14N NQR study of nicotinamide and related compounds

14N nuclear quadrupole resonance (NQR) frequencies have been measured in picolinamide, nicotinamide, isonicotinamide, 2,6-pyridine dicarboxamide, and acetamide by double resonance. The 14N NQR spectra in picolinamide, nicotinamide, isonicotinamide, and 2,6-pyridine dicarboxamide show the presence of two distinct nitrogen positions: the ring position with the quadrupole coupling constant about 4,5 MHz and the amide position with the quadrupole coupling constant about 2.6 MHz. The NQR data are related to the structure of the investigated compounds and to the N--H...O hydrogen bonds.     

14N NQR study of some local anesthetics

Nitrogen-14 nuclear quadrupole resonance (NQR) spectra for eleven local anesthetics in the solid state are reported and analyzed using the Townes and Dailey approach. The changes in the electron distributions at various nitrogen sites, produced by protonating the tertiary amino nitrogen, are discussed and shown to be in general agreement with expectations based on the increased electrophilic character of the protonated amino group.     

NQR and NMR study of hydrogen bonding interactions in anhydrous and monohydrated guanine cluster model: A computational study

In this paper extensive systematic computational study has been carried out to justify hydrogen bonding interactions and their influence on the oxygen, nitrogen and hydrogen NQR and NMR parameters of the anhydrous and monohydrated guanine crystal structures at two different levels, B3LYP and MP2, using 6-311++G** and D95** basis sets. These theoretical data have been compared with experimental NMR and NQR measurements. For further investigation, results of cluster calculation have been compared with that of a single molecule. Our theoretical NQR and NMR parameters of ¹⁷O, ¹⁵N and ²H atoms of anhydrous and monohydrated guanine exhibited extreme sensitivity to electron distribution around mentioned nuclei caused by cooperative influences of various types of hydrogen bonding interactions. Fortunately, our calculated isotropic shielding values and CS tensors for the ¹⁷O and ¹⁵N nuclei as well as obtained ¹⁴N-NQR parameters are in excellent agreement with experimental data. Therefore, we can undoubtedly conclude that for anhydrous and monohydrated guanine tetrameric clusters including intermolecular interactions, our theoretical estimates are in better agreement with observed experimental values than those in which these interactions have been ignored.     

Dependence of (35)Cl NQR on hydrogen bonding and temperature in dichlorophenol-aniline charge transfer complexes

The hydrogen-bonded charge transfer complexes of aniline with pi-acceptors (or proton donors) such as 2,5-, 2,6-, 3,4- and 3,5-dichlorophenol were prepared. The (35)Cl nuclear quadrupole resonance (NQR) frequencies of these charge transfer complexes in the temperature range 77-300 K were measured to ascertain the existence or otherwise of a phase transition upon complex formation. Further, the NQR frequency and asymmetry parameter of the electric field gradient at the site of quadrupole nucleus were used to estimate the chemical bond parameters, namely ionic bond, double bond character of the carbon-chlorine(C--Cl) bond and the percentage charge transfer between the donor-acceptor components in charge transfer complexes. The effect of hydrogen bonding and temperature on the charge transfer process is analysed.     

Theoretical study of intramolecular hydrogen bonding and molecular geometry of 2-trifluoromethylphenol

The conformational behavior of 2-trifluoromethylphenol was investigated by means of theoretical calculations. Four characteristic structures have been found on the potential energy hypersurface of the compound: anti form (local minimum), in which the hydroxy hydrogen points away from the trifluoromethyl group; and three syn forms (the hydrogen points towards the trifluoromethyl group), with different trifluoromethyl torsions (global minimum, one low and another one high lying saddle-point). The geometry of these conformers were optimized by ab initio calculations using 6-31G** basis set. The effects of electron correlation were investigated by MP2 and various DFT methods. To investigate the intramolecular interaction in the syn forms, the electron density distribution was calculated at the MP2 level of theory. In the structure corresponding to the global minimum at the MP2/6-31G** level a bond critical point was found in Bader's sense between the hydroxy hydrogen and a fluorine of the trifluoromethyl group indicating hydrogen bonding interaction. The length of the hydrogen bond, 1.98 Å, corresponds to medium strength interaction. The O(SINGLE BOND)H bond is slightly twisted and the C(SINGLE BOND)F bond, interacting with it, is considerably twisted out of the plane of the benzene ring to the same side of the ring. The most pronounced geometrical consequence of the hydrogen bond is the 0.02-Å lengthening of the C(SINGLE BOND)F bond participating in its formation. All the other geometrical changes in 2-trifluoromethylphenol, as compared with trifluoromethylbenzene and phenol, are also consistent with the phenomenon of resonance-assisted hydrogen bonding. © 1996 by John Wiley & Sons, Inc.     

