UNUSUAL CHANGES IN THE PHOSPHORESCENCE SPECTRA AND ZERO FIELD SPLITTINGS OF THE LOWEST TRIPLET STATE OF THE DIMETHYL PYRAZINES
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Date
1974
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Ohio State University
Abstract
The phosphorescence spectrum and zero field splittings of pyrazine (paradiazabenzene, $C_{4}H_{4}J_{2}$) are well known and have been carefully characterized. Because the $T_{1}$ $\rightarrow$ $S_{o}$ transition is orbitally allowed, it is surprising to find that the phosphorescence of the three dimethyl isomers differs greatly from that of the parent molecule, pyrazine. These spectra measured at $4^{\circ} K$ in the corresponding xylene hosts are reported, and the dramatic differences in vibronic activity, Franck-Condon intensities, and length of progressions are discussed. Using phosphorescence-microwave double resonance, the zero-field parameters are also measured and found to differ markedly. For example, the principal splitting parameter D is found to decrease from 10.362 GHz in pyrazine to only 2.654 GHz in 2,3-dimethyl pyrazine. Shifts of this magnitude are unprecedented in rigid molecules. Ordinary methyl substituent effects are far too small to account for such significant changes in either the spectrum or in the zero-field parameters. We therefore propose that triplet state mixing is responsible, and support this proposal by a simple quantitative model.
Description
Author Institution: Department of Chemistry, Williams College Williamstown; Department of Chemistry, Williams College Williamstown