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

Theoretical calculations on Li+ and Na+ cationized methyl and n-butyl aceta te, and methyl butyrate. were performed in order to explain the very differ ent fragmentation behavior of Li+ and Na+ cationized fatty acid n-butyl est ers in low-energy collisional activation. Li+ cationized n-butyl palmitate shows loss of 1-butene from the ester moiety, while the corresponding Na+ a dduct does not reveal this loss. This elimination of 1-butene can be regard ed as a McLafferty-type rearrangement and since it bears similarity with th e well-known Norrish H photochemical rearrangement of ketones, involving an intramolecular gamma -hydrogen transfer due to an excitation of the carbon yl bond, we postulated that an excitation of the Li+ cationized ester carbo nyl 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 excite d states can be treated by this approach. The results obtained on Li+ and N a+ 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 oxyge n bound to Li+. The n --> pi* transition which is believed to be involved i n McLafferty-type hydrogen rearrangement processes is shown to be energetic ally more favorable in Li- complexes compared to Na+ complexes. This result is thus consistent with the experimental finding that loss of 1-butene occ urs in Li- complexes and not in the corresponding Na+ complexes of fatty ac id n-butyl esters in low-energy collision-induced dissociation. (C) 2001 El sevier Science B.V.