The decay of triplet pyrazine and methylpyrazine in supersonic jets. Substitution effects

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Abstract
We have measured the decay rates of optically excited triplet states of pyrazine and methylpyrazine in a supersonic jet. The excess vibrational energy dependence of the radiationless rate constants in the energy range between the T 1 and the S 1 electronic origins of the isolated molecules was explored. Decay rates between 5×102 and 105 s−1 were found in the 1800 cm−1 range of excess vibrational energy above the origin of the T 1 state in pyrazine. In methylpyrazine the decay rates increase from 8×102 to 3.3×104 in the first 600 cm−1 excess energy range above the T 1 origin. The decay rates are free of mode specificity and rotational effects. The wide dynamic range of the T 1→S 0 radiationless rates of pyrazine is substantially enhanced by methyl substitution. The results support a model which suggests that certain vibrational modes, those which undergo large frequency decreases in the excited state, control the strong vibrational energy dependence of the T 1→S 0 intersystem crossing of pyrazine. These large frequency changes result from the interaction of the near lying 3 nπ* and 3ππ* states (the ‘‘proximity effect’’). The enhancement of the excess energy dependence of the decay rates by methyl substitution indicates a further decrease in the frequency of these modes. This effect of the methyl group is consistent with the ‘‘proximity effect.’’ The frequency reduction of these modes in the S 1manifold of methylpyrazine also support this mechanism.