THE EMPIRICAL-TREATMENT OF SOLVENT SOLUTE INTERACTIONS - 15 YEARS OF PI

The near-UV-visible spectra of 4-nitroanisole (OMe) and N,N-dimethyl-4-nitroaniline (NMe(2)) were obtained at 25.0 +/- 0.1 degrees C in the gas phase and in a set of 229 solvents that includes non-hydrogen-bond donors such as hydrocarbons (aliphatic, alicyclic, ethylenic, and aromatic), perfluorinated and other halogenated hydrocarbons (aromatic and nonaromatic), nitriles, ketones, esters, lactones, anhydrides, amides, ureas, phosphates, HMPA, sulfates, sulfites, sulfoxides, sulfones, pyridines, and tertiary amines as well as weak hydrogen-bond donors such as nitrocompounds and primary and secondary amines. The frequencies of the absorption maxima of these spectra were used to refine and critically examine the pi* scale of solvent effects. The dielectric constants and refractive indexes of the solvents (mostly from this work) were used in an Oshika-Bayliss-McRae (OBM) treatment of these frequencies that led to the following results: (i) The partitioning of dipolarity and polarizability contributions to pi*; (ii) the quantitative assessment of more specific contributions to pi* due to aromatic solvents. This study also showed the limitations of the OBM treatment and the advantages of the empirical approach. General conclusions regarding the physical meaning and the use of scales of solvent effects (excluding hydrogen bonding) were drawn.