Quantitative measures of the capability of solvents for interaction with solutes. Such parameters have been based on numerous different physicochemical quantities, e.g. @[email protected], solvatochromic shifts in @[email protected]/@[email protected] spectra, solvent-induced shifts in @[email protected] frequencies, etc. Some solvent parameters are purely empirical in nature, i.e. they are based directly on some experimental measurement. It may be possible to interpret such a parameter as measuring some particular aspect of solvent–solute interaction or it may be regarded simply as a measure of solvent @[email protected]. Other solvent parameters are based on analysing experimental results. Such a parameter is considered to quantify some particular aspect of solvent capability for interaction with solutes.