Interpretation of rotational constants of the first singlet excited state of substituted benzenes in terms of molecular geometry

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Abstract
The results of a comparison of the rotational constants of the first singlet excited state of five monosubstituted and five p-disubstituted benzenes show that (a) In the monosubstituted benzenes, it is reasonable to assume a common model in which, on electronic excitation, all the bonds in the ring expand as in the 1 B 2u state of benzene but that the carbon-substitutent bond contracts, the contraction being in the order N > O ≈ Br ≈ Cl > F and of approximate magnitudes 0·08, 0·04, 0·04, 0·04, 0·02 Å respectively. There is also a tendency for the ring angle adjacent to the substituent to open by a few degrees. (b) In the p-disubstituted benzenes there is a slight shortening of the molecules along the long axis compared to the geometry of an excited state in which the two ends of the molecule and the C2C3 bond behave as in the corresponding monosubstituted benzenes. This effect is almost independent of the substituents, except in the case of -NH2 where it is largest, and probably results from the ring differing slightly from that of benzene in the 1 B 2u state. (c) Except in the cases of aniline and p-fluoroaniline there is no evidence for non-planarity in the excited state.