Rotational band contour analysis in the 2700 Å system of chlorobenzene

Abstract

The origin band of the 2700 Å system of chlorobenzene has been rotationally analysed by the asymmetric rotor band contour method. It is shown to be a type B band of a prolate asymmetric top which requires the electronic assignment of the system to be ¹ B ₂-¹ A ₁. The excited state rotational constants are: A′=0·1797 ± 0·0001 cm^-1, B′=0·05230 ± 0.00002 cm^-1, C′=0.040512 ± 0.000007 cm^-1. These can be interpreted in terms of a contraction of the C-Cl bond of about 0·04 Å and an expansion of the benzene ring similar to that in the ¹ B _2u excited state of benzene. The origin of the 2700 Å band is shown to be at 37 048·2 ± 0·2 cm⁻¹. Strong type A bands involving the b ₂ vibration v ₂₉ are also assigned from their rotational contours. The 29¹ level is shown to be in Fermi resonance with the 14¹20¹ level. Of the eight prominent sequence intervals associated with the origin band, five are interpreted with considerable certainty.