via the Gaussian 98 package. Nuclear quadrupole coupling constants, C(Q), and asymmetry parameters, eta(Q), are reported for (14)N and (2)H. The chemical shielding anisotropy, Deltasigma, and chemical shielding isotropy, sigma(iso), are also reported for (15)N and (1)H. The difference between the calculated parameters of the monomer and heptameric layer-like cluster 9-MA shows how much H-bonding interactions affect the EFG and CS tensors of each nucleus. This result indicates that N(10) (imino nitrogen) has a major role in H-bonding interactions, whereas that of N(9) is negligible. There is good agreement between the present calculated parameters and reported experimental data. Although some discrepancies were observed, this could be attributed to the different conditions which were applied for calculation and the experiments.     

Study of hydrogen bonds in N-methylacetamide by DFT calculations of oxygen, nitrogen, and hydrogen solid-state NMR parameters

Solid-state nuclear magnetic resonance (NMR) parameters of ¹⁷O, ¹⁴N/¹⁵N, and ²H/¹H nuclei were evaluated in two available neutron crystalline structures of N-methylacetamide (NMA) at 250 and 276 K, NMA-I and NMA-II, respectively. Density functional theory calculations were performed by B3LYP method and 6-311++G** and IGLO-II type basis sets to calculate the electric field gradient (EFG) and chemical shielding (CS) tensors at the sites of mentioned nuclei. In order to investigate hydrogen bonds (HBs) effects on NMR tensors, calculations were performed on four-model systems of NMA: an optimized isolated gas-phase, crystalline monomers, crystalline dimers, and crystalline trimers. Comparing the calculated results reveal the influence of N–H···O=C and C–H···O=C HB types on the NMR tensors which are observable by the evaluated parameters including quadrupole coupling constant, C _Q, and isotropic CS, σ _iso. Furthermore, the results demonstrate more influence of HB on the EFG and CS tensors of NMA at 276 K rather than that of 250 K.     

B24N24 nanocages: a GIAO density functional theory study of 14N and 11B nuclear magnetic shielding and electric field gradient tensors

Abstract  

Density functional theory (DFT) calculations were performed to determine boron-11 and nitrogen-14 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy parameters in the three most stable B₂₄N₂₄ fullerenes for the first time. The considered samples were first allowed to relax entirely, and then the NMR and NQR calculations were performed on the geometrically optimized models. The calculations of the ¹¹B and ¹⁴N nuclear magnetic shielding tensors and electric field gradient tensors employed the Gaussian 98 software implementation of the gauge-including atomic orbital (GIAO) method using the Becke3, Lee-Yang-Parr (B3LYP) DFT level and 6-311G** and 6-311++G** standard basis sets in each of the three optimized forms, and converted the results to experimentally measurable NMR parameters.The calculated NMR chemical shieldings of the three cages show significant differences, providing a way to identify these clusters. The evaluated NQR parameters of the ¹¹B and ¹⁴N nuclei in the clusters are also reported and discussed.     

Theoretical 13C chemical shift, 14N, and 2H quadrupole coupling- constant studies of hydrogen bonding in L-alanylglycine dipeptide

(13)C chemical shieldings and (14)N and (2)H electric field gradient (EFG) tensors of L-alanylglycine (L-alagly) dipeptide were calculated at RHF/6-31 + + G** and B3LYP/6-31 + + G** levels of theory respectively. For these calculations a crystal structure of this dipeptide obtained from X-ray crystallography was used. Atomic coordinates of different clusters containing several L-alagly molecules were used as input files for calculations. These clusters consist of central and surrounding L-alagly molecules, the latter forming short, strong, hydrogen bonds with the central molecule. Since the calculations did not converge for these clusters, the surrounding L-alagly molecules were replaced by glycine molecules. In order to improve the accuracy of calculated chemical shifts and nuclear quadrupole coupling constants (NQCCs), different geometry-optimization strategies were applied for hydrogen nuclei. Agreement between calculated and experimental data confirms that our optimized coordinates for hydrogen nuclei are more accurate than those obtained by X-ray diffraction.     

