Knowledge Bank

High resolution spectra of the $2875\backslash AA(\pi \ast \leftarrow n)$ singlet system of pyridine and perdeuteropyridine have been obtained in polycrystalline benzene and cyclohexane matrices at $4.2\circ K$. These studies were undertaken with the hope of resolving several puzzling features of the pyridine vapor $specrum,1$ most notably the appearance of a medium-strong band lying $139cm-1$ to the blue of the $\pi \ast \leftarrow n$ origin. At low temperatures, this band is either absent or has shifted to higher energies: a band having comparable intensity appears at $352cm-1$ above the origin in benzene and at $331cm-1$ in cyclohexane. In addition, a weak band, having roughly $1/20$ the intensity of the origin band and scen only in the benzene host, appears at $186cm-1$ to the red of the $\pi \ast \leftarrow n$ origin. Although the possibility of site effects or dimer lines cannot be completely ruled out as exmplanations for this weak band, it might also be interpreted as the origin of an electronically forbidden transition whose appearance results from crystal field perturbations. The results for perdeuteropyridine are analagous and will be presented.