The origin of M^+˙ in the chemical ionization mass spectra of polymethylbenzenes

Abstract

The variation of the relative abundance of [MH]' and M" in the methane chemical ionization mass spectra of six polymethylbenzenes [C,H,-m(CH3),; n = 1 4 as a function of methane pressure is reported. The relevance of these results to the suggestion that M" arises from [MH]' by H loss in violation of the even-electron rule is discussed. An analysis of the energetics of three possible routes for M" formation ( H loss from (MH]', charge transfer ionization of M accompanied by dissociation of [CH,]' to CH, and H , and residual electron ionization of M) is presented. The importance of considering residual electron ionization in chemical ionization spectra is emphasized.

A fourier transform-ion cyclotron resonance study of steric effects on proton transfer reactions of polyalkyl benzenes

“…Surprisingly, and in contrast to 1 and 2, the compounds 3 and 4 yield mainly radical cations M'+ instead of MHf under a variety of CI conditions. The fo~ation of radical cations instead of arenium ions during CI experiments with aromatic compounds has been observed before [18]. However, all reasonable reactions a GB(2) = 810kJmol-', GB(3) = 825 + 2kJmol-', GB(4) = 830 f 2kJmol-' (this work).…”

Collision-induced dissociation mass spectra of protonated alkyl amines

“…It also is possible that a portion of the apparent [MH -alkane]+ ions may actually be [M -alkyl radical]+ ions formed by facile dissociation of M+' produced by a small residual electron ionization component in the spectra. The importance of recognizing the possibility of such residual electron ionization components in chemical ionization mass spectra has recently been emphasized (29).…”

A CAD study of isobaric (CH3)3SiArH+ ions (ArH = benzene, toluene) formed by association and protonation

“…In addition there was always a significant M' peak (-60% the intensity of MH+), which has a 2 9~i isotopic component isobaric with MH+. Presumably M+ originated by direct electron impact ionization (8). The possibility was therefore present of metastable decomposition or collisional dissociation of M' interfering with studies of MH+.…”