Local analysis and comparative study of the hydrogen bonds in the linear (HCN)n and (HNC)n clusters

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.

THEORETICALLY EXPLORING METHYLATION EFFECTS ON SOLVATED MULTI-HYDROGEN BONDING AMMONIUM PERCHLORATE SYSTEMS

Methyl substituents are introduced into solvated ammonium perchlorate (AP) systems to explicate the effects of methylation on localized hydrogen-bonding. Methylation systems are modeled using density functional theory (DFT) B3LYP type with 4-31G(d, p), 6-311+(2d, p) basis set methods, indicating that whichever methyl group is substituted at the proton site in an ammonium ion or solvent, both methyl groups cease to H-bond, as evidence by elongation of the neighboring bond and characteristic red-shifts in the frequency of vibration. The energy barriers to the conversion from reactants to products in solvated AP systems are also discussed herein to help to elucidate the relationship solubility and H-bonding.

Cooperativity of hydrogen-bonded networks in 7-azaindole(CH3OH)n (n=2,3) clusters evidenced by IR-UV ion-dip spectroscopy and natural bond orbital analysis

IR-UV ion-dip spectra of the 7-azaindole (7AI)(CH(3)OH)(n) (n=1-3) clusters have been measured in the hydrogen-bonded NH and OH stretching regions to investigate the stable structures of 7AI(CH(3)OH)(n) (n=1-3) in the S(0) state and the cooperativity of the H-bonding interactions in the H-bonded networks. The comparison of the IR-UV ion-dip spectra with IR spectra obtained by quantum chemistry calculations shows that 7AI(CH(3)OH)(n) (n=1-3) have cyclic H-bonded structures, where the NH group and the heteroaromatic N atom of 7AI act as the proton donor and proton acceptor, respectively. The H-bonded OH stretch fundamental of 7AI(CH(3)OH)(2) is remarkably redshifted from the corresponding fundamental of (CH(3)OH)(2) by 286 cm(-1), which is an experimental manifestation of the cooperativity in H-bonding interaction. Similarly, two localized OH fundamentals of 7AI(CH(3)OH)(3) also exhibit large redshifts. The cooperativity of 7AI(CH(3)OH)(n) (n=2,3) is successfully explained by the donor-acceptor electron delocalization interactions between the lone-pair orbital in the proton acceptor and the antibonding orbital in the proton donor in natural bond orbital (NBO) analyses.

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.

Interaction Energies and NMR Indirect Nuclear Spin−Spin Coupling Constants in Linear HCN and HNC Complexes

The cooperativity effects on both the electronic energy and NMR indirect nuclear spin-spin coupling constants J of the linear complexes (HCN)n and (HNC)n (n = 1-6) are discussed. The geometries of the complexes were optimized at the MP2 level by using the cc-pVTZ basis sets. The spin-spin coupling constants were calculated at the level of the second-order polarization propagator approximation with use of the local dense basis set scheme based on the cc-pVTZ-J basis sets. We find strong correlations in the patterns of different properties such as interaction energy, hydrogen bond distances, and spin-spin coupling constants for both series of compounds. The intramolecular spin-spin couplings are with two exceptions dominated by the Fermi contact (FC) mechanism, while the FC term is the only nonvanishing contribution for the intermolecular couplings. The latter do not follow the Dirac vector model and are important only between nearest neighbors.