The Spectroscopy of H2CS: Out-of-Plane Potentials for Singlet States and Geometry Optimization of the 1(π,π), 1(σ,π),1(n0,π2), and Rydberg States

Out-of-plane potentials for 1A′ and 1A″ states of thioformaldehyde, H2CS, at several CS distances, as well as equilibrium geometries of the singlet (n,π*), (π,π*), (σ,π*), (n0,π*2), (n, 4s), and (n,4px,y,z) states were determined by multireference Cl methods. Particular emphasis was given to the S2 surface, which is related to the unusual photoreactivity of thiones. S2 is composed of (π,π*), (σ,π*), (n,4s), and (n0,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(n0,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 S2 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 (n0,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 13B2 state of H2CS and D2CS, and the X̃2B2 state of H2CS+ and D2CS+.