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.     

Combined first-principles computational and experimental multinuclear solid-state NMR investigation of amino acids

13C, 14N, 15N, 17O, and 35Cl NMR parameters, including chemical shift tensors and quadrupolar tensors for 14N, 17O, and 35Cl, are calculated for the crystalline forms of various amino acids under periodic boundary conditions and complemented by experiment where necessary. The 13C shift tensors and 14N electric field gradient (EFG) tensors are in excellent agreement with experiment. Similarly, static 17O NMR spectra could be precisely simulated using the calculation of the full chemical shift (CS) tensors and their relative orientation with the EFG tensors. This study allows correlations to be found between hydrogen bonding in the crystal structures and the 17O NMR shielding parameters and the 35Cl quadrupolar parameters, respectively. Calculations using the two experimental structures for L-alanine have shown that, while the calculated isotropic chemical shift values of 13C and 15N are relatively insensitive to small differences in the experimental structure, the 17O shift is markedly affected.     

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.     

Solid-state 93Nb and 13C NMR investigations of half-sandwich niobium(I) and niobium(V) cyclopentadienyl complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.     

Solid-state 139La and 15N NMR spectroscopy of lanthanum-containing metallocenes

A preliminary set of solid-state 139La and 15N NMR data for lanthanum-containing metallocenes is presented, including (C5H5)3La, (C5Me4H)3La, [(C5Me5)2La]+[BPh4]-, and 15N-enriched [(C5Me4H)2La(THF)]215N2. Broad 139La NMR spectra, with breadths ranging from 600 kHz to 2.5 MHz, were acquired with piecewise QCPMG techniques at 9.4 T. Simulations of the spectra reveal 139La quadrupolar coupling constants (CQ) between 44 and 105 MHz. In addition, the first NMR measurement of a nitrogen chemical shift (CS) tensor for dinitrogen bound side-on to a metal atom is reported for [(C5Me4H)2La(THF)]215N2. The 139La NMR parameters show remarkable sensitivity to changes in metallocene structure and can be interpreted in an intuitive manner. Preliminary RHF and DFT calculations of 139La electric field gradient (EFG) and nitrogen CS tensors are used to provide tensor orientations and to rationalize the origin of the NMR parameters in terms of molecular structure and symmetry. The sensitivity of 139La and 15N NMR tensor parameters to changes in structure and bonding should prove invaluable in future studies of noncrystalline and disordered systems.     

Solid-state (63)Cu and (65)Cu NMR spectroscopy of inorganic and organometallic copper(I) complexes

Solid-state 63Cu and 65Cu NMR experiments have been conducted on a series of inorganic and organometallic copper(I) complexes possessing a variety of spherically asymmetric two-, three-, and four-coordinate Cu coordination environments. Variations in structure and symmetry, and corresponding changes in the electric field gradient (EFG) tensors, yield 63/65Cu quadrupolar coupling constants (CQ) ranging from 22.0 to 71.0 MHz for spherically asymmetric Cu sites. These large quadrupolar interactions result in spectra featuring quadrupolar-dominated central transition patterns with breadths ranging from 760 kHz to 6.7 MHz. Accordingly, Hahn-echo and/or QCPMG pulse sequences were applied in a frequency-stepped manner to rapidly acquire high S/N powder patterns. Significant copper chemical shielding anisotropies (CSAs) are also observed in some cases, ranging from 1000 to 1500 ppm. 31P CP/MAS NMR spectra for complexes featuring 63/65Cu-31P spin pairs exhibit residual dipolar coupling and are simulated to determine both the sign of CQ and the EFG tensor orientations relative to the Cu-P bond axes. X-ray crystallographic data and theoretical (Hartree-Fock and density functional theory) calculations of 63/65Cu EFG and CS tensors are utilized to examine the relationships between NMR interaction tensor parameters, the magnitudes and orientations of the principal components, and molecular structure and symmetry.     

Disiline-doped boron nitride nanotubes: A computational study

The properties of the electronic structure of the Disiline-doped boron nitride nanotubes (Disiline-BNNTs) are investigated by a density functional theory (DFT) calculation. The structural forms are firstly optimized and the CS tensors calculated. Subsequently, the chemical-shielding isotropic (CS^I) and chemical shielding anisotropic (CS^A) parameters are found. The shielding values of boron (B) and nitrogen (N) atoms were calculated by Gauge-Including Atomic Orbital (GIAO), Continuous Set of Gauge Transformations (CSGT) and Individual Gauges for Atoms in Molecules (IGAIM) methods, using B3LYP/6-311+G*. The B3LYP level of theory with IGAIM was the best method to evaluate the theoretical chemical shifts for studied models. The results reveal a significant effect of Disiline doping on the chemical shielding tensors at the sites of those ¹¹B and ¹⁵N nuclei located in the nearest neighborhood of the Disiline-doped ring. Furthermore, the values of dipole moments and HOMO-LUMO gaps change in the Disiline-doped models in comparison with the original pristine model.     

