Abstract/Summary

Time-resolved kinetic studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second order rate coefficients obtained by RRKM-assisted extrapolation to the high pressure limits at each temperature, fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm3 molecule-1 s-1) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol-1)/ RT ln10 End product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O-atom (to form a donor acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps have been explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerisation of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O-atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k∞(π) and k∞(O) which indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O-atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways have been obtained. The lack of a fit at 598K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone.