Asymmetric structure and force field of the 1B2(1A′) state of sulphur dioxide

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Abstract
The vibrational term values, principal rotational constants and centrifugal distortion constants of the 1 B 2 state of SO2 at 42 573 cm-1 (2350 Å) ‡ 1 Å = 10-10 m. 1 d = 1 dyne = 10-5 N. 1 u = 1 unified atomic mass unit ≈ 1·660 531 × 10-27 kg. View all notes are interpreted in a unified treatment using (i) a vibrational hamiltonian based on a double minimum potential in the antisymmetrical stretching coordinate q 3 and strong anharmonic coupling of q 3 with the symmetric stretch q 1; and (ii) a modified treatment of Coriolis coupling and centrifugal distortion, in order to take account of the irregular intervals in the vibrational manifold. This treatment successfully explains a group of observed quantities, including isotopic data, relative intensities and inertial defects, which were not used in determining the constants of the potential function. The analysis indicates that there is a double minimum potential in the antisymmetric mode with a barrier of about 141 cm-1 at the C 2v configuration; the top of the barrier is probably significantly higher, by 43±20 cm-1, than the zero-point vibrational level, corresponding to an asymmetrical structure in this electronic state. Calculated equilibrium values of the short and long distances are respectively 1·491 and 1·639 Å.