Franck-Condon principle as involving torsional oscillations: Height of the potential barrier to internal rotation in an electronically excited state of ethane

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Abstract
The effect of restricted internal rotation on electronic spectra can be considered in the light of the Franck‐Condon principle. For usual values of the temperature and the molecular constants, internal rotation can have a heavy influence on the width and shape of a vibronic line, conceivably even to the extent of masking the shape due to external rotation. These ideas are pursued in the case of the zero‐zero and one other vibronic line of ethane. Spectra at ∼ 1400 Å for ethane and perdeuteroethane from −78−260°C are presented, as are spectra theoretically calculated for various temperatures, and barriers ranging from zero to 27 kcal/mole. A detailed comparison of calculated and actual spectra leads to an educated guess for the excited‐state barrier height as 1.7 kcal/mole, close to the calculated value of Salahub and Sandorfy of 1.3 kcal/mole. In a discussion of the assignment of the electronic transition, the Innes and Pearson finding that the transition is perpendicular is accepted, but it is suggested that the transition lies over an intense parallel transition in the same region. Various previous efforts to assign these transitions can, it is felt, be reconciled.