Basicity of C-substituted pyrazoles in the gas phase: an experimental (ICR) and theoretical study

A study of spectroscopic data on the gas-phase proton affinities (PAs) of 32 C-substituted pyrazoles and N-methyl pyrazoles as well as of 12 C-methyl- substituted pyrazoles reveal a simple additive pattern of substituent effects. Protonation energies calculated by ab initio 6-31G/6-31G methods are shown to correlate linearly with the experimental PAs. The analysis of substituent effects on PAs in terms of polarizability and field and resonance contributions suggests that, except for 3-aminopyrazole, the 3(5)-substituent does not play a major role in pyrazole's tautomeric behavior.