Pyrazoles protonation, basicity

The basicity and the protonation site of 3-amino, 4-amino, 5-amino 1-methyl-, and 1-phenyl-pyrazoles has been determined most protonate on the pyrazole Nj ring nitrogen (only 4-amino-pyrazoles protonate on the amino group) <85JHC997,85MRC367,89JCS(P2)1941>. 

With a pATa of 2.5, pyrazole is significantly less basic than imidazole, whose p/fa is 7.1. In fact, having an adjacent heteroatom near the N atom always has the effect of lowering the basicity of the N because of their inductive effect. Therefore, the N atoms on isothiazole (p/Ca, -3.0) and isoxazoles (pATa, -0.5) are less basic than those of thiazole (pA a, 2.5) and oxazole (pAfa, 0.8), respectively. Nonetheless, pyrazole is basic enough to be protonated with most strong inorganic acids. ... 

Which nitrogen in each molecule is more basic Compare energies of the alternative conjugate acids (Nprotonated imidazole, NH protonated imidazole, N protonated pyrazole and NH protonated pyrazole). Which compound, imidazole or pyrazole, is more basic Compare the energies of protonation (leading to the favored conjugate acid in each case). Rationalize your result. 

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 aromatic sextet is completed by delocalisation of the lone pair from the second heteroatom, 4.4a-e. Consequently, as in pyridine, the nitrogen atoms of the 1,2-azoles have a lone pair available for protonation. However the 1,2-azoles are significantly less basic than the 1,3-azoles because of the electron-withdrawing effect of the adjacent heteroatom. Isoxazole and isothiazole are essentially non-basic heterocycles (pAas <0), and even pyrazole (pAa=2.5) is a much weaker base than the corresponding 1,3-azole imidazole (pAa=7). 

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