Spectral Solvent Shift. I. Paraffin Hydrocarbon Solvent Interactions with Polynuclear Aromatic Hydrocarbons

The electronic shifts for 11 ultraviolet transitions of the chromophores naphthalene, phenanthrene and the polar aromatic hydrocarbon, azulene, have been observed in a series of 17 paraffin hydrocarbon solvents from isopentane to n-tridecane. The contribution to the shift of branching and cyclization of solvent molecules has been observed by including the five structural isomers of hexane, 2,2,4-trimethylpentane and 2,2,5-trimethylhexane together with the cyclo- and methylcyclopentanes and hexanes. The statistical approach is used to interpret the electronic shifts in terms of interbond potentials and the solvent radial distribution function and is compared to the McRae extension of the Bayliss solvent shift theory. Dispersive type potentials suffice to account for the frequency shifts of transitions, except the 1Lb transition of azulene which shifts to the blue from the vapor to hydrocarbon solutions. The behavior can be accounted for by a change in the static dipole moment of azulene on excitation, a phenomenon predicted by quantum mechanical calculations on the molecule.