Collision-induced dissociations of carboxylate negative ions from 2-ethylbutanoic, 2-methylpropanoic, and pivalic acids. An isotopic labelling study

Abstract

Deprotonation of Et₂CHCO₂H yields Et₂CHCO₂^–. On collisional activation this ion forms CO₂^–˙, CH₂CH^–, and MeCHCH^–. In addition, elimination of H˙ and Et˙ yield Et(R)CCO₂^–˙(R = Et and H, respectively). The elimination of Et˙ is not a simple cleavage but occurs by loss of H˙ from a methyl group followed by loss of ethene. The carboxylate ion also rearranges to Et₂CCO₂H; this species decomposes to HO^–, EtCCH₂, and also eliminates the elements of C₃H₈ and CH₄. All fragmentations have been studied using ²H and ¹³C labelling: for example it is proposed that loss of CH₄ from Et₂CCO₂H occurs by a six-centre stepwise process in which the first step (formation of an incipient methyl anion) is rate determining. The collisional activation mass spectra of Et₂CHCO₂^–, Me₂CHCO₂^–, and Me₃CCO₂^– are different, all showing characteristic decompositions. For example, all three ions eliminate methane; the mechanism is different in each case.