THERMODYNAMIC BASICITY VS KINETIC BASICITY OF DIAZOLES (IMIDAZOLES AND PYRAZOLES)

The intrinsic basicity of 24 azoles (pyrazoles, indazoles, imidazoles, benzimidazoles) and 7-methylazaindole was determined by mass spectrometry techniques, ion cyclotron resonance (ICR) and/or chemical ionization (CI) in conjunction with tandem mass spectrometry (MS/MS). A reasonably good agreement (r2 = 0.967) is found between both methods (15 compounds). Thus, it is possible to use Ci/MS/MS to determine the intrinsic basicity of compounds not measurable by ICR for purity or volatility reasons. Some anomalies are interpreted in terms of entropy and steric effects. The basicity data are also discussed by using empirical models (sigma-alpha, sigma-R) and chelation and annelation effects.