Experimental and theoretical studies of 45Sc NMR interactions in solids

Solid-state 45Sc NMR spectroscopy, ab initio calculations, and X-ray crystallography are applied to examine the relationships between 45Sc NMR interactions and molecular structure and symmetry. Solid-state 45Sc (I = 7/2) magic-angle spinning (MAS) and static NMR spectra of powdered samples of Sc(acac)3, Sc(TMHD)3, Sc(NO3)3.5H2O, Sc(OAc)3, ScCl3.6H2O, ScCl3.3THF, and ScCp3 have been acquired. These systems provide a variety of scandium coordination environments yielding an array of distinct 45Sc chemical shielding (CS) and electric field gradient (EFG) tensor parameters. Acquisition of spectra at two distinct magnetic fields allows for the first observations of scandium chemical shielding anisotropy (CSA). 45Sc quadrupolar coupling constants (CQ) range from 3.9 to 13.1 MHz and correlate directly with the symmetry of the scandium coordination environment. Single-crystal X-ray structures were determined for Sc(TMHD)3, ScCl3.6H2O, and Sc(NO3)3.5H2O to establish the hitherto unknown scandium coordination environments. A comprehensive series of ab initio calculations of EFG and CS tensor parameters are in excellent agreement with the observed parameters. Theoretically determined orientations of the NMR interaction tensors allow for correlations between NMR tensor characteristics and scandium environments. Solid-state 45Sc, 13C, and 19F NMR experiments are also applied to characterize the structures of the microcrystalline Lewis acid catalyst Sc(OTf)3 (for which the crystal structure is unknown) and a noncrystalline, microencapsulated, polystyrene-supported form of the compound.     

Alkaline earth chloride hydrates: chlorine quadrupolar and chemical shift tensors by solid-state NMR spectroscopy and plane wave pseudopotential calculations

A series of alkaline earth chloride hydrates has been studied by solid-state (35/37)Cl NMR spectroscopy in order to characterize the chlorine electric field gradient (EFG) and chemical shift (CS) tensors and to relate these observables to the structure around the chloride ions. Chlorine-35/37 NMR spectra of solid powdered samples of pseudopolymorphs (hydrates) of magnesium chloride (MgCl(2).6H(2)O), calcium chloride (CaCl(2).2H(2)O), strontium chloride (SrCl(2), SrCl(2).2H(2)O, and SrCl(2).6H(2)O), and barium chloride (BaCl(2).2H(2)O) have been acquired under stationary and magic-angle spinning conditions in magnetic fields of 11.75 and 21.1 T. Powder X-ray diffraction was used as an additional tool to confirm the purity and identity of the samples. Chlorine-35 quadrupolar coupling constants (C(Q)) range from essentially zero in cubic anhydrous SrCl(2) to 4.26+/-0.03 MHz in calcium chloride dihydrate. CS tensor spans, Omega, are between 40 and 72 ppm, for example, Omega= 45+/-20 ppm for SrCl(2).6H(2)O. Plane wave-pseudopotential density functional theory, as implemented in the CASTEP program, was employed to model the extended solid lattices of these materials for the calculation of their chlorine EFG and nuclear magnetic shielding tensors, and allowed for the assignment of the two-site chlorine NMR spectra of barium chloride dihydrate. This work builds upon our current understanding of the relationship between chlorine NMR interaction tensors and the local molecular and electronic structure, and highlights the particular sensitivity of quadrupolar nucleus solid-state NMR spectroscopy to the differences between various pseudopolymorphic structures in the case of strontium chloride.     

Peptide 17O chemical shielding and electric field gradient tensors

Complete (17)O chemical shielding (CS) and quadrupole coupling (QC) tensors and their molecular orientations were determined for the central residues in two tripeptides Gly-Gly-Val (GGV) and Ala-Gly-Gly (AGG) by single-crystal NMR methods. Tensor orientations in the two peptides are very similar, however, principal components are different. The most shielded CS and smallest magnitude QC components are normal to the peptide plane, while the most deshielded CS and largest QC components are in the peptide plane either at an angle of 17 degrees (CS) or perpendicular (QC) to the C=O bond. Comparisons of principal components from experiment and DFT calculations indicate that the smaller shielding tensor span in GGV (549 ppm) compared to AGG (606 ppm) is likely due to two factors: a shorter "direct" H-bond distance to the peptide carbonyl oxygen and an "indirect" H bond of the peptide NH to a carboxylate rather than a carbonyl. We anticipate that (17)O NMR should be generally useful for probing H-bonding and local electrostatic interactions in proteins and polypeptides. Using the single-crystal data as an accurate reference, we show that a useful subset of the NMR parameters, QC and CS principal components and their relative orientation, can be obtained with reasonable accuracy from a very high-field (21.2 T), stationary sample powder spectrum.     

