Franck–Condon analysis of the 1 1A−g→1 1B+u transition of 1,3-butadiene from absorption and Raman intensities

Abstract

Vibronic activity in the 1 1A−g→1 1B+u electronic transition of 1,3-butadiene is analyzed using both the observed structure in the absorption spectrum and Raman band intensities measured at eight excitation frequencies between 23 522 and 37 026 cm−1. The Raman spectra are dominated by the fundamental of ν4, and ag carbon–carbon stretching mode. From an analysis of the preresonance Raman excitation profiles, a total of six ag modes are found to be active in the electronic transition. The distortion of the 1 1B+u excited state is calculated in terms of displacements in these six normal coordinates by fitting the Raman relative intensities and the absorption Franck–Condon envelope first assuming no excited state mode mixing. A significantly improved fit of the absorption spectrum is obtained when the two most active modes ν4 and ν7 are allowed to mix in the excited state. The geometry of the 1 1B+u state is calculated with and without excited state mode mixing, and both analyses predict bond inversion of the ground state single and double bonds together with an overall in-plane skeletal expansion.