Knowledge Bank

Thioacetaldehyde, CH3CHS displays a banded absorption spectrum at 615 nm which is attributed to the $T1\leftarrow So,a¯3A\prime \prime \leftarrow X1A\prime$ electronic transition which arises through $n\to \pi \ast$ excitation. The spectrum in the To region consists of a cluster of bands which was assigned to the activity of the methyl torsional mode $V15$. Substitution of deuterium for the aldehyde hydrogen produces an isotope shift which demonstrates that a strong coupling exists between the aldehyde wagging mode $V14$ and the methyl torsion $15$ Ab initio SCF methods were used to generate the potential surface defined by 0(internal rotation) and a(hydrogen wagging) in both electronic states from which the potential $V(\theta ,\alpha )$ was derived as a Fourier expansion. The band spectrum of $CH3CHS$ was synthesized from the energy levels and Franck Condon factors of a two-dimensional torsional-inversion Hamiltonian in which the torsion and inversion motions were treated as $C3$ and $C1$ rotations respectively.