The Spectroscopy of H2CS: Out-of-Plane Potentials for Singlet States and Geometry Optimization of the 1(π,π*), 1(σ,π*),1(n0,π*2), and Rydberg States

Out-of-plane potentials for 1 A ′ and 1 A ″ states of thioformaldehyde, H 2 CS, at several CS distances, as well as equilibrium geometries of the singlet ( n ,π*), (π,π*), (σ,π*), ( n 0 ,π* 2 ), ( n , 4 s ), and ( n ,4 p x,y,z ) states were determined by multireference Cl methods. Particular emphasis was given to the S 2 surface, which is related to the unusual photoreactivity of thiones. S 2 is composed of (π,π*), (σ,π*), ( n ,4 s ), and ( n 0 ,π* 2 ). The former three states have very similar equilibrium energies and are separated by low barriers; therefore, internal conversions and vibronic couplings should give rise to complicated spectra. The (diabatic) 1 (π,π*) state is shown to be planar, whereas 1 (σ,π*) and 1 ( n 0 ,π* 2 ) are nonplanar. The latter state has a small HCH and large out-of-plane angle. The 1 (σ,π*) state is considered to be the photoactive state in S 2 since (σ,π*) lies slightly lower than (π,π*), can be easily populated by internal conversion from (π,π*), and has a low oscillator strength with the ground state. Evidence suggesting that ( n 0 ,π* 2 ) is the "dark state," a nonemissive state capable of being populated by internal conversion from (π,π*), is not conclusive. Vibrational frequencies were calculated by the HF/6-311++G** method for the 1 3 B 2 state of H 2 CS and D 2 CS, and the X 2 B 2 state of H 2 CS + and D 2 CS + .