Origin bands of electronically forbidden spectra made allowed by centrifugal distortion

Watson's theory of pure rotational spectra in nonpolar molecules is adapted to show that, if appropriate symmetry conditions are met, centrifugal distortion can lead to the appearance of the forbidden 0-0 origin band in an electronically forbidden spectrum. The intensity of such bands will be low; in the case of the benzene Ã1B2u-X˜1A1g system the maximum intensity of the forbidden origin is estimated to be about 10−7 of that of the 610 band in the main vibronic spectrum. The prospects for the experimental detection of such weak bands are discussed.