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

✍ Scribed by Rodinei Augusti; Maciej Turowski; Hao Chen; R. Graham Cooks

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 144 KB
• Volume: 39
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.625

✦ Synopsis

Abstract

Ionized benzophenones ([PhC(O)PhY]^+•^; Y = 4 − NO~2~, 4 − CF~3~, 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(Δ__S__), 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. Copyright © 2004 John Wiley & Sons, Ltd.