Ultraviolet Absorption Spectra and the Quasi-planarity of Pyridine and its d$_{5}$ Isotopomer in its S$_{1}(\pi $,$\pi $*) Excited State

The ultraviolet absorption spectra of pyridine-d$_{0}$ and --d$_{5}$ vapor have been recorded and analyzed in the 32,000 to 38,000 cm$^{-1}$ region. The electronic band origins are at 34,767 (d$_{0})$ and 34,945 cm$^{-1}$ (d$_{5})$ for the two isotopomers. For both molecules series of transitions for $\nu_{18}$, the out-of-plane ring-bending vibration, in the excited electronic state can be observed, and a one-dimensional potential energy function of the form V = ax$^{4}$ -- bx$^{2}$ can be determined, where x is the out-of-plane vibrational coordinate. In the S$_{0}$ electronic ground state pyridine is rigid and planar with $\nu_{18}$ at 403 cm$^{-1}$. In the S$_{1}(\pi $,$\pi $*) excited state $\nu_{18}$ drops to 59.5 cm$^{-1}$ and the molecule becomes floppy with a tiny barrier to planarity of 3 cm$^{-1}$ resulting in a quasi-planar structure.