Structure and vibrational spectra of H+(H2O)(8): Is the excess proton in asymmetrical hydrogen bond?

The energetics, structure, and vibrational spectra of a wide variety of H+( H2O)(8) structures are calculated using density functional theory and secon d-order Moller-Plesset ab initio methods. In these isomers of H+(H2O)(8) th e local environment of the excess proton sometimes resembles a symmetric H5 O2+ structure and sometimes H3O+, but many structures are intermediate betw een these two limits. We introduce a quantitative measure of the degree to which the excess proton resembles H5O2+ or H3O+. Other bond lengths and, pe rhaps most useful, the position of certain vibrational bands track this mea sure of the symmetry in the local structure surrounding the excess proton. The general trend is for the most compact structures to have the lowest ene rgy. However, adding zero-point energy counteracts this trend, making predi ction of the most stable isomer impossible at this time. At elevated temper atures corresponding to recent experiments and atmospheric conditions (150- 200 K), calculated Gibbs free energies clearly favor the least compact stru ctures, in agreement with recent thermal simulations [Singer, McDonald, and Ojamae, J. Chem. Phys. 112, 710 (2000)]. (C) 2000 American Institute of Ph ysics. [S0021- 9606(00)30735-8].