Ultraviolet absorption spectra of pyridine-d 0 and -d 5 and their ring-bending potential energy function in the S 1(n,π ∗) state

Abstract

The ultraviolet absorption spectra of both pyridine and its pyridine-d 5 isotopomer have been recorded and assigned with a focus on the low-frequency vibrational modes in the S 1(n,π ∗) electronic excited state. More than a dozen absorption bands associated with the ν 18 (formerly labeled ν 16b) bending mode were observed. An energy map for the low-frequency modes was constructed and the data for the ν 18 mode allowed a highly anharmonic one-dimensional potential energy function to be determined for the S 1 excited state. In this state pyridine is quasi-planar and very floppy with a barrier to planarity of 3 cm −1.