Gas-phase reactions of silyl cation (2P) with small hydrocarbon molecules: formation of silicon-carbon bonds

Reactions of ground-state Si + ( 2 P) ions have been investigated with the hydrocarbon molecules CH 4 , C 2 H 6 , C 2 H 4 , C 2 H 2 , CH 2 CCH 2 , CH 3 CCH, and C 4 H 2 proceeding in a helium bath gas at 0.35 Torr and a temperature of 296±2 K with the selected-ion flow tube (SIFT) technique. A range in the nature and degree of reactivity was observed. Methane reacts slowly to form an adduct ion. Results of quantum chemical calculations performed at the UMP4(SDTQ)/6-31G**//UHF/6-31G** level are presented which provide insight into the possible structure of this adduct ion. Adduct formation also is a strong feature in the reactions of Si + with C 2 H 2 and C 2 H 4 in which it competes with condensation resulting in H atom elimination. The reaction with C 2 H 6 predominantly leads to Si―C bond formation in the product ions together with elimination of CH 3 , CH 4 , or H 2 . For these three reaction channels possible mechanisms are proposed involving C―C and C―H bond insertion. The reactions with allene and propyne are rapid and have similar product distributions