Quadrupole coupling constants in linear (HCN)nclusters: Theoretical and experimental evidence for cooperativity effects in C–H⋅⋅⋅N hydrogen bonding

Fundamental Vibrational Analyses of the HCN Monomer, Dimer and Associated Isotopologues

In this work we provide high level ab initio treatments of the structures, vibrational frequencies, and electronic energies of the HCN monomer and dimer systems along with several isotopologues. The plethora of information related to this system within the literature is summarized and serves as a basis for comparison with the results of this paper. The geometry of the dimer and monomer are reported at the all electroncoupled-cluster singles, doubles, and perturbative triples level of theory [AE-CCSD(T)] with the correlation consistent quadruple-zeta quality basis sets with extra core functions (cc-pCVQZ) from Dunning. The theoretical geometries and electronic structures are further analyzed through the use of the Natural Bond Orbital (NBO) method and Natural Resonance Theory (NRT). At the AE-CCSD(T)/cc-pCVQZ level of theory, the full cubic with semi-diagonal quartic force field for nine dimer and four monomer isotopologues (the parent isotopologue along with 15 N, 13 C, and D derivatives) were obtained to treat the anharmonicity of the vibrations via second order vibrational perturbation theory (VPT2). Lastly, the enthalpy change associated with the formation of the dimer from two monomer units was determined using the focal point analysis. Computations including coupled-cluster through perturbative quadruples as well as basis sets up to six-zeta quality, including core functions (cc-pCVXZ, X=D,T,Q,5,6) were used to extrapolate to the AE-CCSDT(Q)/CBS energy associated with this hydrogen-bond forming process. After appending anharmonic zero-point vibrational, relativistic, and diagonal Born-Oppenheimer corrections, we report a value of -3.93 kcal mol-1 for the enthalpy of formation. To our knowledge, each set of results (geometries, vibrational frequencies, and energetics) reported in this study represents the highest-level and most reliable theoretical predictions reported for this system.

Characterization of cooperative effects in linear alpha-glycylglycine clusters

The aspects of N-H...O=CNH, N-H...O=CO and C-H...O=CNH interactions are analyzed by applying ab initio and DFT methods as well as Bader theory. We investigated geometry, binding energies, (17)O, (15)N chemical shift tensors, and Atoms in Molecules (AIM) properties of alpha-glycylglycine (alpha-glygly) clusters, via MP2, B3LYP and PW91(XC) methods. Dimer stabilization energies and equilibrium geometries are studied in various levels of theory. MP2 and DFT calculations reveal that for alpha-glygly clusters, stability of N-H...O and C-H...O hydrogen bonds are enhanced significantly as a result of cooperativity effects. Furthermore, a covalent nature is also detected for some hydrogen bondings. The n-dependent trend of (17)O and (15)N chemical shift tensors was reasonably correlated with cooperative effects in hydrogen-bond interactions. Regarding the various N-H...O=CNH, N-H...O=CO and C-H...O=CNH hydrogen bondings, capability of the alpha-glygly clusters for electron localization at the N-H...O and C-H...O bond critical points, depends on the cluster size. This leads to cooperative changes in the hydrogen-bond length and strength as well as (17)O and (15)N chemical shift tensors.

Influence of N–H…O and O–H…O hydrogen bonds on the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen: A density functional theory study

A computational investigation was carried out to characterize the (17)O, (15)N and (13)C chemical shielding tensors in crystalline acetaminophen. We found that N-H...O and O-H...O hydrogen bonds around the acetaminophen molecule in the crystal lattice have different influences on the calculated (17)O, (15)N and (13)C chemical shielding eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the B3LYP method and 6-311++G(d, p) and 6-311+G(d) standard basis sets using the Gaussian 98 suite of programs. Calculated chemical shielding tensors were used to evaluate the (17)O, (15)N, and (13)C NMR chemical shift tensors in crystalline acetaminophen, 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 chemical shielding tensors of each nucleus. The computed (17)O chemical shielding tensor on O(1), which is involved in two intermolecular hydrogen bonds, shows remarkable sensitivity toward the choice of the cluster model, whereas the (17)O chemical shielding tensor on O(2) involved in one N-H...O hydrogen bond, shows smaller improvement toward the hydrogen-bonding interactions. Also, a reasonably good agreement between the experimentally obtained solid-state (15)N and (13)C NMR chemical shifts and B3LYP/6-311++G(d, p) calculations is achievable only in molecular cluster model where a complete hydrogen-bonding network is considered. Moreover, at the B3LYP/6-311++G(d, p) level of theory, the calculated (17)O, (15)N and (13)C chemical shielding tensor orientations are able to reproduce the experimental values to a reasonably good degree of accuracy.

Theoretical Study of HCN and HNC Neutral and Charged Clusters

A theoretical study of linear and cyclic clusters of (HCN)n and (HNC)n (up to n = 10) has been carried out by means of DFT and MP2 ab initio methods. The transition states linking the cyclic clusters show high energetic barriers that prevent the spontaneous transformation of the high-energy clusters, (HNC)n, into the low-energy ones, (HCN)n. The effect of the protonation/deprotonation of the linear clusters has also been explored. The results show that (HNC)n clusters with n values larger than six are thermodynamically more stable as charged systems than as neutral ones. The geometrical results have been analyzed using a Steiner-Limbach plot. The electron density and its Laplacian at the bond critical points correlate with the corresponding bond distances by means of two exponential functions, one for the open shell and another for the closed shell cases.

Electronic changes due to thermal disorder of hydrogen bonds in liquids: pyridine in an aqueous environment

Combined Metropolis Monte Carlo computer simulation and first-principles quantum mechanical calculations of pyridine in water are performed to analyze the role of thermal disorder in the electronic properties of hydrogen bonds in an aqueous environment. The simulation uses the NVT ensemble and includes one pyridine and 400 water molecules. Using a very efficient geometric-energetic criterion, the hydrogen bonds between pyridine and water C5H5N---H2O are identified and separated for subsequent quantum mechanical calculations of the electronic and spectroscopic properties. Statistically uncorrelated configurations composed of one pyridine and one water molecule are used to represent the configuration space of the hydrogen bonds in the liquid. The quantum mechanical calculations on these structures are performed at the correlated second-order perturbation theory level and all results are corrected for basis-set superposition error. The results are compared with the equivalent electronic properties of the hydrogen bond in the minimum-energy configuration. Charge transfer, dipole moment, and dipole polarizabilities are calculated for the thermally disordered and minimum-energy structures. In addition, using the mean and anisotropic polarizabilities, the Rayleigh depolarizations are obtained. All properties obtained for the thermally disordered structures are represented by a statistical distribution and a convergence of the average values is obtained. The results indicate that the charge transfer, dipole moment, and average depolarization ratios are systematically decreased in the liquid compared to the optimized cluster. This study quantifies, using ab initio quantum mechanics and statistical analysis, the important aspect of the thermal disorder of the hydrogen bond in a liquid system.