The kinetic method as a structural diagnostic tool: ionized alpha-diketones as loosely one-electron bonded diacylium ion dimers

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Eur. J. Mass Spectrom. 9, 295 - 304 (2003)
DOI: 10.1255/ejms.563

The kinetic method as a structural diagnostic tool: ionized α-diketones as loosely one-electron bonded diacylium ion dimers

Eduardo C. Meurer,^a Fabio C. Gozzo,^a R. Augusti^b and Marcos N. Eberlin^*a
^aState University of Campinas UNICAMP, Institute of Chemistry CP6154, 13083-970 Campinas, SP Brazil. E-mail:
^bFederal University of Minas Gerais UFMG, Department of Chemistry, CP 702, 31270-901 Belo Horizonte, MG Brazil

ABSTRACT:

The kinetic method is used to corroborate the description of ground state ionized α-diketones as loosely electron-bonded acylium ion dimers: R¹C=O⁺---e---⁺O=CR². 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.

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