Effect of Solvent Polarizability on the Ultra-Violet Spectral Shifts of Aromatic Compounds

Abstract

IT is generally recognized that non-polar solvents cause a long wave-length shift of the optical spectral maxima of absorbing solutes, and that this shift tends to increase with the polarizability of the solvent. Bayliss¹ has proposed that: where Δν is the long wave-length shift, compared to the dilute vapour spectral position, f is the oscillator strength (that is, the number of ‘classical’ absorbing electrons), a is the radius of the absorbing molecule, n is the refractive index of the solvent, and the other symbols have their usual significance. Longuet-Higgins and Pople² have proposed the relationship: where the shift is expressed in energy (frequency × h); α_A and α_B are the molecular polarizabilities of the solute and solvent, respectively, and M and E are the dipole moment and energy of the transition. Each solute molecule is regarded as being surrounded by z solvent molecules at a mean distance R̄. This relationship predicts that the long wave-length shift will be proportional to (n ² − l)/(n ² + 2). Both equations predict that Δνcm.⁻¹) or Δλ (Δλ = λ_s – λ₀, where the subscripts refer to solvent and vapour respectively; over the very short wave-length range to be considered here, Δλ = Δνλ² to a high degree of approximation) will approach zero only in the vapour state, and that in any solvent a substantial shift should be observed. Similar but more elaborate theoretical treatments have been given by Ooshika³ and McRae⁴.