Loss of methane and ethene from long-lived gaseous xylenium ions (protonated xylene) after “composite” scrambling

Abstract

The unimolecular fragmentation of long-lived gaseous xylenium ions, CH 3C 6H 5 +CH 3, has been studied in detail using 13 C -labeling, in addition to deuterium labeling, in combination with mass-analyzed ion kinetic energy (MIKE) spectrometry. Metastable xylenium ions generated from the EI-induced loss of COOR (R=H, CH 3) from 1,4-dimethyl-1,4-dihydrobenzoic acid or its methyl ester or by protonation of para-xylene under CI(CH 4) conditions eliminate H 2, CH 4 and, most remarkably, C 2H 4. The kinetic energy release characteristics of these fragmentation processes are reported. 2 H -labeling reveals that the loss of methane occurs by protonolytic cleavage of the C αC ipso bonds, excluding intra-complex benzylic hydride abstraction by CH 3 + ions. 13 C -labeling confirms the composite scrambling behavior preceding both methane and ethene losses. CH 4 loss of a major fraction (92%) of xylenium ions occurs without C scrambling but with some concomitant hydrogen exchange (H α/H ring) via nonclassical tolylmethonium ions, whereas a minor fraction (8%) undergoes complete C and H scrambling. C 2H 4 loss is preceded by different sequences of reversible ring expansion and ring contraction reactions involving methyldihydrotropylium ions (protonated methylcycloheptatriene) which, by irreversible ring contraction, eventually form ethylbenzenium ions (protonated ethylbenzene). A major fraction (66–75%) of the ions exhibits “specific” behavior, with the one of the methyl carbons being incorporated specifically into the ethene fragment and the other only after randomization with the carbons of the tolyl unit. A minor fraction (34–25%) of the ions undergoes complete C and H scrambling prior to ethene loss, involving both methyl groups. 1,2-CH 3 shifts are invoked to occur in the xylenium and/or dihydrotropylium ions.