B<Subscript>24</Subscript>N<Subscript>24</Subscript> nanocages: a GIAO density functional theory study of <Superscript>14</Superscript>N and <Superscript>11</Superscript>B nuclear magnetic shielding and electric field gradient tensors

Abstract  Density functional theory (DFT) calculations were performed to determine boron-11 and nitrogen-14 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy parameters in the three most stable B₂₄N₂₄ fullerenes for the first time. The considered samples were first allowed to relax entirely, and then the NMR and NQR calculations were performed on the geometrically optimized models. The calculations of the ¹¹B and ¹⁴N nuclear magnetic shielding tensors and electric field gradient tensors employed the Gaussian 98 software implementation of the gauge-including atomic orbital (GIAO) method using the Becke3, Lee-Yang-Parr (B3LYP) DFT level and 6-311G** and 6-311++G** standard basis sets in each of the three optimized forms, and converted the results to experimentally measurable NMR parameters.The calculated NMR chemical shieldings of the three cages show significant differences, providing a way to identify these clusters. The evaluated NQR parameters of the ¹¹B and ¹⁴N nuclei in the clusters are also reported and discussed. Graphical abstract        

Characterization of intermolecular interactions in crystalline aspirin: A computational NQR study

A computational study at the level of density functional theory was carried out to characterize the ¹⁷O and ²H nuclear quadrupole resonance (NQR) spectroscopy parameters in crystalline aspirin. To include O? H···O and C? H···O hydrogen bonding effects in the calculations, the most probable interacting molecules with the target molecule in the crystalline phase were considered through a pentamer cluster. The NQR calculations were performed with BLYP, B3LYP, and M06 functionals employing 6‐311++G** and Jensen's polarization‐consistent pcJ‐1 basis sets. Linear correlations are observed between the calculated ¹⁷O and ²H NQR parameters and the hydrogen bond strengths, suggesting the possibility of estimating hydrogen bonding information from calculated NQR data. Different contributions of various nuclei to hydrogen bonding interactions and observed trends of calculated NQR parameters are well justified by atoms in molecules analyses at the BCPs of these interactions. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Study of hydrogen bonds in crystalline 5-nitrouracil. Density functional theory calculations of the O-17, N-14, and H-2 nuclear quadrupole resonance parameters

Hydrogen bond interaction properties of backbone uracil was studied in crystalline structure of 5-nitrouracil. To this aim the electric field gradient tensors were calculated at the level of density functional theory in two single (non-hydrogen bonded) and cluster (hydrogen-bonded four-molecule) models of 5-nitrouracil. The electric field gradient tensors at the sites of O-17, N-14, and H-2 nuclei were converted to the experimentally measurable nuclear quadrupole resonance spectroscopy parameters, quadrupole coupling constant and asymmetry parameter. The results indicated different hydrogen bond interaction properties at the sites of various nuclei and also the protective role of ?NO₂ group for contribution of O1 to hydrogen bond interactions in comparison with uracil. The density functional theory calculations were performed using GAUSSIAN 98 package employing B3LYP method and 6-311G** and 6-311++G** basis sets.     

A systematic investigation of hydrogen-bonding effects on the 17O, 14N, and 2H nuclear quadrupole resonance parameters of anhydrous and monohydrated cytosine crystalline structures: a density functional theory study

A systematic computational study was carried out to characterize the 17O, 14N, and 2H nuclear quadrupole resonance (NQR) parameters in the anhydrous and monohydrated cytosine crystalline structures. To include the hydrogen-bonding effects in the calculations, the most probable interacting molecules with the central molecule in the crystalline phase were considered in the pentameric clusters of both structures. To calculate the parameters, couples of the methods B3LYP and B3PW91 and the basis sets 6-311++G** and CC-pVTZ were employed. The mentioned methods calculated reliable values of 17O, 14N, and 2H NQR tensors in the pentameric clusters, which are in good agreements with the experiment. The different influences of various hydrogen-bonding interactions types, N-H...N, N-H...O, and O-H...O, were observed on the 17O, 14N, and 2H NQR tensors. Lower values of quadrupole coupling constants and higher values of asymmetry parameters in the crystalline monohydrated cytosine indicate the presence of stronger hydrogen-bonding interactions in the monohydrated form rather than that of crystalline anhydrous cytosine.     

Investigation of the effects of ionic hydrogen bonds (COH–C and N–HOC) in crystalline dl-proline by ab initio and DFT calculated NQR parameters

In this paper, we have calculated the nuclear quadrupole resonance (NQR) parameters of the quadrupole nuclei involved in the hydrogen bonds (CO⁻H–C and ⁺N–H⁻OC) in the monomer and pentameric cluster of dl-proline by HF and B3LYP methods and basis sets of 6-311+G^* and 6-311++G^**. These computations are performed on the basis of X-ray diffraction structural data of dl-proline. The results indicate that the calculations including hydrogen-bonding (HB) interactions (in pentamer) are in better agreement with the experimental data than those in which these interactions are neglected (in monomer). The quantum chemical calculations show that the intermolecular hydrogen-bonding interactions play an important role in determination of the NQR parameters of ¹⁴N, ²H of group and ¹⁷O.     

