Vinyl- and ethynylsilanes, -germanes and -stannanes. A new case of dissociative proton attachment

Abstract

The gas-phase protonation of H2C=C(H)XH3 and HC≡CXH3 (X = Si, Ge, Sn) compounds was investigated through the use of high-level density functional theory methods. The structures of neutral and protonated species were optimized at the B3LYP/6-31G* level of theory, while the final energies were obtained by single-point B3LYP/6-311 + G(3df,2p) calculations. In the gas phase, vinyl- and ethynylsilanes, -germanes and -stannanes behave as carbon bases of moderate strength, with the only exception of vinylstannane, which is predicted to be about 20 kJ mol-1 more basic than ammonia. In all cases Cα protonation is the most favorable process. This protonation is followed by a C-X bond cleavage, so that the protonated form corresponds to a tightly bound complex between ethylene (or acetylene) and the corresponding XH3+ cation. This implies that dissociative proton attachments can be observed when the basic center is an atom of low electronegativity, provided that the other atoms bonded to it are much less electronegative than the basic center itself, and that the fragments formed as products of the dissociation are intrinsically stable. Copyright © 2002 John Wiley & Sons, Ltd.