Hydrogen bonding of 1- and 2-substituted tetrazoles with conventional proton donors

Dissociation constants of H-complexes formed by 1- and 2-substituted tetrazoles with 4-fluorophenol in carbon tetrachloride and methylene chloride were determined by Fourier-transform IR spectroscopy. The nature of substituents on the endocyclic nitrogen and carbon atoms was found to weakly affect the pK _HB value. By contrast, the position of the N-substituent is significant. For example, 1-methyl-5-phenyl-1H-tetrazole (pK _HB = 0.66 in CH₂Cl₂) is a stronger proton acceptor than 2-methyl-5-phenyl-2H-tetrazole (pK _HB = 0.05 in CH₂Cl₂). The proton affinity of the examined compounds appreciably decreases in going from less polar carbon tetrachloride to more polar methylene chloride. No general correlation was revealed between the pK _HB values of substituted 1H- and 2H-tetrazoles and shifts of the OH stretching vibration frequency of methanol (Δν) upon formation of H-complexes.