An index of the @I03215@ of a solvent based on the frequency of the longest @W06659@ electronic absorption maximum of 1-ethyl-4-methoxycarbonylpyridinium iodide in the solvent. The Z-value is defined by: \[Z = \frac{2.859\,\times\,10^{4}}{\lambda}\] where \(Z\) is in \(\text{kcal mol}^{-1}\) and \(\lambda \) is in \(\text{nm}\).