The Solvatochromic Comparison Method. 6. The π* Scale of Solvent Polarities1

Seventy solvents are arranged in a π* scale of solvent polarities, so named because it derives from and best correlates solvatochromic effects on p → π and π → π electronic spectral transitions. Solvent effects on vmax values of seven primary indicator compounds are employed in the initial construction of the π* scale, and correlations with 40 additional spectral indi-cators are used to expand and refine the data base. Standard deviations in the 47 correlation equations of vmax with solvent π* values average O.l l kK, which compares well with the 0.10 kK precision limit of the solvatochromic comparison method. A number of stratagems are employed to exclude or minimize hydrogen bonding effects in determining π* values of HBA (hydrogen bond acceptor) and amphiprotic HBA-D (hydrogen bond acceptor-donor) solvents. Values of s in the solvatochromic equation, vmax = vo + sπ*, show logical variations with indicator structure, lending confidence that this new solvatochromic parameter will come to serve as a convenient and meaningful indicator of the interaction of a chromophore with its cybotactic environment. Poor correlation of vmax values for Dimroth's betaine, 4-(2,4,6-triphenylpyridinium)-2, 6-diphenylphenoxide with the π* scale is rationalized in terms of differing polarity and polarizability contributions to overall solvent effects. © 1977, American Chemical Society. All rights reserved.