Theoretical study of the internal rotational barriers in some N-substituted nitropyrroles

The molecular geometries and internal rotational barriers of the nitro group of various N-substituted 2-and 3-nitropyrrole (X-2NPy(I) and X-3NPy(II); X = H(l), CH3(2), OH(3), NH2(4), and NO2(5)) were calculated by a DFT method at the B3LYP/6-31 lG** level. Since there were no experimental data available on the molecular geometries and the internal rotational barriers in title molecules, the calculated result were compared with corresponding substituted nitrobenzene. The tendency of substituents affecting the molecular geometries and internal rotational barriers are similar to the corresponding substituted nitrobenzenes. The optimized geometries show that the I-1, I-3, II-1; and II-5 are planar molecules, the others are not. The steric effect plays a determining role to the twist of the substituted group when the neighboring group exists. Internal rotational barriers of the nitro group for all title molecules were calculated as the V-2 barrier, and the NO2 torsonal potential for each molecule were given. The values of the V-2 barrier of nitropyrrole derivatives depend on the resonance, inductive, conjugate, hydrogen bond, and steric effects between the substituted group and the pyrrole ring. I-5 has the lowest V-2 barrier as 3.43 kcal/ml; however, I-3 indicates the highest one as 14.91 kcal/mol. (C) 2003 Elsevier B.V. All rights reserved.