Electronic transition moments in the spectra of substituted naphthalenes

Abstract

The absorption spectrum of naphthalene is drastically perturbed as a result of substitution, and the perturbation depends on the nature and position of the substituents (Baba and Suzuki, 1961; Suzuki et al., 1973). In the present communication we report the effect of substitution on the orientation of electronic transition moments in naphthalene. The most reliable experimental method of determining this parameter is based on the rotational contour analysis of the electronic-origin band in the absorption spectrum of molecular vapor recorded under high resolution. This has been done for the near ultraviolet spectra of naphthalene and several of its monoderivatives (Hollas and Thakur, 1973, 1974; Singh and Thakur, 1978). The orientation of the electronic transition moment determined by this method is, however, not unambiguous and one has to choose between the two orientations on the basis of molecular-orbital calculations. The choice of the angle between the transition moment vector and the long in-plane axis of the naphthalene ring in the case of 2-hydroxynaphthalene (Hollas and Thakur, 1973) was made on the basis of the MO calculations of Nishimoto (1967). The orientation angle rejected on this basis is, however, found to be in good agreement with a later MO calculation of Suzuki et al. (1973). This ambiguity between the two different sets of calculations is resolved if we remember that the transition moment is represented by a double-headed arrow and that the MO calculations give either the acute or the obtuse angle between this arrow