MASS-SPECTROMETRIC AND MNDO STUDY OF ELECTRON-IMPACT INDUCED FRAGMENTATION OF SOME NORBORNANE-CONDENSED AND NORBORNENE-CONDENSED PYRIMIDINONES AND OXAZINONES

The main electron impact induced fragmentation processes of nine norbornane- or norbornene-condensed 2-aryl-1,3-pyrimidin-4(3H)-ones and six oxazinone analogues were established by means of mass spectrometry and MNDO calculations. The fragmentations involve the common and main process of selective and simple bond cleavage in the norbornane skeleton, followed by McLafferty H-rearrangement, and a competing and hidden retro Diels-Alder (RDA) reaction of the hetero ring. The norbornenes additionally undergo an RDA reaction directed by the C = C double bond. The decomposition schemes deduced from mass spectrometric measurements were supported by the theoretical results. MNDO bond orders and charge densities calculated for neutral and ionized model molecules proved to be informative for the interpretation of the observed fragmentations. MNDO heats of formation for neutral and ionized products also indicate the high probability of the suggested main fragmentation processes.