Effect of the silyl substitution on structure and vibrational spectra of hydrogen-bonded networks in dimers, cyclic trimers, and tetramers

Geometries of the hydrogen bound clusters (ROH)(n) (n = 1-4, R = H, CH3, and SiH3) are optimized at the B3LYP/DZP+diff level of theory. The reliability of the theoretical method employed for the description of the electronic structure of the hydrogen-bonded network is assesed by a comparison of the predicted geometry parameters and the BSSE corrected association energies with the existing experimental parameters and the higher level theoretical estimates for a water dimer and methanol oligomers. Cyclic structures with S-4 symmetry, similar in the hydrogen-bond arrangement to that of the water tetramer, were-found as minima at the potential energy surface of methanol and silanol tetramers. The cyclic silanol trimer has the C-3h symmetry, in contrast to water and methanol trimers which are characterized by the nonplanar asymmetric structures. The O-O atomic separations decrease and association energies per monomer increase with n and in going from H to SiH3. The silanol tetramer has the highest association energy per monomer among the analyzed systems. This finding is in keeping with the dominance of tetrameric cyclic structures in the experimentally studied crystal structures of silanols. Vibrational frequency shifts with n in the water-methanol-silanol systems are discussed and frequencies for methanol and silanol trimers and tetramers are predicted from the ab initio force fields scaled with factors refined to fit experimental vibrational frequencies of monomers and a water dimer. The main feature of the frequency change with n, along with the lowering of the OH stretch, is the upward shift of the OH bend, which is exceptionally high in silanols.