CHARACTERIZATION AND DIFFERENTIATION OF HETEROCYCLIC ISOMERS .2. MASS-SPECTROMETRY AND MOLECULAR-ORBITAL CALCULATIONS ON PYRROLO[1,2-A][1,4]BENZODIAZEPIN-4-ONE, PYRROLO[1,2-A][1,4]BENZODIAZEPIN-6-ONE, AND PYRROLO[1,2-A][1,4]BENZODIAZEPIN-4,6-DIONE

Pyrrolo[1,2-a][1,4]benzodiazepin-4-one (1), -6-one (2), and -4,6-dione (3), which are starting materials for the synthesis of pharmacologica lly interesting compounds that are active as neurotropic agents, have been characterized in the gas phase. The application of different mass spectrometric techniques, such as electron ionization, high-resolutio n, and tandem mass spectrometry, has allowed the structural characteri zation and differentiation of their molecular ions and most abundant f ragment ions formed in the source. In particular, the two positional i somers 1 and 2 produce quite different mass spectra, and their molecul ar and the most intense fragment ions yield different metastable mass- analyzed ion kinetic energy spectra. Furthermore, high-resolution mass spectrometry and accurate mass measurements have revealed different e lemental compositions and abundances for isobaric fragment ions produc ed by isomers 1 and 2. From these data and from the comparison with th ose relevant to compound 3, it has been possible to evaluate the influ ence of the position of the carbonyl group on the fragmentation pathwa ys. Semiempirical molecular orbital calculations carried out by both t he modified neglect of differential overlap and Austin 1 methods have provided useful information on the characterization of the neutrals as well as the molecular ions of compounds 1-3.