Positive-ion electron ionization (EI), methane chemical ionization (CI) and collision-induced dissociation (CID) mass spectra are reported for a series of pyridone-imines generated from 1-alkyl substituted aminopyridinium salts in a direct-probe inlet. The series comprises 1-methyl-2-imino-and 1-ethyl-2imino-1,2-dihydropyridine, the respective 4-imino isomers and 1,2,6-trimethyl-4-imino-1,4-dihydropyridine. Also compared is 1-methyl-4-methylene-1,4-dihydropyridine generated from 1,4-dimethylpyridinium salts. Reference CID data are obtained for 1-methylpyrrole.
Comparisons of the relative abundances of the fragments resulting from EI and of the adducts, molecular ions and fragments from CI reveal substituent and positional effects. From these, and with supporting CID evidence, fragmentation mechanisms are inferred. EI fragmentation of the 2-and 4-imines involves mainly initial loss of HNC with ring contraction to the same 1-alkylpyrrole structure. This is followed by loss of H atom. The 1-ethyl-2-iminodihydropyridine is unique here in undergoing an ortho-induced McLafferty rearrangement with major effects on its EI spectrum. Under CI, adduct-ion formation occurs at the imine site, but for the methylene compound there is significant charge exchange to give the molecular ion.