Proximity effect in spin sublevel dynamics: Possible influence of a low-lying3 A2 (nπ*) state: On singlet → triplet intersystem crossing of Pyrimidine

Comparison of spin sublevel population rates in pyrimidine and 5-methylpyrimidine suggests that the vibronic perturbation of ³B₂ and/or ³A₁ ππ* states by a higher-lying ³A₂nπ* state plays an important role in S₁ → T₁ intersystem crossing of pyrimidine. The result also implies that the n₊π*—n_?π* splitting in pyrimidine is significantly smaller than the corresponding gap in pyrazine. The appearance of the 0.0 band in the τ_x spectrum indicates that the triplets state symmetry is lower tha C_2v.