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

According to the density functional theory calculations, the X···H···N (X?N, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the ^1hJ(N,H) and ^2hJ(N,N) coupling constants across the N? H···N hydrogen bond and an increase of the ¹J(N,H) coupling constant across the N? H covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the N? H···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the N? H···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the ^1hJ(N,H) and ^2hJ(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρ_H···N at the hydrogen bond critical point and Laplacian of this electron density ?²ρ_H···N. The natural bond orbital analysis suggests that the additional N? H···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*_{N? H} antibonding orbital across hydrogen bond weakening of the ^1hJ(N,H) and ^2hJ(N,N) trans‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s‐character percentage of the N? H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the ¹J(N,H) coupling constant across the N? H covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.     

Interpolymer complex formation between helical poly(L-proline) and random-coil poly(methacrylic acid) through hydrogen bonding

Interpolymer complex formation between poly(L -proline) (PLP) with helical structure and poly(methacrylic acid) (PMAA) with random-coil structure through hydrogen bonding in aqueous medium has been studied by several experimental techniques, e.g., viscometry, turbidimetry, potentiometry, conductometry, scanning electron microscopy, and x-ray diffraction methods. The decreases in reduced viscosity of the solution on addition of an increasing quantity of PLP to a constant amount of PMAA reveals the formation of a complex between PLP and PMAA. The minimum in reduced viscosity at a unit-mole ratio [PLP]/[PMAA] = 1.0 suggests a 1 : 1 complex formation. A distinct change in the curves for turbidity, pH, and conductance versus [PLP]/[PMAA] supports this conclusion. A scanning electron micrograph for the 1 : 1 PLP–PMAA complexes shows that the PLP/PMAA complex has the shape of entangled long fibers. An x-ray diffraction pattern for the PLP/PMAA complexes gives no diffraction patterns which appear in pure PLP, indicating the destruction of the helical structure of PLP due to the interpolymer complexation. Mixtures of PMAA with poly(γ-hydroxy-L -proline) (PHLP) which has a similar conformation as PLP, but involves intra- or intermolecular hydrogen bonds, has also been investigated by vicometry measurements. The reduced viscosity of a solution of the mixed polymers increases with increasing [PHLP], indicating no complex formation. All the results reveal that the magnitude and the nature of the forces acting in the polymers play an important role in interpolymer complexation.     

Competition between van der Waals and hydrogen bonding interactions: structure of the trans-1-naphthol/N(2) cluster

The excitation energy in the multiphoton ionization spectrum of the trans-1-naphthol/N(2) cluster shows only a small red shift with respect to isolated naphthol, indicating a van der Waals pi-bound structure rather than a hydrogen-bonded one. To confirm this interpretation, high-level electronic structure calculations were performed for several pi- and hydrogen-bonded isomers of this cluster. The calculations were carried out at the second order M?ller-Plesset (MP2) level of perturbation theory with the family of correlation consistent basis sets up to quintuple-zeta quality including corrections for the basis set superposition error and extrapolation to the MP2 complete basis set (CBS) limit. We report the optimal geometries, vibrational frequencies, and binding energies (D(e)), also corrected for harmonic zero-point energies (D(0)), for three energetically low-lying isomers. In all calculations the lowest energy structure was found to be an isomer with the N(2) molecule bound to the pi-system of the naphthol ring carrying the OH group. In the CBS limit its dissociation energy was computed to be D(0) = 2.67 kcal/mol (934 cm(-1)) as compared to D(0) = 1.28 kcal/mol (448 cm(-1)) for the H-bound structure. The electronic structure calculations therefore confirm the assignment of the experimental electronic spectrum corresponding to a van der Waals pi-bound structure. The energetic stabilization of the pi-bound isomer with respect to the hydrogen-bonded one is rather unexpected when compared with previous findings in related systems, in particular phenol/N(2).     

