Solvent effects on the relative stability of 4-nitro-1H-imidazole and 5-nitro-1H-imidazole.: Ab initio and density functional theory calculations

Ab initio and density functional theory (DFT) calculations were carried out to investigate the relative stability of 4-nitro-1H-imidazole (1) and 5-nitro-1H-imidazole (2) in the gas phase and in solutions with different polarities. Solvent effects were examined by means of both the self-consistent isodensity polarized continuum method (SCI-PCM) and the self-consistent reaction field (SCRF) method. In the gas phase, both QCISD(T)/6-311++G** and CCSD(T)/6-311++G** single point calculations at the MP2/6-311++G** optimized geometries estimated that 1 is more stable than 2 by 0.7 kcal/mol. On the other hand, B3LYP calculations, even with the 6-311++G(3df, 3pd) basis set, predicted that 2 is more stable than 1 by about 0.3-0.8 kcal/mol. In solutions, all the calculations employed in this study agreed that the relative stability of 1 over 2 gets significantly larger as the solvent becomes more polar. However, the relative stability calculated by the SCRF method was substantially larger than those computed by the SCI-PCM method. Origins of the solvent effects were examined by analyzing the changes of: (1) electrostatic potentials; and (2) the dipole moments due to the change of a solvent polarity. (C) 2000 Elsevier Science B.V. All rights reserved.