Imidazoles, calculations aromaticity

Calculation (INDO) of the energy barriers to the [l,5]-sigmatropic shift of a proton from nitrogen to an adjacent carbon to give a IH- or 4Af-imidazole (non-aromatic tautomers of imidazole) shows that activation energies above 210 kJ mol are required <86BSF(2)429>. Amino-substituted 2H- and 4//-imidazoles, however, may be more stable than the 1//-forms <84AHC(35)375>. 

Imidazole is aromatic. The pyrrole-like N-atom contributes two electrons to the r-electronic sextet. The pyridine-like N-atom and the C-atoms each donate one electron. As can be deduced from the NMR spectra, the r-electrons are largely delocalized, whereas the nonbonding electron pair is localized onto the pyridine-like N-atom. The following electron densities have been calculated by SCF/MO methods ... 

Pyrazole, like its structural isomer imidazole, is aromatic [126]. The following r-electron densities and r-bond orders were calculated by LCAO/MO methods ... 

Aromaticity varies with change of state because of the influence of the molecular environment on the interactions that determine aromaticity quantitatively, particularly in nonsymmetrical heterocycles. Comparative calculations of aromaticity indices for molecules in the gas phase and in condensed media with dielectric constants > 1, with or without hydrogen-bonding, showed coherent results for a set of nitrogen heterocycles, including imidazole, pyrrole, pyrazole, 1//-1,2,4-triazole, and benzimidazole. The... 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Many papers (see, for instance [16, 17]) and some books [18] have been devoted to discussion of the aromaticity of heterocycles. However, a simple problem such as knowing where the imidazolium cation (1) and imidazolate anion (2) are more or less aromatic than imidazole itself (3) is not solved, and probably never will be because it depends on the chosen aromaticity criteria. The NICS(l) values (NICS at 1 A above the ring plane, calculated at the GIAO//B3LYP/6-311++G level) are... 

The important field of ionic liquids, in most cases 1,3-dimidazolium salts (143) has been studied by IR and Raman spectroscopies supported by B3LYP calculations. Additionally, calculations of the heteroaromaticity (HOMA index) showed that cation formation causes a decrease of the aromaticity of the imidazole ring. However, when the R groups at positions 1,3 are benzyl or adamantyl, the aromatic nature of the heterocyclic moiety increases. Moreover, the electron distribution performed using the GAPT method indicated the positive charge delocalization in the imidazolium ring [143],... 

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). 

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