The Kinetic Method as a Structural Diagnostic Tool: Ionized α-Diketones as Loosely One-Electron Bonded Diacylium Ion Dimers

Abstract

The kinetic method is used to corroborate the description of ground state ionized α-diketones as loosely electron-bonded acylium ion dimers: R1–C=O+—e−—+O=C–R2. The abundance ratio of both the acylium ion fragments R1CO+ and R2CO+ (summed to those of their respective secondary fragments) formed upon low energy (5 eV) collision-induced dissociation of several ionized α-diketones is found to correlate linearly with the ionization energies ( IEs) of the corresponding R1CO• and R2CO• free radicals as predicted by density functional theory calculations at the B3LYP/6-311++G(d,p) level. However, when these abundances are taken from 70 eV electron ionization mass spectra, lower and sometimes inverted ratios (2,3-pentanedione and 2,3-hexanedione) are observed. Inverted ratios are also observed via charge-exchange mass spectrometry/mass spectrometry (MS/MS) experiments for ionized 2,3-pentanodione formed with relatively high internal energies. Ionized α-diketones are found to display an effective temperature of 1705 K, which indicates an intermediate loosely-bonded nature. B3LYP/6-311++G(d,p) optimized geometries and charge and spin densities also corroborate the description of ground state ionized α-diketones as loosely electron-bonded diacylium ion dimers.