Pyrazole dipole moment measurement

Experimental values are collected in the McClellan book (B-63MI40400) and in a review on dipole moments and structure of azoles (71KGS867). Some selected values are reported in Table 3. The old controversy about the dipole moment of pyrazole in solution has been settled by studying its permittivity over a large range of concentrations (75BSF1675). These measurements show that pyrazole forms non-polar cyclic dimers (39) when concentration increases and, in consequence, the permittivity value decreases. 

This five-membered heterocycle contains one pyrrole-like and two pyridine-like N-atoms in the 1,2,3-positions. It was known as v-triazole (v meaning vicinal). Since all ring atoms are sp -hybridized, its six available electrons are in delocalized r-MOs. Thus, 1,2,3-triazole is aromatic. Its ionization energy, measured by photoelectron spectroscopy, amounts to 10.06 eV, which is greater than that of imidazole (8.78 eV) and pyrazole (9.15 eV), i.e. the HOMO of 1,2,3-triazole is lower [150]. The dipole moment in benzene is 1.82 D and is similar to that of pyrazole. 

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