- 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,*) electronic excited state. More than a dozen absorption bands associated with the 18 (formerly labeled 16b) bending mode were observed. An energy map for the low-frequency modes was constructed and the data for the 18 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. 2008 Elsevier B.V. All rights reserved.