The [formula omitted] absorption spectrum of thioformaldehyde : A vibrational and rotational analysis

Abstract

The results of a vibrational and rotational analysis of the banded a ̃ 3A 2 ← X ̃ 1A 1 transition in CH 2S CD 2S are presented. Only three of the six vibrational modes are active in the spectrum with ν′ 2 = 1320 1012 , ν′ 3 = 859 798 , and 2ν′ 4 = 711 516 cm −1 . The spin forbidden transition gains intensity primarily by a mixing of the 1A 1(π ∗,π) and 3A 2(π ∗,n) states. This is confirmed by a rotational analysis of the 0 0 0 band of both isotopes. The rotational analysis shows that the coupling in the a ̃ 3A 2 state is near Hund's case b and that the spin constants are nearly 10 times greater than those observed for CH 2O. A CNDO 2 calculation shows that this difference is due to the greater spin orbit coupling of S in CH 2S and to the smaller energy differences between the B ̃ 1A 1(π ∗,π) , b ̃ 3A 1(π ∗,π) , X ̃ 1A 1 , and the a ̃ 3A 2(π ∗,n) states. The r 0 structure calculated from the rotational constants is r CS = 1.683 A ̊ , r CH = 1.082 A ̊ , β HCH = 119.6°, and α (out of plane) = 16.0°. A simultaneous fit of the vibrational levels in ν′ 4 of CH 2S and CD 2S to a double minimum potential function yielded a barrier to molecular inversion of 13 cm −1 and an equilibrium out-of-plane angle of 15°.