Excitation processes in alkali-cationized esters: a molecular orbital study

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DOIResolve DOI: http://doi.org/10.1016/S1387-3806(01)00409-2
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TypeArticle
Journal titleInternational Journal of Mass Spectrometry
Volume210-211
Pages2130; # of pages: 10
SubjectAlkali adducts; Collision-induced dissociation; Fatty acids; Theoretical calculations
AbstractTheoretical calculations on Li+ and Na+ cationized methyl and n-butyl acetate, and methyl butyrate, were performed in order to explain the very different fragmentation behavior of Li+ and Na+ cationized fatty acid n-butyl esters in low-energy collisional activation. Li+ cationized n-butyl palmitate shows loss of 1-butene from the ester moiety, while the corresponding Na+ adduct does not reveal this loss. This elimination of 1-butene can be regarded as a McLafferty-type rearrangement and since it bears similarity with the well-known Norrish II photochemical rearrangement of ketones, involving an intramolecular γ-hydrogen transfer due to an excitation of the carbonyl bond, we postulated that an excitation of the Li+ cationized ester carbonyl bond in Li+ adducts of fatty acid n-butyl esters is the trigger for the loss of 1-butene in low-energy collisional activation. For the theoretical calculations using density-functional theory was considered because excited states can be treated by this approach. The results obtained on Li+ and Na+ cationized methyl and n-butyl acetate and methyl butyrate indicate that the inductive effect of Li+ is stronger than that of Na+ and that the ionic effect promotes less accumulation of negative charge on the carbonyl oxygen bound to Li+. The n → π∗ transition which is believed to be involved in McLafferty-type hydrogen rearrangement processes is shown to be energetically more favorable in Li+ complexes compared to Na+ complexes. This result is thus consistent with the experimental finding that loss of 1-butene occurs in Li+ complexes and not in the corresponding Na+ complexes of fatty acid n-butyl esters in low-energy collision-induced dissociation.
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LanguageEnglish
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NPARC number12338896
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Record identifier7981a840-df6c-4ddb-91b7-94fedff56a93
Record created2009-09-11
Record modified2016-05-09
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