Hydrogen Bonds of Peptide Group in Four Acetamide Derivatives: DFT Study of Oxygen and Nitrogen NQR and NMR Parameters

A computational study was conducted to examine hydrogen bond (HB) properties of peptide group in four derivatives of acetamide by density functional theory (DFT) calculations of nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) parameters at the sites of oxygen and nitrogen nuclei of peptide groups. The available crystalline structures of four derivatives; 2,2,2-trifluoro-N-(2-hydroxy-5-nitrophenyl)acetamide, N-(2-acetylphenyl)acetamide, 2-chloro-N-(4-nitrophenyl) acetamide, and N-(4-fluorophenyl)acetamide were obtained from literature. Following the influence of HB interactions, calculations were done on non-hydrogen bonded (single) and hydrogen bonded (cluster) models of derivatives. The results revealed different behaviors of peptide group in contributing to HB interactions in different derivative structures. HB interactions are the strongest in 2-chloro-N-(4-nitrophenyl)acetamide. However, the strengths of HB interactions in all of the four derivatives are still less than that of acetamide. The calculations are done at the level of B3LYP method and 6-311++G** standard basis set using GAUSSIAN 98 package of program.     

Investigation of the electronic structure and the structural stability of selected penicillins by density functional calculations of 14N nuclear quadrupole resonance parameters

A computational study was carried out by density functional theory (DFT) to investigate the relative stability and reactivity in three selected penicillins: penicillin-G, penicillin-V and carbenicillin. The geometry of the investigated molecules was optimized at the B3LYP/6-31G(d) level of theory. Then, the nuclear quadrupole resonance (NQR) parameters of ¹⁴N and ²H nuclei and natural bond orbital (NBO) analysis in these molecules were calculated on the geometrically optimized models at the B3LYP level using 6-311++G(d,p) basis sets in the gas phase. The NBO analysis shows that the occupancy of the LP(N) decreases with increasing p character of the lone pair of nitrogen. A comparison between the results obtained for these penicillins and related 6-APA structures of them indicates that the presence of a bulky side group in the acyl side chain can lead to more stability of the β-lactam ring. On the other hand, NBO analysis was applied to rationalize the ¹⁴N NQR parameters in the charge distribution around nitrogen atoms. Inspection of the present results illustrates that the largest component of EFG tensor (q _zz ), the nuclear quadrupole coupling constant, C _Q, and the NQR frequency values of nitrogens decrease with decreasing occupancy values of LP(N). We suggest that the reason for this trend can be found in increasing contribution of delocalized electrons of nitrogen in the intramolecular interactions and hence stability of these structures increases in the order: PG < PV < CA. Finally, a good relationship is found between most of the calculated ²H NQR parameters and the related intramolecular hydrogen bonds.     

A Study of Transition‐Metal Organometallic Complexes Combining 35Cl Solid‐State NMR Spectroscopy and 35Cl NQR Spectroscopy and First‐Principles DFT Calculations

A series of transition‐metal organometallic complexes with commonly occurring metal? chlorine bonding motifs were characterized using ³⁵Cl solid‐state NMR (SSNMR) spectroscopy, ³⁵Cl nuclear quadrupole resonance (NQR) spectroscopy, and first‐principles density functional theory (DFT) calculations of NMR interaction tensors. Static ³⁵Cl ultra‐wideline NMR spectra were acquired in a piecewise manner at standard (9.4 T) and high (21.1 T) magnetic field strengths using the WURST‐QCPMG pulse sequence. The ³⁵Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. ³⁵Cl NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of ³⁵Cl SSNMR spectra. ³⁵Cl EFG tensors obtained from first‐principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a ³⁵Cl SSNMR spectrum of a transition‐metal species (TiCl₄) diluted and supported on non‐porous silica is presented. The combination of ³⁵Cl SSNMR and ³⁵Cl NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine‐containing transition‐metal complexes, in pure, impure bulk and supported forms.     

Evaluation of 27Al and 51V electric field gradients and the crystal structure for aluminum orthovanadate (AlVO4) by density functional theory calculations

