Theoretical characterization of photoisomerization channels of dimethylpyridines on the singlet and triplet potential energy surfaces
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Photoexcitations and phstoisomerizations due to low-lying n pi*; and pi pi* 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 Mobius 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(pi pi*)/S-0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S-2(pi pi*)/S-1(n pi*) 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 kcalmol(-1) at the B3LYP/6-311G**: level, respectively. In the suggested dtriplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T-1(B-3(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 kcalmol(-1) is a key step during the triplet migration process leading to another out-of-plane distorted structure 27 Subsequent re aromatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.
CitationCHEMISTRY-A EUROPEAN JOURNAL，2001,7（9）：1927-1935