Low-Lying Pi—Sigma Transition in the Spectrum of Norbornadiene

Abstract

A vibrational analysis of the first electronic band of norbornadiene (NBD), with origin at 45 120 cm⁻¹, shows that the band is electronically allowed, the accompanying vibrations being totally symmetric. This is contrary to earlier theoretical work which assigned the band as N→V₁, forbidden. A comparison with the spectra of cyclopentene and norbornene (NBE) suggests that the first band of norbornadiene is to be associated with the weak band of uncertain assignment found at approximately 45 000–50 000 cm⁻¹ in all olefinic substances. Gross changes in the vibrational spacing and shape of the vibronic envelope are observed on freezing the NBD and NBE molecules in low‐temperature matrices. The spectral changes appear to be due to modifications of the upper‐state potential surfaces as a consequence of the rigidity of the matrix. There is some evidence in the NBD spectrum for weak vibronic coupling between the two halves of the diene molecule, and it is shown how the observed splitting pattern might be explained on such an assumption.