Time-Resolved Gas-Phase Kinetic, Quantum Chemical and RRKM Studies of Reactions of Silylene with Cyclic Ethers

Abstract

Time-resolved kinetic studies of silylene, SiH(2), generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reactions with oxirane, oxetane, and tetrahydrofuran (THF). The reactions were studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at four or five temperatures in the range 294-605 K. All three reactions showed pressure dependences characteristic of third-body-assisted association reactions with, surprisingly, SiH(2) + oxirane showing the least and SiH(2) + THF showing the most pressure dependence. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equations where the error limits are single standard deviations: log(k(infinity)oxirane/cm3 molecule(-1)s(-1))=(-11.03+/-0.07) + (5.70+/-0.51)kJ mol(-1) ln 10, log(k(infinity)oxetane/cm3 molecule(-1)s(-1))=(-11.17+/-0.11)+(9.04+/-0.78) kJ mol(-1)/RT ln 10, log(k(infinity)THF/cm3 molecule(-1)s(-1))=(-10.59+/-0.10)+(5.76+/-0.65) kJ mol(-1)/RT ln 10. Binding-energy values of 77, 97, and 92 kJ mol-1 have been obtained for the donor-acceptor complexes of SiH2 with oxirane, oxetane, and THF, respectively, by means of quantum chemical (ab initio) calculations carried out at the G3 level. The use of these values to model the pressure dependences of these reactions, via RRKM theory, provided a good fit only in the case of SiH2 + THF. The lack of fit in the other two cases is attributed to further reaction pathways for the association complexes of SiH2 with oxirane and oxetane. The finding of ethene as a product of the SiH2 + oxirane reaction supports a pathway leading to H2Sid=O + C2H4 predicted by the theoretical calculations of Apeloig and Sklenak.