A crossed molecular beams investigation of the reactions of atomic silicon (Si(³P)) with C₄H₆ isomers (1,3-butadiene, 1,2-butadiene, and 1-butyne)

The bimolecular gas phase reactions of ground state silicon (Si(³P)) with the C₄H₆ isomers 1,3-butadiene, 1,2-butadiene, and 1-butyne were investigated under single collision conditions in a crossed molecular beams machine at collision energies of about 15 kJ mol⁻¹. Our data suggest each reaction proceeds indirectly via SiC₄H₆ intermediates that decompose by elimination of molecular hydrogen through tight exit transition states. In the Si(³P) plus 1,3-butadiene system, multiple product channels are open as evidenced by the observation of molecular hydrogen, hydrogen deuteride, and molecular deuterium losses in experiments utilizing isotopologues of 1,3-butadiene. Non-adiabatic reaction dynamics likely dominate the reaction mechanism in each Si(³P)-hydrocarbon system via intersystem crossing from the triplet to the singlet manifold. These systems are unique in that the reactions proceed at relatively low collision energies and yield products in overall exoergic reactions, unlike the reactions of Si(³P) with the C1-C3 hydrocarbon which have highly endoergic product channels.