Ab initio and density functional studies on bonding nature of the N-N bonds in 1,2,5-trinitroimidazole and 1,2,4,5-tetranitroimidazole

The bonding nature of the N-N bonds in 1,2,5-trinitroimidazole (I) and 1,2, 4,5-tetranitroimidazole (II) was examined with various levels of ab initio and density functional (DF) theories. The second-order Moller-Plesset pertu rbation method (MP2) with the 6-31G** basis set has predicted significantly long N-N bond lengths in I and II, that is, 1.737 and 1.824 Angstrom, resp ectively. Two DF theories, BLYP/G-31G** and BP86/6-31G**, provided similar results to those of MP2/6-31G**. On the other hand, Hartree-Fock (HF) calcu lation with the 6-311 + + G** basis set evaluated these bond lengths of I a nd II to be 1.443 and 1.414 Angstrom, respectively. Bond properties includi ng the bond critical density are strongly dependent on the equilibrium bond length. Thus, accurate prediction of geometric parameters is of particular importance to derive reliable bond properties. Especially, a substantial d ifference in bonding properties is observed when the electron correlation e ffect is included. According to our analyses with bonding natures and CHELP G charges at the MP2 level, (1) the N-N bonds of I and II appear to have a significant ionic nature, and (2) the l-nitro group bears a considerable po sitive charge and has attractive electrostatic interactions with O atoms of adjacent nitro groups. Although all the theories utilized in this study pr edict that both I and II are stable in their potential-energy surfaces, sig nificantly long N-N bond lengths calculated with MP2 and DF theories imply a strong hyperconjugation effect, which may explain a tendency to form a sa lt in these compounds easily. (C) 1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 145-154, 1999.