Abstract

European Journal of Mass Spectrometry
Volume 15 Issue 2, Pages 261–273 (2009)
doi: 10.1255/ejms.959

Small (poly)unsaturated oxygen containing ions and molecules: a brief assessment of their thermochemistry based on computational chemistry

John L. Holmes^a,*, Karl J. Jobst^b and Johan K. Terlouw^b,*
aChemistry Department, Ottawa University, Ottawa, ON K1N 6N5. Canada
^bChemistry Department, McMaster University, Hamilton, ON L8S 4M1. Canada

The CBS-QB3, CBS-APNO and Gaussian-3 model chemistries have been used to determine the ionic and neutral heats of formation and the adiabatic ionization energies (IE_a) derived therefrom, for the ca 30 principal isomers of the C₃H₂O^•+ and the C₄H₄O^•+ families of radical cations. Theory and experiment are in excellent agreement for those molecules whose experimental IE_a has been accurately measured. In contrast, large deviations from the computed values were found for a great many ionic heats of formation reported in the literature. These deviations largely arise from the uncertainty in the heat of formation of the corresponding neutral species for which often only a rough estimate is available. A useful by-product of this study is that it permits the evaluation of new Benson-type group additivity (GA) terms appropriate for highly unsaturated oxygen containing molecules. Several new GA terms are proposed but it is also argued that a single GA term for the ketene function cannot be defined.

Keywords: computational chemistry, enthalpies, group additivity, ionization energies, unsaturated oxy-molecules and ions

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