THERMOCHEMICAL PROPERTIES AND GAS-PHASE ION CHEMISTRY OF PHENYLSILANEINVESTIGATED BY FT-ICR SPECTROMETRY - IDENTIFICATION OF PARENT-ION AND FRAGMENT-ION STRUCTURAL ISOMERS BY THEIR SPECIFIC REACTIVITIES

Gas-phase positive ion-molecule chemistry in phenylsilane, in phenylsi lane/benzene-d6 mixtures, and in several binary phenylsilane/hydrocarb on mixtures has been examined by the technique of Fourier transform io n cyclotron resonance spectrometry for the species C6HxSi+ (x = 5-8) f ormed by electron impact ionization of phenylsilane. Reaction pathways and rate constants have been determined for the reactions of the C6Hx Si+ ions with neutral phenylsilane and with neutral benzene-d6. Electr on impact ionization of phenylsilane yields isomeric C6HxSi+ (x = 6-8) ions. The variation of the yields of unreactive C6H6Si+ and C6H7Si+ i ons with electron impact energy enable their identification as Si+-ins erted seven-member rings. Observations of Si+ transfer from C6H6Si+ to C6D6 and SiH2+ transfer from C6H8Si+ to C6D6 in phenylsilane/benzene- d6 mixtures indicate that C6H6Si+ and C6H8Si+ have structures correspo nding to an Si+-benzene complex and an SiH2+-benzene complex, respecti vely. Comparison of the reactions Of C6H6Si+ and C6H8Si+ in phenylsila ne with those of the complex ions C6D6Si+ and C6D6H2Si+ in phenylsilan e/benzene-d6 mixtures enable additional reactive C6H6Si+ and C6HgSi+ i somers to be identified for which a C6H5-SiH+ and a C6H5-SiH2+ structu re, respectively, are suggested. In phenylsilane/hydrocarbon mixtures, hydride-transfer reactions are examined for the two isomers of C6H7Si +. DELTAH-degrees-298 of the H- transfer from phenylsilane to the 2-me thylbutyl cation has been determined to be -1.0+/-0.4 kcal mol-1. Usin g this value, the H- affinity of the phenylsilyl cation, DH-degrees 29 8(PhSiH2+-H-), has been determined to be 229.8+/-0.6 kcal mol-1. The H - affinity of the silacycloheptatrienyl cation has been determined to be less than that of the cycloheptatrienyl cation. Related thermochemi cal results provided by the present study include estimates for the Si -H bond energy in PhSiH3+, DH-degrees 298(PhSiH2+-H), 38 kcal mol-1, a nd the ionization potential of the phenylsilyl radical, IP(PhSiH2), 6. 89 eV.