Solvent Effects on n→π Transitions in Pyrazine

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Abstract
The T→S and S→S′ spectra of pyrazine and 2,5‐dimethylpyrazine in hydrocarbon and EPA glasses has been measured. The S→S′ and S→T spectra of the same compounds in ethanol and isopentane are also reported. When due correction is made for the effect of the rigid media, solvent shifts (from hydrocarbon to EPA) of 60 cm—1 to the red in emission and the same amount to the blue in absorption are obtained for the 0–0 band. The absorption maximum is shifted by approximately 1200 cm—1 to the blue and the emission maximum by about 600 cm—1 to the red. The data is explained in terms of the weak hydrogen bonding and the Franck‐Condon strain resulting therefrom. It is suggested that due to the change in the extension of the n orbitals in nitrogen atoms of N heterocyclics, a Franck‐Condon destabilization energy results in protonic solvents. The role of the Franck‐Condon principle in the n→π* blue shift phenomenon is found to be of considerable importance in accordance with Pimentel's views. The previously reported discrepancy of 500 cm—1 in the 0–0 bands of the T→S emission and S→T absorption of these compounds is entirely explained by media effects.