Norcaradiene intermediates in mass spectral fragmentations of tropone and tropothione

Abstract

The fragmentation of mass spectroscopy of tropone (1) was compared with that of tropothione (2). The electron-impact mass spectroscopy of 1 afforded almost exclusively the benzene cation radical (m/z = 78). Mass spectroscopy of 2 gave similarly the m/z = 78 base peak along with an unusual (M⁺ − 1) peak. This hydrogen elimination was found to occur at the C(α)–H bond by the use of a ²H-labeled analogue of 2. Ab initio calculations (UB3LYP/6-31G*) showed that a π cation radical of 1 and a σ cation radical of 2 were converted to norcaradiene intermediates. Their further isomerizations led to [the benzene cation radical (m/z = 78) + CO] and [(m/z = 78) + CS], respectively. The fragmentation channel of 1 was calculated to have sufficiently small activation energies of intervening transition states to give almost exclusively the m/z = 78 peak. For a σ radical of 2, an α hydrogen moved to the sulfur atom. The resultant thiol was isomerized to a second norcaradiene and its further isomerization led to a thioketene like cation and a hydrogen atom corresponding to the unusual (M⁺ − 1) peak. The difference in fragmentation patterns of 1 and 2 is discussed in terms of their electronic structures.