A vibronic analysis of the lower à 1A″←X̃ 1A′ singlet–singlet and ã 3A″←X̃ 1A′ triplet–singlet band systems of thioacrolein (2‐propenethial)

The visible spectrum of thioacrolein has been photographed under conditions of long path lengths 98 m and modest dispersion (15 nm cm−1). Two electronic systems have been identified in the spectrum, namely, the a 3A″←X 1A′ spin forbidden but orbitally allowed n→π* transition and its fully allowed companion transition A 1A″←X 1A′. The vibrational fine structure in both transitions is assigned in terms of a single quantum of ν10(CS), ν11(CC), ν12(CCS), and sequence transitions in ν18 (torsion). The structural changes which accompany the electronic excitation are interpreted in terms of a shortening of the CC bond, a lengthening of the CS bond and a stiffening of the skeleton to torsional distortion. No evidence for cis thioacrolein has been found.