Add time:07/26/2019         Source:sciencedirect.com

We have examined the molecular structure of imidazole (I), 4-nitroimidazole (II), 5-nitroimidazole (III) and 4,5-dinitroimidazole (IV) with semi-empirical, ab initio and density functional theory (DFT) calculations. Compared with experimental data, both B3LYP and MP2 calculations with the 6-31G∗ basis set furnish excellent geometric features with average errors of less than 0.01 Å in bond lengths and of less than 1 ° in bond angles. Although the SCF results are slightly worse than those with electron correlation effects, HF/3-21G and HF/6-31G∗ calculations also provide reasonable geometries and are probably useful in a practical sense. Geometries obtained from semi-empirical methods have large errors in some of bond lengths and angles. Rotational barriers of the nitro group in II and III have been estimated to be 6.4 and 10.8 kcal/mol, respectively, at the QCISD(T)/6-31G∗//HF/6-31G∗ level. These barriers are largely overestimated at the SCF and B3LYP levels, although utilizing an extremely large basis set such as 6-311++G(3pd,3df) at the B3LYP level reduces the barriers to a comparable range with those of QCISD(T)/6-31G∗//HF/6-31G∗. On the other hand, both AM1 and PM3 underestimate these barriers considerably. According to the analysis of natural bond orbitals, a substantial difference in rotational barriers of between II and III is attributed to the relative stabilization energy due to π orbital conjugations between the C4C5 bond and NO bonds. In the global minimum of IV predicted by both ab initio and DFT calculations, both nitro groups are skewed, with more deformation of the nitro group attached to the C5 atom, although the degree of twisted angles in nitro groups is quite variant depending on the calculational levels. Both semi-empirical methods predict that the nitro group attached to the C4 atom is eclipsed and the one attached to the C5 atom is planar. The orientation of nitro groups in IV can be understood by a compromise between 1.(1) an increase of electrostatic repulsions as two electronegative nitro groups approach and2.(2) a destabilization due to less π orbital overlaps as nitro groups are skewed.

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