COOPERATIVE (NONPAIRWISE) EFFECTS IN WATER TRIMERS - AN ABINITIO MOLECULAR-ORBITAL STUDY

High levels of ab initio molecular orbital theory were used to study the structures and binding energies of water trimers. These calculations included HF/6-311 + + G(2df,2p) geometry optimizations for the 17 structures considered. Harmonic vibrational energies were obtained at the HF/6-311 + + G (2d,2p) level. The HF potential energy surface present three minima whose geometries were refined at the MP2/6-31 1 + G (dp) level. The global minimum corresponds to an asymmetric cyclic structure which presents significant cooperative effects with respect to the C(s) dimer. To properly describe these nonpairwise effects, ZPE (zero point energy) and correlation corrections must be taken into account. They are reflected in a stiffer intermolecular potential, shorter 0-O distances, longer donor 0-H bond lengths, larger energies per hydrogen bond (HB), and greater shifts of the donor 0-H bond stretching frequencies than the C(s) dimer. Contrarily, the other two local minima present HBs which are weaker than those of the dimer. These nonpairwise effects upon trimerization are mirrored in the topological characteristics of the electronic charge distributions of these clusters and there is a good linear relationship between the energies per HB and the charge density at the HB critical point. The binding energies for the three minima were evaluated at the MP4SDQ/6-31 + G (2d,2p) level using the MP2 optimized geometries. For the remaining structures considered they were obtained at the MP2/6-311 + + G (2d,2p) level using the corresponding HF optimized geometries. We have also shown that nonpairwise effects can be qualitatively explained in terms of acid-base arguments.