The Effect of the Electrostatic Polarization of the Solvent on Electronic Absorption Spectra in Solution

Regarding the solvent as a continuous dielectric medium, it is shown that its effect on the Franck-Condon absorption of light by solute molecules must be expressed in terms of the electronic polarization part of its dielectric constant, K = n2. Using methods based both on quantum theory and on classical dispersion theory, it is shown that the red shift of absorption in solution depends directly on f, the oscillator strength, and inversely either on a3 (a is the radius of the spherical solute molecule) or the polarizability α. The expression Δν(cm−1)=const. (f/νa3)(n2−1)/(2n2+1) with two possible values of the constant, and alternatively with the substitution of α for a3, is tested on experimental data for isoprene, benzene, bromine, and iodine. Good quantitative agreement is obtained for the (V, N) transitions of isoprene and benzene. If the strong ultraviolet absorption of bromine and iodine solutions is regarded as the displaced (V, N) transition, the quantitative agreement is poor, although qualitatively in accordance with the theory. The weak λ2600 system of benzene, and the visible continua of bromine and iodine, show the expected smaller Δν with smaller f, although quantitative comparison with theory is prevented by the superposition of other solvent effects which become important in weak absorption bands.