Thermal decomposition of the benzyl radical to fulvenallene (C7H6) + H.

We show that the benzyl radical decomposes to the C7H6 fragment fulvenallene (+H), by first principles/RRKM study. Calculations using G3X heats of formation and B3LYP/6-31G(2df,p) structural and vibrational parameters reveal that the reaction proceeds predominantly via a cyclopentenyl-allene radical intermediate, with an overall activation enthalpy of ca. 85 kcal mol(-1). Elementary rate constants are evaluated using Eckart tunneling corrections, with variational transition state theory for barrierless C-H bond dissociation in the cyclopentenyl-allene radical. Apparent rate constants are obtained as a function of temperature and pressure from a time-dependent RRKM study of the multichannel multiwell reaction mechanism. At atmospheric pressure we calculate the decomposition rate constant to be k [s(-1)] = 5.93 x 10(35)T(-6.099) exp(-49,180/T); this is in good agreement with experiment, supporting the assertion that fulvenallene is the C7H6 product of benzyl decomposition. The benzyl heat of formation is evaluated as 50.4 to 52.2 kcal mol(-1), using isodesmic work reactions with the G3X theoretical method. Some novel pathways are presented to the cyclopentadienyl radical (C5H5) + acetylene (C2H2), which may constitute a minor product channel in benzyl decomposition.