Time resolved studies of the addition reactions of silylenes and unsaturated hydrocarbons in the gas phase (an investigation of the strain energies of silirane and silirene rings)

This thesis reports the measurement of absolute rate constants for number of silylene addition reactions with unsaturated hydrocarbons. The reactions of SiH2, SiD2 and Me2Si with alkene and alkynes were studied. The silylenes were formed, in situ, by the photolysis of an organosilicon precursor, and the rate constants obtained by the direct observation of the absorption decay of the silylene reactant. The reactions were studied in the gas phase and their temperature and pressure dependence investigated. The reaction of SiH2 and 1,3-butadiene was investigated and found to be pressure dependent. The following Arrhenius equation was yielded at infinite pressure; log(k∞/cm3molecule-1s-1) = (-9.57 ± 0.05) + (3.22 ± 0.35) kJmol-1 /RT ln 10. The reaction was found to proceed via a two-channel pathway, leading to the products vinylsilirane and silacyclopentane. RRKM modelling of the system was carried out and led to the calculation of the strain energy of vinylsilirane as 185 kJ mol-1. The reactions of SiH2 and SID2 with 2-butyne were studied and found to be pressure dependent, a secondary isotope effect was observed. The Arrhenius equations for the reactions of SiH2 and SID2 respectively were; Log(k∞/cm3molecule-1s-1) = (-9.65 ± 0.05) + (1.55 ± 0.32) kJmol-1 / RT ln10; Log(k∞/cm3molecule-1s-1) = (-9.59 ± 0.06) + (1.57 ± 0.47) kJmol-1 / RT ln10. The low-pressure kinetics of the reaction were also studied, and along with RRKM modelling suggest that the reaction proceeds via a single collisionally stabilised pathway. The strain energy of 2,3-dimethylsilirene was calculated as 225 kJ mol-1. The rate constants for the reaction SiD2 and ethyne were measured together with low-pressure rate constants for the analogous SiH2 reaction. Both reactions were found to be pressure dependent. The Arrhenius equation for the reaction of SiD2 and ethyne found had the following form; Log(k∞/cm3molecule-1s-1) = (-1019 ± 0.11)+ (4.89 ± 1.03) kJmol-1 / RT ln10. The low-pressure study of the SiH2 and ethyne reaction failed to conclusively find the low-pressure limit of the reaction. The reaction of Me2Si and ethyne was investigated and found to be pressure dependent. The following Arrhenius equation was yielded; Log(k∞/cm3molecule-1s-1) = (-11.28 ± 0.10) + (5.02 ± 0.69) kJmol-1 / RT ln10. Experimental and theoretical evidence suggested that the reaction proceeds via a single channel third body assisted pathway. Good fits were obtained using RRKM modelling to experimental the fall-off curves and these allowed calculation of the strain energy of 1,1-dimethylsilirene, found to be 211 kJmol-1. Investigation of the Me2Si and tetramethylethylene reaction showed the reaction to be pressure independent. The following Arrhenius equation was calculated; Log(k∞/cm3molecule-1s-1) = (-11.16 ± 0.09) + (5.72 ± 0.61) kJmol-1 / RT In10. End product analysis provided no evidence for a second reaction pathway so the reaction is thought to proceed via a single channel to yield hexamethylsilirene. (author)