Excitation processes in alkali-cationized esters: a molecular orbital study
Faculty of Pharmaceutical, Biomedical and Veterinary Sciences. Pharmacy
International journal of mass spectrometry. - Amsterdam
, p. 21-30
University of Antwerp
Theoretical 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 Á-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 ¨ Î* 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.