1. H. C. Longuet-Higgins andJ. N. Murrell, Proc. Phys. Soc.68 A, 601 (1955).
2. J. N. Murrell, J. chem. Soc.1956, 3779.
3. The presence of bulkyortho-substituent(s) may lower theintensity of the E.T. band to a considerable extent, or even remove the band completely from the spectrum. This effect may be accompanied by a wavelength shift (blueor red) and has been studied in a unified molecular orbital treatment byE. Heilbronner andR. Gerdil [Helv. chim. Acta39, 1996 (1956)]. The present treatment allows examples of severe steric repulsion to be recognised only with respect to wavelength shift, but does not appear to be unduly limited by such considerations.
4. J. Tanaka andS. Nagakura, J. chem. Phys.24, 1274 (1956).
5. Indeed, such a correlation was sought in the valuable study of substituted benzenes byDoub andVandenbelt 6. However, these authors used a hypothetical base line of 180 mµ as a means of deriving substitutional parameters, implying no fundamental difference between benzenoid L. E. and E. T. absorption. Further, no allowance was made for the varying effect of the disposition of substituents in theortho- orpara-position with respect to a −M group.