Imidazole dipole moment measurement

Imidazoles.—Dipole-moment measurements suggest that N-acetyl-imidazole, -benzimidazole, and -indole exist as equilibrium mixtures of (Z)- and ( )-conformers. ... 

I.P. stands for ionization potential italic characters denote dipole moments measured in gas phase (imidazole, 1,2,4-triazole and tetrazole) and for the two last compounds (value below the first one) also in 1,4-dioxane solution electron densities are from (03MI)... 

The dipole moment is a fundamental property of a molecule (or any dipole unit) in which two opposite charges are separated by a distance . This entity is commonly measured in debye units (symbolized by D), equal to 3.33564 X 10 coulomb-meters, in SI units). Since the net dipole moment of a molecule is equal to the vectorial sum of the individual bond moments, the dipole moment provides valuable information on the structure and electrical properties of that molecule. The dipole moment can be determined by use of the Debye equation for total polarization. Examples of dipole moments (in the gas phase) are water (1.854 D), ammonia (1.471 D), nitromethane (3.46 D), imidazole (3.8 D), toluene (0.375 D), and pyrimidine (2.334 D). Even symmetrical molecules will have a small, but measurable dipole moment, due to centrifugal distortion effects. Methane " for example, has a value of about 5.4 X 10 D. 

Pozharskii et al.s have criticized the high dipole moments obtained by Hiickel170 for imidazole and 4-methylimidazole, suggesting that the measurements were made in concentrated solutions where association would be significant. A similar conclusion had been reached by earlier workers171 (see Table I). 

There has been considerable interest in the measurement of the dipole moments of imidazole and its derivatives since the values obtained should provide valuable information about the structures of the molecules (70AHC 12)103, 80AHC(27)24l, 66RCR122). 

The best known of the potential mercaptoimidazoles are the imidazoline-2- and benz-imidazoline-2-thiones, which resemble imidazolin-2-ones in that the tautomeric form (53 X = S) is the preferred form. The crystal structure and the HNMR spectrum of 1,3-dimethyl-3H-imidazoline-2-thione have been interpreted as showing partial double bond character in the N—C—N system, but no aromaticity (70CC56). However, the preference for a betaine structure (56) rather than (57) or (58) should be accepted with caution since it is really only a resonance structure similar to others which undoubtedly contribute to the overall structures of oxo-, thio- and amino-imidazoles. Measurement of the piSTa values for a series of imidazoline-2-thiones substituted variously on C-4, C-5, N-1 and N-3 by hydrogen, phenyl or methyl shows that all of the values are similar. Approximate Kr values calculated show that these compounds exist even more in the thione forms (53, X = S 58) than do the corresponding thiazoline-2-thiones and oxazoline-2-thiones. The UV spectra in aqueous solution support thione structures, as do dipole moment and X-ray studies (76AHC(S1)280, p. 400). 

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. 

---