Polarity and basicity of solvents. Part 2. Solvatochromic hydrogen-bonding shifts as basicity parameters

Abstract

The solvatochromic hydrogen-bonding shifts of p-nitrophenol and of p-nitroaniline have been measured by the thermosolvatochromic comparison method for an extended sample of oxygen, nitrogen, carbon, halogen, and sulphur bases. Their significance as a hydrogen-bonding parameter has been tested by their correlation with formation constants, n.m.r. shifts, vibrational shifts, and enthalpies for hydrogen-bonding formation. Family-dependent correlations are generally found between the above properties. The correlation of a hydrogen-bonding property for an OH donor versus the same property for an NH donor is family (polar oxygen bases, ethers, pyridines, and tertiary aliphatic amines) dependent. The only significant family-independent correlation is for the solvatochromic shift of p-nitrophenol versus the enthalpy of hydrogen-bond formation of p-fluorophenol. It is shown that the β scale is mainly a scale of NH hydrogen-bond acceptor basicity. The averaging process used to define β is criticized and it is recommended that correlation analysis of basicity is undertaken with clearly defined models.