Photoexcitations and photoisomerizations due to low-lying np* and pp* excited states of dimethylpyridines are investigated by density functional theory, CASSCF, CASPT2 and MRCI methodologies. Mechanistic details for the formation of Dewar dimethylpyridines and the interconversions of dimethylpyridines are rationalized through the characterization of minima and transition states on the singlet and triplet potential energy surfaces of relevant intermediates. Our present theoretical schemes suggest that Möbius dimethylpyridine intermediate 14 and azabenzvalene intermediate 10 can serve as possible precursors to Dewar dimethylpyridines and singlet phototransposition
products, respectively. The calculations suggest that an S 1 (pp*)/S 0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S 2 (pp*)/S 1 (np*) interaction followed by an S 1 /S 0 decay in dimethylpyridine 6. Calculated barriers of isomerizations from 14 to Dewar dimethylpyridine 7 and from 10 to 4 are 8.4 and 28.5 kcal mol À1 at the B3LYP/6 ± 311G** level, respectively. In the suggested triplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T 1 ( 3 B 1 ) excited state of 3,5-dimethylpyridine 6 is a precursor of the interconversion of 6 to 2,4-dimethylpyridine 4. The formation of a triplet azaprefulvene 21 with a barrier of 20.7 kcal mol À1 is a key step during the triplet migration process leading to another out-of-plane distorted structure 27. Subsequent rearomatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.