Three sets of crystal-structure data reported for AlVO(4) from two powder-XRD studies and a density functional theory (DFT) investigation, employing the Vienna ab initio simulation package (VASP), have been examined and refined using the DFT structure-optimization scheme implemented in the WIEN2k software. The crystal structures are evaluated on the basis of (27)Al and (51)V quadrupole coupling parameters recently reported for AlVO(4), employing the corresponding electric-field gradient (EFG) tensor elements obtained from the DFT calculations. The DFT calculations provide a reliable assignment of the (27)Al/(51)V resonances from three distinct Al and three V environments to the specific crystallographic sites in the asymmetric unit for AlVO(4). An improved agreement between experimental quadrupole tensor elements and calculated EFG tensors is achieved after the DFT structure optimizations and consistent results are obtained using the three different structures as starting points. The improvement of the structural data is also supported by an evaluation of the Al-O and V-O bond lengths before and after DFT structure optimization. The (51)V nuclear quadrupole moment, |Q((51)V)| = 4.8 +/- 0.1 fm(2), derived from the present analysis, represents a value of higher accuracy than earlier reported Q((51)V) values. The origin of the (27)Al and (51)V EFGs are investigated by an evaluation of the orientations of the EFG tensors in the crystal frame and by an examination of the individual contributions from the valence electrons and the surrounding lattice. The latter investigation shows that the magnitude and orientation of the tensors are largely determined by the p-p((27)Al) and p-p, d-d((51)V) orbital contributions to the valence electrons, while the lattice part only gives a minor contribution for both nuclei.     

Assessment of substituent effects on the parameters of 35Cl nuclear quadrupole resonance in para‐substituted benzene‐sulphenyl chloride via quantum chemical calculations

In this research, substituent effects on the parameters of ³⁵Cl nuclear quadrupole resonance (NQR) in para‐substituted benzene‐sulphenyl chloride were studied at M062X/6‐311G(d,p) theory level. The ³⁵Cl NQR parameters of the quadrupole coupling constant (QCC) and electric‐field gradient (EFG) tensor, as well as an asymmetric parameter, were shown to be correlated with Hammett constant following their calculations. The frontier orbital energy levels, HOMO‐LUMO gaps, hardness, electrophilicity, and chemical potential values of these molecules were calculated as well. natural bond orbital (NBO) analysis was applied for calculating natural populations at chlorine atoms.     

Calculation of the 14N and 2H quadrupole coupling tensor in trimeric imidazole

The ¹⁴N and ²H quadrupole coupling tensors at both nitrogen and at all hydrogen sites, have been calculated by ab initio SCF MO methods, in a trimeric imidazole molecule having the geometry found in the solid state. The ¹⁴N changes produced at N(1)H by hydrogen bonding are correctly reproduced to a noteworthy degree of precision, as is the ²H quadrupole interaction at the same site. The changes in the values of the three principal components of the tensor, which have been inferred from NQR measurements when imidazole passes from the gas to the solid state, are supported by the calculations, both for N(1)H and N(3).     

Theoretical 14N and 17O nuclear quadrupole resonance parameters for tirapazamine and related metabolites

Density functional theory (DFT) calculations were performed to realize the effects of the N–O group on the reactivity and electronic properties of 3-amino-1,2,4-benzotriazines. The electric field gradient, EFG, tensors of ¹⁴N and ¹⁷O nuclei and natural bond orbital (NBO) analysis in the tirapazamine (TPZ) and its four derivatives were calculated at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) method in the gas phase. The NBO analysis reveal that the bond strength, proton affinity and position of N–O group in the heterocyclic ring have major influence on the reactivity of considered molecules. Accordingly, we suggest that the TPZ and 4-oxide (d) structures due to having a weaker N–O bond with larger negative charge on the oxygen atom at the 4-position are more active than the other ones. Calculated ¹⁴N and ¹⁷O EFG tensors were used to evaluate nuclear quadrupole coupling tensors, χ, and asymmetry parameters, η _Q . Results showed that oxidation of a nitrogen atom at any position have significant influence on its ¹⁴N nuclear quadrupole resonance (NQR) parameters. Also, the occupancy of nitrogen lone pair plays an important role in determination of the q _zz and χ values of mentioned nuclei. It is found that the η _Q and χ are appropriate parameters to study the contribution of lone pair electrons of nitrogen atom in the formation of chemical bond or conjugation with the aromatic system. Finally, a linear correlation is observed between the χ(¹⁴N) and χ(¹⁷O) values in the N–O bond which may be associated with the reactivity of these compounds.     

DFT calculations as a powerful technique to probe the crystal structure of Al(acac)3

(27)Al, (17)O and (13)C chemical shieldings of aluminum acetylacetonate complex, Al(acac)(3), were calculated at some Density Functional Theory (DFT) levels of theory. In these calculations the X-ray structures of its different polymorphs were used. Using these calculated data observed discrepancies between the X-ray crystallography and solid state NMR experiment were explained in terms of the quality of the NMR data. In this survey we resorted to the simulated spectra using our calculated chemical shifts. In order to confirm our conclusions, electric field gradient (EFG) tensors of the (27)Al and (17)O nuclei were calculated at the same levels of theory as used in the chemical shielding calculations. On the other hand, these calculated chemical shifts and nuclear quadrupole coupling constants (NQCCs) made a correlation between X-ray crystallography and solid state NMR experiments.