Electron-induced (EI) mass fragmentation is directed by intra-molecular H-bonding in two isomeric benzodipyran...

The striking differences observed in the electron-induced (EI) mass fragmentation pathways of two isomeric benzodipyrans are attributable to hydrogen bonding in these molecules. In the "angular" isomer, 6-butyryl-5-hydroxy-2,2,8,8-tetramethyl-3,4,9,10-tetra-hydro-2H,8H-benzo[1,2-b:3,4-b(1)]dipyran (2), H-bonding occurs between the aromatic OH group and the alpha carbonyl moiety contained in the ortho-phenone group, whereas in the"linear" isomer, 10-butyryl-5-hydroxy-2,2,8,8-tetramethyl-3,4,6,7-tetrahydro-2H,8H-benzo-[1,2-b:5,4-b(1)]dipyran (3), the aromatic OH group is para to the phenone moiety, effectively precluding any H-bonding. Semi-empirical molecular orbital calculations (AM1) were used to compare predicted sites of ionization with associated fragmentation patterns. In both molecules, the highest occupied molecular orbital (HOMO) was located predominantly on the aromatic moiety. Similarly, in the radical cation species of both compounds, maximum spin density was located over the aromatic rings. Neither the HOMO nor the spin density maps provided a rational explanation for the differences in fragmentation patterns of the two benzodipyran isomers. The H-bonding favors EI alpha aromatic ring C-O bond cleavage in the"angular" benzodipyran and in 5,7-dihydroxy-2,2-dimethyl-8-butyryl chroman (1), a related monochroman also containing a hydrogen proximal to the aromatic ring C-O bond. In contrast,fragmentation of the "linear" benzodipyran followed a different route, which was exhibited by its base peak resulting from the loss of a propyl group from the butyryl side-chain.