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.

Hybrid Hartree−Fock/density functional theory (HF/DFT) explicitly including all electrons has been employed to study the three iron carbonyls Fe(CO)5, Fe2(CO)9, and Fe3(CO)12. The geometries are fully optimized by the BP86 and B3LYP methods in conjunction with two basis sets, including as many as 507 contracted Gaussian functions for Fe3(CO)12. Our theoretical geometries and harmonic vibrational frequencies are carefully compared with available experimental results. For Fe3(CO)12 most of the fundamental vibrational frequencies have never been assigned experimenally. Several of the experimental assignments for Fe2(CO)9 and Fe3(CO)12 are dubious.