Charge-remote fragmentations of closed-shell ions. a thermolytic analogy

Mechanisms for the slow (metastable) decompositions of closed-shell ions of 0-hydroxyalkenoic acids, an alcohol, and an ester are determined by deuterium-labeling, linked-scanning mass spectrometry (B/E), and tandem mass spectrometry (MS/MS, MS/CA-MS, and CA-MS/CA-MS). The parent ions, desorbed by fast atom bombardment as either (M + Li)⁺, (M + 2 Li-H)⁺, or (M-H)⁻ ions, decompose both in the ion source and as metastable ions to lose either H₂O or an aldehyde by mechanisms that do not involve the charge site (i.e., by “charge-remote” decompositions). Loss of the aldehyde occurs by a charge-remote, pericyclic O-hydro-C-allyl elimination, and loss of H₂O occurs by a charge-remote hydro-hydroxy elimination. Both eliminations occur without electronic interaction with the charge site, and thus the mechanisms for these gas-phase elimination reactions are identical with pericyclic Ei mechanisms of analogous thermolytic elimination reactions of neutral molecules. Evidence is also provided that supports the previously proposed charge-remote, pericyclic 1, 4-elimination mechanism for the collision-induced losses of C_nH_2n+2 from long-chain carboxylate ions. These studies show that charge-remote fragmentations are not limited to high-energy collisional activation and they may be the only class of mass spectrometric decompositions that are directly comparable to gas-phase thermolysis.