Dissociation of ionized benzophenones investigated by the kinetic method: Effective temperature, steric effects and gas-phase CO^+. affinities of phenyl radicals

Ionized benzophenones ([PhC(O)PhY]^+•; Y = 4 - NO₂, 4 - CF₃, 4-F, 4-Br, 4-Me, 3,4-diMe, 4-OH, 4-OMe, 2-Cl, 2-Me, 2-OH, 2,6-diMe) undergo competitive dissociation upon collision-induced dissociation (CID) at 20 eV collision energy to generate benzoyl cations ([PhCO]⁺ and [YPhCO]⁺) and phenyl radicals (Ph^• and YPh^•). For the para-substituted benzophenones, the natural logarithm of the abundance ratio of the benzoyl cations [ln([PhCO⁺]/[YPhCO⁺])] is found to correlate linearly with the calculated CO^+• affinities of the phenyl radicals Ph^• and YPh^•. A deviation from linearity is observed for the ortho-substituted isomers. This is probably due to a significant intramolecular steric interaction between the carbonyl group and the ortho substituent which prevents the formation of a stable planar system. An observed shift in the intercept relative to the origin is interpreted as the result of a systematic error in the calculated CO^+• affinities and this effect is minimized by calculations at a higher level. The dissociation of ionized para-substituted benzophenones is associated with a relatively high effective temperature of 1816 ± 41 K, calculated from the slope of the kinetic method plot, a value that is consistent with a covalent bond in the activated ion. In addition, Δ(ΔS_co⁺), the dissociation entropy of the benzoyl cations to form CO^+• and the aryl radical, is found to be about 4 J mol^-1 K^-1 by employing the extended version of the kinetic method.