Thioformaldehyde. Vibrational analysis of the Ã1A 2- X∼1A 1 visible absorption system

The electronic absorption spectra of thioformaldehyde and thioformaldehyde- d₂ have been obtained. A vibrational analysis of the discrete band system in the 6100-4400-Å region is reported. The type A origin bands are at {16 394}/{16 484}cm ^-1 for {CH ₂S }/{CD ₂S }, and are magnetic dipole allowed. The electronic transition is ùA ₂- X∼¹A ₁ under the C_{2 v} point group. Most of the intensity of the system is in type B bands, and is due to vibronic mixing with higher ¹B₂ states when the inversion mode ν₄ is excited. The molecule in the excited ¹A₂ state is "floppy-planar," having a broad potential function with a barrier of the order of 20 cm ^-1 to the inversion motion.