ABSTRACT
The electronic properties, specifically, the dipole and quadrupole moments and the ionization energies of benzene (Bz) and hydrogen cyanide (HCN), and the respective binding energies, of complexes of Bz(HCN)(1-4), have been studied through MP2 and OVGF calculations. The results are compared with the properties of benzene-water complexes, Bz(H(2)O)(1-4), with the purpose of analyzing the electronic properties of microsolvated benzene, with respect to the strength of the CH/π and OH/π hydrogen-bond (H-bond) interactions. The linear HCN chains have the singular ability to interact with the aromatic ring, preserving the symmetry of the latter. A blue shift of the first vertical ionization energies (IEs) of benzene is observed for the linear Bz(HCN)(1-4) clusters, which increases with the length of the chain. NBO analysis indicates that the increase of the IE with the number of HCN molecules is related to a strengthening of the CH/π H-bond, driven by cooperative effects, increasing the acidity of the hydrogen cyanide H atom involved in the π H-bond. The longer HCN chains (n ≥ 3), however, can bend to form CH/N H-bonds with the Bz H atoms. These cyclic structures are found to be slightly more stable than their linear counterparts. For the nonlinear Bz(HCN)(3-4) and Bz(H(2)O)(2-4) complexes, an increase of the binding energy with the number of solvent molecules and a decrease of the IE of benzene, relative to the values for the Bz(HCN) and Bz(H(2)O) complexes, respectively, are observed. Although a strengthening of the CH/π and OH/π H-bonds, with increasing n, also takes place for the Bz(H(2)O)(2-4) and Bz(HCN)(3-4) nonlinear complexes, Bz proton donor, CH/O, and CH/N interactions are at the origin of this decrease. Thus CH/π and OH/π H-bonds lead to higher IEs of Bz, whereas the weaker CH/N and CH/O H-bond interactions have the opposite effect. The present results emphasize the importance of both aromatic XH/π (X = C, O) and CH/X (X = N, O) interactions for understanding the structure and electronic properties of Bz(HCN)(n) and Bz(H(2)O)(n) complexes.
Subject(s)
Benzene/chemistry , Electrons , Hydrogen Cyanide/chemistry , Water/chemistry , Hydrogen Bonding , Quantum TheoryABSTRACT
Ab initio results for the electronic properties of sodium-ammonia [Na(NH(3))(n);n=1-8] and the corresponding ionized structures [Na(+)(NH(3))(n)] are reported and compared with those for neutral ammonia clusters [(NH(3))(n)]. Emphasis was placed on the analysis of polarization effects and calculation of vertical and adiabatic ionization potentials. The theoretical discussion is based on second order Møller-Plesset perturbation theory and Green's function or electron propagator theory calculations. Our results for the ionization energies (IEs) of Na(NH(3))(n) clusters are in very good agreement with experimental information. The relationship between the dependence of the IEs on the number of ammonia molecules (n), polarization effects, and hydrogen bond formation is investigated. The presence of a hydrogen bond acceptor-only ammonia molecule that binds a delocalized excess electron in Na(NH(3))(6-7) clusters is discussed.
ABSTRACT
A one-body decomposition approach for investigating the electronic absorption spectra of molecular systems was proposed and applied to water clusters (H2O)N including up to N = 80 water molecules. Two specific aspects of the present implementation are the inclusion of the coupling between excited states and a simplified representation for the N-body Coulombic effects. For smaller clusters, the results based on the one-body decomposition scheme are in good agreement with full EOM-CCSD calculations. Two different regimes can be identified in the electronic absorption profile of larger water clusters. The first low-energy regime is dominated by local excitonic states on the cluster surface, whereas the higher-energy excitations associated with the second one are of delocalized nature.
ABSTRACT
The effect of the environment on the properties of water in the bulk and at the surface of a cluster is studied by all-electron Born-Oppenheimer molecular dynamics. The vibrational spectrum of surface and bulk water is interpreted in terms of the molecular orientation and the local changes in the H-bond network of the cluster. Our results show that, in spite of the presence of a surface moiety of "acceptor-only" molecules, the H-bond network is significantly more labile at the surface than in the bulk part of cluster, and single donor-acceptor arrangements are largely dominant at the interface. Further, although surface water molecules depict in average a single H atom protruding into the vapor, molecules exhibit significant orientational freedom. These results explain the apparently opposite experimental observations from infrared sum frequency generation and X-ray spectroscopy of the liquid-vapor interface. The dipole moment, intramolecular geometry and surface relaxation are also analyzed at light of the different H-bond regions in the cluster.
