The 3400 Å band system of sulphur dioxide

The contour of the band of SO₂ at 29937 cm^?1 has been shown to be of type-c, and an approximate excited state structure derived as r_SO = 1.50 Å, valence angle = 112°. For a number of reasons it is proposed that the principal bands between 3150 and 3400 Å correspond to vibronically induced B₁-A₁ transitions of a ¹A₂-¹A₁ forbidden electronic transition rather than to an allowed ¹B₁-¹A₁ transition.