Two large-amplitude motions in triatomic molecules. Force field of the 1B2 (1A′) state of SO2

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Abstract
A program has been developed to calculate the energy levels associated with the two large‐amplitude stretching vibrations ν1 and ν3 of a bent triatomic molecule in which the ν3 oscillation occurs in a double minimum potential. Employing the two large‐amplitude Hamiltonian H0s(ρ1,ρ3) obtained earlier by Brand and Rao. [J. Mol. Spectrosc., 61, 360 (1976)], the vibrational energy levels (v1,v3even/odd) of SO2 molecule in its 1B2 (1A′) excited state are calculated. The nine parameters of the potential function V0(ρ1,ρ3) are then adjusted to give a least‐square fit to the 12 observed vibrational term values corresponding to the levels (v1,v3even) of S16O2 and S18O2. A three‐dimensional picture of the potential surface V0(ρ1,ρ3) using the final set of force constants is also presented. The saddle point of this surface is at (ρO1=1.5525 Å, ρO3=0.0 Å) and the absolute minima occur at (ρe1=1.5644 Å, ρe3=±0.0745 Å). Barrier height, i.e., the height of the saddle point above the absolute minima, is 140 cm−1.