Welham, Kevin John; (1992) The role of charge localisation in mass spectrometry. Doctoral thesis (Ph.D.), University College London.

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Abstract

Ab-Initio molecular orbital calculations have been used to investigate the degree of charge localisation associated with the formation of ground state molecular radical cations upon electron impact for some simple amides, thio-amides, urea, thiourea, and their N-methyl substituted analogues, and guanidine. Some of the thio-amides which were not commercially available, were synthesised for the mass spectrometric studies. Ionisation energies have been calculated from the molecular orbital energies, using Koopmans' Theorem, and related to the predicted site of charge location in the molecular radical cations. The molecular orbital data was also used to study the effect of N-methylation on ionisation energy. The results obtained were found to be in close agreement with results obtained from photoelectron spectroscopy and electron impact mass spectrometry. Mulliken population analysis was used to obtain total atomic charges from the molecular orbital calculations so that the charge distributions in the neutral molecules and the radical cations could be compared. The molecular orbital calculations predict the major change in charge distribution to be equally shared between each nitrogen atom of urea, upon ionisation, with very little charge located on the oxygen; however for thiourea the change in charge distribution is largely located on the sulphur atom with very little change in the charge on either of the nitrogen atoms. These results are in agreement with previous predictions based on observed experimental data. The amides show more delocalisation with the charge more evenly distributed between nitrogen and oxygen, although N-methylation causes the charge to be preferentially located on nitrogen. The thio amides show the charge preferentially located on sulphur throughout. Molecular orbital calculations have also been used to investigate the energetics of the major fragmentation reaction in some of these compounds in relation to the predicted site of charge location in both the ground state molecular radical cations, and the fragment ions. This study has enabled an attempt to be made at rationalising the observed electron impact mass spectra on the basis of the calculated change in charge distribution upon ionisation of the compounds studied.

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