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

Rodinei Augusti, Maciej Turowski, Hao Chen, R. Graham Cooks

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

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(ΔSco+), 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.

Original languageEnglish (US)
Pages (from-to)558-564
Number of pages7
JournalJournal of Mass Spectrometry
Volume39
Issue number5
DOIs
StatePublished - May 2004
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Spectroscopy

Keywords

  • Agostic bonding
  • Effective temperature
  • Ion affinities
  • Kinetic method
  • Linear free energy
  • Phenyl radicals
  • Steric effects

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