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 pKHB value. By contrast, the position of the N-substituent is significant. For example, 1-methyl-5-phenyl-1H-tetrazole (pKHB = 0.66 in CH2Cl2) is a stronger proton acceptor than 2-methyl-5-phenyl-2H-tetrazole (pKHB = 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 pKHB values of substituted 1H- and 2H-tetrazoles and shifts of the OH stretching vibration frequency of methanol (Δν) upon formation of H-complexes.