The 341 nm band system of pyridine-N-oxide. Analysis of the in-plane vibrational structure

The near-ultraviolet absorption of pyridine-N-oxide has been photographed at high resolution in the range 295–365 nm, and a weak, long-axis polarized subsystem of A-type bands has been identified in the spectrum for the first time. The presence of both A- and B-type bands in the system establishes conclusively that the transition is $\text{π}{}^1\text{← π [}{}^1\text{B}{}_2\text{←}{}^1\text{A}{}_1\text{]}$.The complete set of a₁ and b₂ fundamental frequencies has been identified in the ground and excited states of the transition, using infrared and Raman measurements to supplement the data from the electronic analysis. Physical structure in the excited state has not been determined but appears qualitatively to involve a generalized increase in the dimensions of the ring, coupled with a decrease in the NO distance. The structure of the band system is very similar to that of isovalent aromatic molecules, including phenol and chlorobenzene.