Near ultra-violet spectra of the<i>s-trans</i>and a second rotamer of acrolein vapour

Abstract

The rotational contour of the 0-0 band of the 1 A″(nπ*)-1 A′ system of s-trans acrolein has been analysed. Interpretation of the excited state rotational constants shows that the changes in angles CCC and CCO, if assumed to be equal, are between +3° and +4° over a large range of Δr C=C, Δr C=O and Δr C-C and, if Δr C=C Δr C=O are assumed equal, then Δr C-C≈ - Δr C=C over a large range. Weaker bands, to low wavenumber of the 1 A″(nπ*)-1 A′ system of s-trans acrolein, are sorted by their temperature dependence and characteristic contours into two systems: the 4120 Å system is the 3 A″(nπ*)-1 A′ system of s-trans acrolein while the 4060 Å system is shown to be the 1 A″(nπ*)-1 A′ system of s-cis (or possibly ggauche) acrolein. Evidence for this has been obtained from ground state vibration wavenumbers, rotational band contours and temperature effects. Temperature dependence of the intensity of bands has been used to show that the separation of the s-trans and s-cis zero-point levels in the ground electronic state is 770±40 cm-1 and that the torsional vibration wavenumber of the s-cis form is 230±40 cm-1 in this electronic state. It is estimated that there is about 4 per cent of s-cis acrolein at 20°C, 7 per cent at 100°C and 12 per cent at 200°C.