The Solvatochromic Comparison Method. I. The β-Scale Of Solvent Hydrogen-Bond Acceptor (HBA) Basicities

Abstract

The solvatochromic comparison method is outlined. Magnitudes of enhanced solvatochromic shifts in HBA (hydrogen-bond acceptor) solvents are determined for 4-nitroaniline (1) relative to N,N-diethyl-4-nitroaniline (2) [—ΔΔv(1-2)B←H2N] and for 4-nitrophenol (3) relative to 4-nitroanisole (4) [—ΔΔv(3-4)B ←Ho]· The —ΔΔv values for the HBD (hydrogen-bond donor) substrates 1 and 3 in corresponding HBA solvents are shown to be proportional to one another, proportional to limiting 19F NMR shifts of hydrogen-bonded complexes of 4-fluorophenol with the same HBA's, and linear with log association constants of hydrogen-bonded complexes between 4-fluorophenol (pKHB) and phenol and the same HBA molecules. The LFE relationships are used to establish a β-scale of solvent HBA basicities. © 1976, American Chemical Society. All rights reserved.