Thermal Decomposition of the Benzyl Radical to Fulvenallene (C₇H₆) + H

Abstract

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.

A consistent chemical mechanism for oxidation of substituted aromatic species

“…Because of the weak benzyl-H bond, the formation of benzyl radicals dominates the formation of phenyl and methylphenyl at lower temperatures. However, even at moderate temperatures, the phenyl-H and phenyl-CH 3 bond dissociation reactions become important as noted by da Silva et al [20]. The rate constants for these decomposition pathways have been taken from Klippenstein et al [21], where rate constants for different pressure regimes are reported.…”

Time- and frequency-resolved photoionization of the C A22 state of the benzyl radical, C7H7

“…Photodissociation forming fulvenallene ͑C 7 H 6 ͒ + H is endothermic by +355 kJ/ mol ͑3.69 eV͒. 35 The photoexcitation described here deposits more than 4 eV of energy in the molecule and the reaction is expected to occur in the electronic ground state, but on a nanosecond time scale. Since we do not observe any ion signals at the mass of C 7 H 6 at delay times up to 20 ps, we consider a photochemical reaction from the electronically excited state unlikely.…”

Computational Kinetics of Hydroperoxybutylperoxy Isomerizations and Decompositions: A Study of the Effect of Hydrogen Bonding

“…Enthalpies of formation, for both the HB and NHB structures, of each species and transition state were determined using ChemRate, estimated an uncertainty in the pre-exponential portion ( ) of a factor of 2 36 and the uncertainty in the activation energy will be similar to that of the barrier height (1.2 kcal/mol) reported above.…”