TIME RESOLVED KINETIC-STUDIES OF THE GAS-PHASE REACTIONS OF DIMETHYLSILYLENE WITH SOME O-DONOR MOLECULES .1. ROOM-TEMPERATURE STUDIES

The reactions of dimethylsilylene, SiMe2, generated by laser flash photolysis from pentamethyldisilane and octamethyltrisilane, and detected via its 457.9 nm absorption, have been investigated with a number of oxygen containing molecules, including oxygen itself, carbon monoxide, water, alcohols, and ethers at 295 K. Some substrate molecules were deuterated. Apart from CO, all molecules reacted with SiMe2 with second order rate constants (in cm3 molecule-1 s-1) in the range 7.7 x 10(-14) to 4.7 x 10(-11). While most of the reactions of SiMe2 studied went to completion those with dimethylether and tetrahydrofuran were partially reversible. The reaction of SiMe2 with oxirane was pressure dependent. The data are generally consistent with a mechanism involving initial formation of a donor-acceptor, zwitterionic complex. In the case of reaction with alcohols, there is a small overall isotope effect, suggesting formation of a complex followed by rapid H-transfer, which only marginally affects the rate. In the case of ethers, complexes are formed which (i) may react further (Me2Si...oxetane), (ii) may react further but also reversibly decompose (Me2Si...oxirane), or (iii) do not seem to react further (Me2Si...OMe2, Me2Si...THF) but partially redissociate. The donor-acceptor bond strength has been estimated in these last cases to be ca. 40 kJ mol-1. Supporting arguments and mechanistic details of all reactions investigated are presented.