Imidazole cation -complexes, calculations

More recent calculations of cr-complex energies and total electron densities for the imidazole cation, anion and neutral molecule demonstrate that, where electrophilic attack is believed to involve a cr-complex, good agreement can be obtained with the experimentally observed site of substitution. SCF calculations of rr-electron distributions for the ground... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

The metal activators Mg2+ and K+ alone produced smaller decreases in the pK value of his (3) suggesting that the binding of P-enolpyruvate lowers the pK of his(3) by strengthening its interaction with the metal activators. Direct evidence for the proximity of the divalent cation to his (3) was obtained by the paramagnetic effects of the activator Ni2+ on 1/Ty and 1/T2 values of the C-2 proton of his(3) in the pyruvate kinase-K+, Ni2+ complex at pH 6.0. The calculated Ni2+-lH distance of 6 A is consistent with a second sphere imidazole complex. The presence of P-enolpyruvate increases the paramagnetic effect of Ni2+ on I/T2 of this proton by a factor of 8.7, suggesting direct coordination of his(3) by Ni2+ in the substrate complex (14). 

The interactions between tetramethylammonium (TMA) and tt systems may be described as strong CH/tt interactions, as these systems have short contacts between the C - H bonds and the tt system. However, the physical origin of the attraction in the TMA complex is completely different from that in the CH/tt interaction between neutral molecules [124,125). The MP2-level interaction energies of TMA complexes with benzene, pyrrole, furan and imidazole are -8.45, - 10.02, -6.98 and - 16.37 kcal/mol, respectively. The calculations show that the electrostatic interaction is the major somce of the attraction in these complexes, while dispersion is mainly responsible for the attraction in the CH/tt interaction between neutral molecules. The physical origin of the attraction between the TMA and tt systems is the same as that of other cation/Tr complexes therefore, the interaction between TMA and tt systems is essentially a cation/Tr interaction (see the next section). 

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