Structure and vibrational frequencies of di- and monomethyl aluminum, zinc, and boron derivatives on a chemically modified SiO2 surface

The geometrical and electronic structure of (H3SiO)(3)Si-O-R clusters (R=H, B(CH3)(2), Al(CH3)(2), and ZnCH3; n = 0, 1, or 2) modeling a -OR group chemisorbed on a SiO2 surface was studied theoretically with the use of ab initio quantum chemical calculations at the MP2 and B3LYP levels. Various modes of coordination of the organometallic groups at the SiO2 surface were considered. For the Al-containing surface group, two stable structures were found: an open structure and a cyclic structure with the Al atom involved in additional coordination with one of the neighboring oxygen atoms. At the best computational level, only one stable structure was located for the B- and Zn-containing surface fragments, in which the central atom of the surface group (Zn or B) only weakly interacts with the second surface oxygen atom. The calculated vibrational frequencies were compared with the experimental ones and, on this basis, the possible reaction pathways of chemical modification were discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.