COMPARATIVE-STUDIES OF TRIPLET MONOCYCLIC AROMATIC DIAZINES UNDER PRESSURE

We report a zero-field optically detected magnetic resonance study und er high pressure for triplet pyrimidine in benzene, pyrazine in benzen e and in p-dichlorobenzene, and s-tetramethylpyrazine in durene. Gener ally the pressure sensitivity of the zero-field splitting (ZFS) parame ter D, partial derivative D/partial derivative P, for these compounds is much higher than that for quinoxaline. This is rationalized in term s of a smaller pi-electron cloud in the monocyclics than in quinoxalin e. For pyrazine and pyrimidine, the (3)n pi nature of the lowest trip let leads to a larger spin-orbit contribution to the pressure shift. W e observed a larger change in the ZFS parameter E for pyrazine in benz ene than in dichlorobenzene. This is explained by the difference in cr ystalline packing between the two host lattices. There is a large chan ge in D and a multiplet splitting under high pressure for tetramethylp yrazine. These are ascribed to the presence of a pseudo-Jahn-Teller in teraction in this molecule. (C) 1996 American Institute of Physics.