Site of protonation in the chemical ionization mass spectra of olefinic methyl esters

Abstract

Abstract The H 2 and CH 4 chemical ionization mass spectra of the olefinic esters methyl acrylate, methyl methacrylate, methyl crotonate, methyl 3‐butenoate, methyl 2‐methyl‐2‐butenoate, methyl 3‐methyl‐2‐butenoate and methyl cinnamate have been determined. In addition to the expected loss of CH 3 OH from [MH] + , in many cases the protonated molecules also show loss of CO or CH 2 CO with methoxy group migration to the positive ion centre, indicative of protonation at the double bond. These rearrangement reactions, which have analogies in electron impact mass spectra, result in chemical ionization mass spectra of isomeric molecules which show more substantial differences than the electron impact mass spectra. In the case of methyl cinnamate, isotopic labelling experiments show considerable interchange of the added proton with the ortho and meta phenyl hydrogens prior to CH 3 OH or CH 2 CO loss, although the extent of interchange is not the same for both cases.