Rotational band contour analysis of the O00bands of theÃ1A′-X1A′ electronic systems ofp-fluoro- andp-chlorostyrene: Hybrid character and geometry changes

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Abstract
The O0 0 bands of the à 1 A′-X 1 A′ systems of p-fluorostyrene and p-chlorostyrene have been rotationally analysed by the method of rotational band contour analysis. In p-fluorostyrene the band is a (50 ± 10) per cent type A—(50 ∓ 10) per cent type B hybrid and, in p-chlorostyrene, an (80 ± 10) per cent type A—(20 ∓ 10) per cent type B hybrid. These contrast with a (97·5 ± 2·5) per cent type A—(2·5 ∓ 2·5) per cent type B hybrid in styrene (Hartford and Lombardi [1]). These figures show that the -CH = CH2 group in styrene has a large effect on the polarization of the electronic transition moment vector compared to C 2v monosubstituted benzenes and that the fluorine and chlorine atoms have further, but different, effects. An attempted interpretation of the changes of rotational constants from the ground to the excited states in styrene [1], p-fluorostyrene and p-chlorostyrene suggests that, in all three molecules there is an increase in the ethylenic double bond length of about 6 pm (0·06 Å) and a decrease of about 4 pm (0·04 Å) in the single bond joining the ring to the substituent. The C-C = C angle changes by no more than about 1°.