A solid-state 17O NMR study of L -phenylalanine and L -valine hydrochlorides

We have presented an experimental investigation of the oxygen-17 chemical shielding (CS) and electric-field-gradient (EFG) tensors for alpha-COOH groups in polycrystalline amino acid hydrochlorides. The 17O CS and EFG tensors including the relative orientations between the two NMR tensors are determined in [17O]-L-phenylalanine hydrochloride and [17O]-L-valine hydrochloride by the analysis of the 17O magic-angle-spinning (MAS) and stationary NMR spectra obtained at 9.4, 11.7, 16.4, and 21.8 T. The quadrupole coupling constants (CQ) and the span of the CS tensors are found to be 8.41-8.55 MHz and 7.35-7.41MHz, and 548-570 ppm and 225-231 ppm, for carbonyl and hydroxyl oxygen atoms, respectively. Extensive quantum chemical calculations using density functional theory (DFT) have been also carried out for a hydrogen-bonding model. It is demonstrated that the behavior of the dependence of hydrogen-bond distances on 17O NMR tensors for the halogen ions is different from those for the water molecule.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline amino acids

We have presented a systematic experimental investigation of carboxyl oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in crystalline amino acids. Three 17O-enriched amino acids were prepared: L-aspartic acid, L-threonine, and L-tyrosine. Analysis of two-dimensional 17O multiple-quantum magic-angle spinning (MQMAS), MAS, and stationary NMR spectra yields the 17O CS, EFG tensors and the relative orientations between the two tensors for the amino acids. The values of quadrupolar coupling constants (CQ) are found to be in the range of 6.70-7.60 MHz. The values of deltaiso lie in the range of 268-292 ppm, while those of the delta11 and delta22 components vary from 428 to 502 ppm, and from 303 to 338 ppm, respectively. There is a significant correlation between the magnitudes of delta22 components and C--O bond lengths. Since C--O bond length may be related to hydrogen-bonding environments, solid-state 17O NMR has significant potential to provide insights into important aspects of hydrogen bonds in biological systems.     

A theoretical investigation of hydrogen bonding effects on oxygen and hydrogen chemical shielding tensors of aspirin

A computational investigation was carried out to characterize the ¹⁷O and ¹H chemical shielding (CS) tensors in crystalline aspirin. It was found that O–H⋯O and C–H⋯O hydrogen bonds around the aspirin molecule in the crystal lattice have a different influence on the calculated ¹⁷O and ¹H CS eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the BLYP, B3LYP, and M06 functionals employing 6-311++G(d,p) standard basis set. Calculated CS tensors were used to evaluate the ¹⁷O and ¹H chemical shift isotropy (δ_iso) and anisotropy (Δσ) in crystalline aspirin, which are in reasonable agreement with available experimental data. The difference between the calculated NMR parameters of the monomer and molecular clusters shows how much hydrogen-bonding interactions affect the CS tensors of each nucleus.     

Computational Analysis of 47/49Ti NMR Shifts and Electric Field Gradient Tensors of Half‐Titanocene Complexes: Structure–Bonding–Property Relationships

Metal NMR shielding and electric‐field gradient (EFG) tensors are examined by quantum‐chemical calculations for a set of 14 titanium(IV) complexes. Benchmarks are performed for titanocene chlorides that have been characterized recently by solid‐state NMR experiments, focusing on the dependence of Ti^IV NMR parameters on the computational model in terms of the choice of the density functional, and considering molecular clusters versus infinite‐periodic solid. Nearest‐neighbor and long‐range effects in the solid state are found to influence NMR parameters in systems without spatially extended ligands. Bulky ligands increase the fraction of local structure and bonding information encoded in the EFG tensors by reducing intermolecular interactions. Next, Ti shielding constants and EFG tensors for a variety of olefin (co)polymerization catalysts are analyzed in terms of contributions from localized molecular orbitals representing Lewis bonds and lone pairs. Direct links between the observed theoretical trends and the local bonding environment around the Ti metal center are found. A specific dependence of the Ti EFG tensors on the exact arrangement and type of surrounding bonds is demonstrated, providing a basis for further studies on solid‐supported titanium catalytic systems.     

