We have carried out a detailed investigation of the nature of the π-H interaction in the ethene–H2O, benzene–H2O, and benzene–(H2O)2 complexes using large basis sets (ranging from 6-31+G* to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Mo/ller–Plesset (MP2) level of theory. The binding energy of the ethene–H2O complex is only about 1 kcal/mol lower than that of the benzene–H2O complex. In the benzene–(H2O)2 complex, the interaction of benzene with the π-bonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interaction energies were decomposed into the individual interaction energy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies and geometric preferences of these complexes. A detailed analysis of the vibrational frequencies of these complexes has also been done to examine the changes in the frequencies of the monomers upon complexation. It is found that changes in the out-of-plane bending frequencies of benzene and ethene can be correlated to the interaction energies of these complexes, in particular the exchange energy.

Quantum mechanical ab initio calculations at the MP2/6-311++G** level of theory have been used to predict the binding energies and geometries of benzene−BX3 and ethene−BX3 (X = H, F, Cl) complexes. Single point calculations at a much higher level of correlation (MP4) and larger basis sets (6-311++G(2df,p) + diffuse(d,p)) have also been carried out. The calculations reveal interesting trends in their binding energies and geometries. The binding energies indicate that all of them are weakly bound van der Waals complexes with the exception of the C2H4−BH3 complex. While complexes involving BF3 are the weakest (binding energies) in cases of both ethene and benzene, there is a reversal in the relative order of the binding energies as one moves from ethene to benzene. Thus C6H6−BCl3 is more tightly bound than C6H6−BH3. The geometry exhibited by the lowest energy conformer in cases of complexes involving benzene is different from those involving ethene. In contrast to most weak van der Waals interactions involving benzene, H−π, and aromatic−aromatic, the boron atom lies directly over one of the benzene carbons. This observation has been explained by comparing the geometries obtained in complexes involving both benzene and ethene. More importantly, there is strong evidence of an unusual increase in the nucleophilicity of one of the benzene carbons in the lowest energy conformer of systems involving benzene, which implies that Lewis acid−aromatic ring interactions have an important role in electrophilic aromatic substitution reactions.

High level ab initio calculations have been performed on the benzene-HCl and benzene-HF systems using the second-order Mo/ller-Plesset perturbation theory. In contrast to existing theoretical studies, the calculated binding energies indicate that HCl binds more strongly to benzene than HF. This is in accordance with the limited experimental data available on these systems. An explanation has been forwarded for the above observation by performing a molecular orbital analysis of both C6H6⋯HF and C6H6⋯HCl. In the global minimum of C6H6⋯HF, HF lies inclined to the benzene ring with the hydrogen atom pointing either towards a benzene carbon or the center of carbon-carbon bond. In the C6H6⋯HCl complex, HCl is found to lie along the C6 axis of the benzene ring for smaller basis sets, but it also tends to lie inclined to the benzene ring for a very large basis set. The quantum mechanical probabilistic characterization of the structure of the C6H6⋯HCl complex provides a more realistic description of the experimental equilibrium structure. The van der Waals modes have also been characterized, and the modulation of these modes as one progresses from HF to HCl has also been studied.

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In spite of decades of extensive studies of water, the experimental information of water clusters larger than the trimer is hardly available yet. To aid the better analysis of certain cluster properties in an experiment, we studied the small-water-cluster distribution (particularly for the minimum-energy structures) in the gas phase. Utilizing the thermodynamic information in the range from the water monomer to the octamer (except for the heptamer) by ab initio calculations, we investigated the mole fractions of the water clusters along the vapor pressure of the condensed phase. These mole fractions increase with increasing temperature or pressure, while the higher clusters increase still more. The entropy increment of the cyclic pentamer relative to the cyclic tetramer is particularly small; thus, the cyclic pentamer shows thermodynamically unusual characteristics. For the trimer, the cyclic structure is more stable than the linear structure at temperatures lower than -400 K, while above this temperature, the latter is more stable due to the entropy effect. Similar phenomena are also expected for the higher clusters. The mole fractions of the higher cyclic clusters are found to be very small in a vapor unless they are uncondensed with insufficient water molecules.PACS number(s): 36.40. +d, 31.20.Ej, 31.20.Tz, 36.90.+f