H-1, C-13 and N-15 NMR and IR studies of halogen-substituted tetrazolo[1,5-a]pyridines

The tetrazole-azide tautomerization of some halogen-substituted tetrazolo[1 ,5-a]pyridines was examined by IR spectroscopy at ambient temperature and b y H-1,C-13 and N-15 NMR spectroscopy at various temperatures. The tetrazolo pyridines can exhibit equilibrium between the azide and the tetrazole forms . For some of them slow exchange occurs on the NMR time-scale, such that it is possible to estimate equilibrium constants. The position and nature of the substituent in the pyridine ring result in stabilization or destabiliza tion of the tetrazole form and exert a strong influence on the values found for the equilibrium constants. A saturation transfer experiment was carrie d out for 5-bromotetrazolo[1,5-a]pyridine and the rate constants were estim ated. Moreover, based on the van't Hoff equation, the enthalpy Delta H degr ees and entropy Delta S degrees for the tautomerization were calculated. Ab initio calculated energies and charge distribution are in good agreement w ith differences observed in the tetrazole-azide equilibrium constants. Copy right (C) 1999 John Wiley & Sons, Ltd.