Solid-state 91Zr NMR spectroscopy studies of zirconocene olefin polymerization catalyst precursors

(91)Zr (I = 5/2) solid-state NMR (SSNMR) spectra of the zirconocene compounds, Cp(2)ZrCl(2), Cp*(2)ZrCl(2) (1), Cp(2)ZrBr(2) (2), (Me(3)SiC(5)H(4))(2)ZrBr(2) (3), O(Me(2)SiC(5)H(4))(2)ZrBr(2) (4), (1,3-C(5)H(3))(SiMe(2)OSiMe(2))(2)(1,3-C(5)H(3))ZrBr(2) (5), Ind(2)ZrCl(2) (6), Cp(2)ZrMeCl (7), Cp(2)ZrMe(2) (8), and [Cp(2)ZrMe][MeB(C(6)F(5))(3)] (9) have been acquired. Static (91)Zr SSNMR spectra have been acquired for all complexes at magnetic fields of 9.4 and 21.1 T. Cp(2)ZrCl(2) and complexes 1 to 5 possess relatively narrow central transition powder patterns which allows for magic-angle spinning (MAS) (91)Zr solid-state NMR spectra to be acquired at a moderate field strength of 9.4 T. Complexes 6 to 9 possess ultrawideline central transition SSNMR spectra necessitating piece-wise acquisition techniques. From the static and MAS (91)Zr SSNMR spectra, it is possible to measure (91)Zr electric field gradient (EFG) and chemical shift (CS) tensor parameters, as well as the Euler angles which describe their relative orientation. Basis sets and methods for the accurate quantum chemical calculation of (91)Zr EFG and CS tensors have been identified. The origin of the observed EFG and CS tensor parameters are further investigated by visualization of the EFG and CS tensor orientations within the molecular frames. Correlations between the observed and calculated NMR tensor parameters and molecular symmetry and structure are made. All of these observations suggest that (91)Zr SSNMR spectroscopy can be utilized to probe the molecular structure of a variety of homogeneous and heterogeneous olefin polymerization catalysts.     

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.     

Probing local structure in zeolite frameworks: ultrahigh-field NMR measurements and accurate first-principles calculations of zeolite 29Si magnetic shielding tensors

The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.     

An 17O NMR and quantum chemical study of monoclinic and orthorhombic polymorphs of triphenylphosphine oxide

Solid-state (17)O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of (17)O-enriched triphenylphosphine oxide, Ph(3)PO. Precise data on the orientation-dependent (17)O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the (17)O nuclear quadrupolar coupling constants (C(Q)) are essentially identical for the two polymorphs (C(Q) = -4.59 +/- 0.01 MHz (orthorhombic); C(Q) = -4.57 +/- 0.01 MHz (monoclinic)), the spans (Omega) of the CS tensors are distinctly different (Omega = 135 +/- 3 ppm (orthorhombic); Omega = 155 +/- 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar sigma-bonded R(3)P(+)-O(-) end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between (31)P and (17)O is observed directly in (17)O magic-angle-spinning (MAS) NMR spectra as well as in (31)P MAS NMR spectra. Ab initio and density-functional theory calculations of the (17)O EFG, CS, and (1)J((31)P,(17)O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed (17)O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.     

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.     

Solid-state 25Mg NMR spectroscopic and computational studies of organic compounds. square-pyramidal magnesium(II) ions in aqua(magnesium) phthalocyanine and chlorophyll a

We report a solid-state (25)Mg NMR spectroscopic study of two magnesium-containing organic compounds: monopyridinated aqua(magnesium) phthalocyanine (MgPc.H(2)O.Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a square-pyramidal geometry. Solid-state (25)Mg NMR spectra for MgPc.H(2)O.Py were obtained at 11.7 T (500 MHz for (1)H) for a (25)Mg-enriched sample (99.1% (25)Mg atom) using both Hahn-echo and quadrupole Carr-Purcell Meiboom-Gill (QCPMG) pulse sequences. Solid-state (25)Mg NMR spectra for Chla were recorded at (25)Mg natural abundance (10.1%) at 19.6 T (830 MHz for (1)H). The (25)Mg quadrupole parameters were determined from spectral analyses: MgPc.H(2)O.Py, C(Q) = 13.0 +/- 0.1 MHz and eta(Q) = 0.00 +/- 0.05; Chla, C(Q) = 12.9 +/- 0.1 MHz and eta(Q) = 1.00 +/- 0.05. This work represents the first time that Mg(II) ions in a square-pyramidal geometry have been characterized by solid-state (25)Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee-Fock (RHF), density functional theory (DFT), and second-order M?ller-Plesset perturbation theory (MP2) levels for both compounds. Computed (25)Mg nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for MgPc.H(2)O.Py is also reported.