All organic electronic spectra in solution are subject to a generalized polarization red shift which is due to solvent polarization by the transition dipole and which depends on the solvent refractive index. This can be obscured by the effect of dipole-dipole and dipole-polarization forces if the solute is polar, when the application of the Franck-Condon principle shows that the solvent cage around the excited solute molecule is momentarily strained. Orientation strain ana packing strain are defined, of which the former is more important. The absorption frequencies of polar solutes are shifted to the red in solution if the dipole moment increases during the transition; they may be shifted to the blue (relative to the gas) if the dipole moment decreases. Four cases are discussed according to whether solute and solvent are polar or non-polar. The place of ir* *n transitions is discussed.

known x* •*n transitions are blue shift, but that the converse is not necessarily true, quoting the case of the v* *-2600 A. transition of benzene, which is blue shift in his terminology. Now the case of benzene has been shown definitely by Bayliss and Hulme9 to be a polarization red shift (cases I and II), and in fact all polarization red shifts are "blue" shifts in McConnell's sense, since the refractive indices decrease in the order paraffin, alcohol, water. McConnell's definition fails to make the important distinction between (a) "blue shifts" that are red shifts compared with the gas as in cases I and II, and (b) "blue shifts" that are blue • shifts compared with the gas as in cases Ilia and IVa.However it is readily seen that a * +-n transi-tion is likely always to conform to cases Illa and IVa, since it involves a non-bonding (n) electron localized on a hetero-atom which is usually exposed or terminal to a conjugated system. During the transition it is excited into a x* orbital associated with a neighboring double bond or with a non-localized conjugated system. There is thus an electron displacement away from the hetero-atom, and this will usually decrease the dipole moment or the H-bonding capacity. The cases of acetone, crotonaldehyde, nitromethane and nitrobenzene are instructive in this regard, and are described in the following paper.7

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.

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