Indazoles protonation, calculations

The indazole molecular structure (Figure 11) shows the tautomeric proton bonded to N-1 (1//-indazole. Section 4.04.1.5.1). A linear correlation between the bond lengths and the bond orders calculated by the CNDO/2 method was observed (74T2903). 

All of the azoles showed a linear variation of these values except the pyrazoles, which belong to a parallel line 4.5 pK units apart. Fully optimized INDO geometries have been calculated for 12 azoles, as well as their cations and anions, both isolated and specifically solvated by five water molecules166. Evaluation of the protonation and deprotonation energies of the solvated molecules indicates a behaviour similar to that found experimentally in solution. In particular, the difference between pyrazoles (and indazoles) and all the other azoles is a consequence of the protonation of the nitrogen contiguous to NH, that is due to a difference in basicity. 

The acid-base properties of 4-nitroso-5-aminopyrazoles 50 have been studied. In particular, protonation sites have been determined where salts 51 are formed [66], The effects produced on II, 13C, and 15N chemical shifts by protonation and by hydrogen-bonding solvents on five azoles (imidazole, 4,5-dimethylimidazole, pyra-zole, 3,5-dimethyl-pyrazole, and 4,5-dihydro-3-methyl-2//-benz[g]indazole) have been determined experimentally. Phase effects on the 13C chemical shifts of the C-4 atom of pyrazole were 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, was also studied theoretically [88],... 

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