Ultraviolet absorption spectra of pyridine-d0 and -d5 and their ring-bending potential energy function in the S1(n,π∗) state

The ultraviolet absorption spectra of both pyridine and its pyridine-d5 isotopomer have been recorded and assigned with a focus on the low-frequency vibrational modes in the S1(n,π∗) electronic excited state. More than a dozen absorption bands associated with the ν18 (formerly labeled ν16b) bending mode were observed. An energy map for the low-frequency modes was constructed and the data for the ν18 mode allowed a highly anharmonic one-dimensional potential energy function to be determined for the S1 excited state. In this state pyridine is quasi-planar and very floppy with a barrier to planarity of 3 cm−1.