Density functional theory study on ethanol dimers and cyclic ethanol trimers

The structure and the relative stability of the ethanol dimer and the cycli c ethanol trimer were studied using density functional theory methods. The geometries of the different dimers and trimers were optimized at the B3LYP/ 6-311+G(d,p) level of theory, while the final energies were obtained at the B3LYP/6-311+G(3df,2p) level. Four different (ethanol)(2) complexes were fo und to be local minima of the potential energy surface, the global minimum being that in which both monomers exhibit a trans conformation. The hydroge n bond (HB) in ethanol dimer is slightly stronger than in methanol dimer, r eflecting the enhanced intrinsic basicity of ethanol with regards to methan ol. The OH donor stretch appears redshifted by 161 cm(-1), while the redshi fting undergone by the OH acceptor stretch is negligibly small. The relativ e stability of the trimers is a function of the number of monomers with a g auche conformation, the global minimum being that in which the three monome rs have a trans conformation. As for water and methanol trimers, the three HBs in the cyclic ethanol trimer are not strictly equivalent. Consistently, the redshiftings of the OH stretching frequencies are different. Cooperati ve effects are sizably large, as reflected in the O ... O distances, the el ongation of the OH donor groups, the charge density at the bond critical po ints, the frequency shiftings of the OH stretches, and the additivity inter action energy. The most significant features of the vibrational spectra of the monomers, the dimers, and the trimers in the 800-1200 cm(-1) region are reasonably well reproduced by our calculations. (C) 1999 American Institut e of Physics. [S0021-9606(99)30133-1].