Ab initio calculations on the structure of pyridine in its lowest triplet state

Recently we have experimentally shown that pyridine-d5, as a guest in a single crystal of benzene-d6, adopts a boatlike structure upon excitation into the lowest triplet state T0. Here MRDCI ab initio calculations are presented that reveal that the observed nonplanarity of the molecule is not caused by the crystal field but is an intrinsic property of pyridine in T0. The origin of the distortion may be found in the nature of the p-electron system. In the lowest triplet state it is no longer a 6p-electron system as in the ground state but, in first approximation, a 7p-electron system, in which the extra electron occupies a p* orbital that is strongly antibonding between the nitrogen atom and the ortho carbon atoms. The lowest triplet state can not be simply interpreted as an np* or a pp* state.