A study of solvatochromic linear energy relationships in a homologous series of n-alkanes and n-alkylnitriles

This is the first paper to systematically study solvatochromic effects of a large number of Kamlet-Taft indicator solutes in a homologous series of solvents. The transition energies of 12 indicators in 12 n-alkanes (pentane to hexadecane) and 10 indicators in 10 n-alkylnitriles (acetonitrile to pentadecanenitrile) are reported. In addition, data for a number of related solvents including cyclopentane, cyclooctane, and isooctane are presented. The results are used to examine the effect of solvent distortional and orientational polarizability on solvatochromism. The transition energy shifts and the Kamlet-Taft π* parameters are shown to be linear functions of the Onsager reaction field function or, equivalently, a modified Block and Walker reaction field model we recently developed in n-alkanes. However, transition energy shifts in n-alkylnitriles depend in a complex fashion on the solvent refractive index and dielectric constant. The Kamlet-Taft reducing function model, which depends upon solvent refractive index and dipole moment, predicts a trend in solute transition energy for the nitrile series which is contrary to observation. In contrast, a reaction field model previously employed by us correctly predicts the trends in the nitrile series. Finally, variations (both systematic and nonsystematic which are well beyond experimental uncertainty) in solute sensitivity to changes in solvent strength show a single lumped estimate of solvent polarity/polarizability (e.g. π*, R_T, Z) cannot be applied to the liquids studied here.