Evidence of intramolecular hydrogen bonding in a complex N,N-disubstituted formamide

Intramolecular hydrogen bonding has been determined to be the controlling factor in the cis—trans isomer ratio in N-(2-hydroxy-5-nitrobenzyl)-N-(1-methylthio-2-propenyl)formamide. Both ¹H NMR and i.r. show the presence of intramolecular hydrogen bonding in this compound.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

Strong asymmetric hydrogen bonding in 2‐(oxamoylamino)ethylammonium oxamate–oxamic acid (1/1)

The title compound, C₄H₁₀N₃O₂⁺·C₂H₂NO₃⁻·C₂H₃NO₃, contains at least 11 distinct hydrogen‐bond interactions showing a great variety of bond strengths. The shortest and strongest hydrogen bond [O...O = 2.5004 (12) Å] is found between the uncharged oxamic acid molecule and the oxamate monoanion. The grouping formed by such a strong hydrogen bond can thus be considered as a hydrogen bis(oxamate) monoanion. It lacks crystallographic symmetry and the two oxamate groups have different conformations, showing an asymmetric hydrogen‐bond interaction. Significantly, the asymmetry allows us to draw a direct comparison of site basicity for the two inequivalent carboxylate O atoms in the planar oxamate anion. The constituent molecular ions of (I) form ribbons, where all amide and carboxylate groups are coplanar. Graph‐set analysis of the hydrogen‐bonded networks reveals the R²₂(10) and R²₂(9) homodromic nets as important structure‐directing motifs, which appear to be a common feature of many oxamate‐containing compounds.     

Analysis of the hydrogen bonding in (2-hydroxypropyl)cellulose-poly(acrylic acid) composites by Raman spectroscopy

Novel anisotropic polymer composites obtained by the photopolymerisation of acrylic acid (AA) in lyotropic solutions of (2-hydroxypropyl)cellulose (HPC) were investigated. Raman spectroscopy was used in the study of hydrogen bonding formed by carboxylic groups of poly(acrylic acid) (pAA) between themselves and with the hydroxyl groups of HPC macromolecules. These investigations show that the supramolecular structure of pAA obtained by photopolymerisation contains mainly inner oligomeric forms and cyclic dimer forms, as a consequence of the “matrix polymerisation” of AA. The presence of HPC affects the process of the photopolymerisation of AA, by a redistribution of hydrogen bonding within the system, leading to a different supramolecular structure of pAA. With increasing amounts of HPC the inner oligomeric forms characteristic of bulk pAA disappear. In the composites, the pAA chains probably form the terminal oligomeric structure, with some free COOH groups, as well as hydrogen bonds with HPC.     

14N and 35Cl NQR studies of organophosphorus compounds

¹⁴N and ³⁵Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N? P bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the N? P bond due to the p_π? d_π interaction and p_π? σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N? P bond populations is observed in X ? PR_n (R₂N)_3-n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P?Se > P?S > P?O > P.     

Theoretical study of hydrogen bonded clusters of water and cyanic acid: Hydrogen bonding in terms of the molecular structure

Ab initio and density functional calculations were used to analyze the interaction between a molecule of cyanic acid (HOCN) and up to 4 molecules of water at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) computational levels. The cooperative effect (CE) is increased with the increasing size of the studied clusters. Red shifts of the H–O stretching frequency for complexes involving HOCN as an H-donor were predicted. The strength of the hydrogen bonds in terms of molecular structures could be deduced from a comparison of HOCN–H₂O with HCNO–H₂O, HONC–H₂O and HNCO–H₂O HB clusters. The atom in molecules (AIM) method was used to analyze the cooperative effects on topological parameters.     

Control of molecular architecture by hydrogen bonding: mononuclear versus dinuclear copper(II) complexes with tridentate N2O donor Schiff base isomers

Two isomeric Schiff bases, HL ¹  = 1-[(2-dimethylamino-ethylimino)-methyl]-naphthalen-2-ol and HL ²  = 1-[(2-ethylamino-ethylimino)-methyl]-naphthalen-2-ol, have been used to prepare copper(II) complexes in presence of thiocyanate. HL ¹ forms a mononuclear complex [Cu(L ¹ )NCS] with terminal thiocyanate, whereas the isomeric Schiff base HL ² , which is capable of hydrogen bonding, gives a dimeric complex, [Cu₂ (L ² ) ₂(NCS)₂], with double μ-1,1-NCS bridges. Both complexes are characterized by physico-chemical and spectroscopic methods as well as by single crystal X-ray diffraction studies.