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Abstract
Electron paramagnetic resonance spectra have been obtained of tetramethylpyrazine in its lowest (ππ*) triplet state in several rigid matrices. Some unusual observations were made :
| 1. | (1) In a durene crystal host the in-plane zero-field principal axes are not parallel to the molecular symmetry axes. | ||||
| 2. | (2) In rigid glass solution the zero-field parameters are strongly dependent on solvent polarity. | ||||
Both effects are discussed in terms of pseudo Jahn-Teller interaction in conjunction with a crystal-field influence.
In a durene matrix the zero-field parameters are 
The relatively small value of 
, as compared with that of the corresponding aromatic hydrocarbon, probably results from vibronic mixing of the lowest 3ππ* with the near-by 3nπ* state.
Hyperfine structure was observed during measurements at 1·95 K ; it is in agreement with the ππ* character of the lowest triplet state and is due to coupling between the unpaired electrons and protons in the non-rotating methyl groups.