Comparative ab initio study of the structures, energetics and spectra of X-center dot(H2O)(n=1-4) [X=F, Cl, Br, I] clusters

X-.(H2O)(n=1-4) [X=F, Cl, Br, I] have been studied using high level ab init io calculations. This extensive work compares the structures of the differe nt halide water clusters and has found that the predicted minimum energy ge ometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures ca n change depending on temperature due to the entropy effect. As the potenti al surfaces are flat, the wide-ranging zero point vibrational effects are i mportant at 0 K, and not only a number of low-lying energy conformers but a lso large amplitude motions can be important in determining structures, ene rgies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O-H stretc hing frequencies are reported, and compared with the experimental data avai lable. A distinctive difference between F-.(H2O)(n) and X-.(H2O)(n) (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negl igible hydrogen bonding between water molecules, while the latter favors su rface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O-H spectra of the cluste rs. However, the spectra are forced to be very sensitive to the temperature , which explains some differences between different spectra. In case of F-. (H2O)(n), a significant charge transfer is noted in the S-0 ground state, w hich results in much less significant charge transfer in the S-1 excited st ate compared with other hydrated halide clusters which show near full charg e transfers in the S-1 excited states. Finally, the nature of the stabiliza tion interactions operative in these clusters has been explained in terms o f many-body interaction energies. (C) 2000 American Institute of Physics. [ S0021-9606(00)31537-9].