Protonation and phase effects on the NMR chemical shifts of imidazoles and pyrazoles: experimental results and GIAO calculations

Abstract

The effects produced on ¹H, ¹³C and ¹⁵N chemical shifts by protonation and by hydrogen-bonding solvents on five azoles have been determined experimentally. The following compounds have been studied: imidazole, 4,5-dimethylimidazole, pyrazole, 3,5-dimethylpyrazole and 4,5-dihydro-3-methyl-2H-benz[g]indazole. Phase effects on the ¹³C chemical shifts of the C-4 atom of pyrazole are discussed based both on empirical models and on GIAO calculations of absolute shieldings in different complexes. The special case of the chemical shifts of pyrazoles in the solid state, where they form multiple N–H⋯N hydrogen bonds, has also been studied theoretically.