Gas phase formation of 1-phenylcyclobuten-3-yl and 1-phenylallyl anions and a determination of the allylic C–H acidities and bond dissociation energies of 1-phenylcyclobutene and (E)-1-phenylpropene

Abstract

1-Phenylcyclobuten-3-yl and 1-phenylallyl anions (1a and 2a, respectively) were prepared in the gas phase by deprotonating their conjugate acids in a Fourier transform mass spectrometer. The acidities of both compounds were measured by determining equilibrium constants with standard reference acids [ΔG°_acid(1) = 369.5 ± 0.7 kcal mol⁻¹ and ΔH°_acid(1) = 377.5 ± 0.7 kcal mol⁻¹; ΔG°_acid(2) = 361.5 ± 2.1 kcal mol⁻¹ and ΔH°_acid(2) = 368.1 ± 2.1 kcal mol⁻¹]. Electron affinities of the corresponding radicals were measured by the bracketing technique [EA = 0.94 ± 0.11 (1r) and 1.06 ± 0.07 eV (2r)] and allylic C–H bond dissociation energies were obtained via a thermodynamic cycle [BDE = 85.6 ± 2.6 (1) and 78.9 ± 2.6 (2) kcal mol⁻¹]. These results are contrasted to density functional theory and ab initio calculations on cyclobutene, 1-phenylcyclobutene, (E)-1-phenylpropene and propene.