A computational study of structural and reactive properties of imidazolidine, piperazine, and their N,N-dinitro derivatives

Abstract

Structural features, relative stabilities and reactive properties of the five- and six-membered diaza alicyclic systems imidazolidine and piperazine and their N,N-dinitro derivatives have been studied using an ab initio self-consistent-field molecular orbital approach at the 3-21G level. Both imidazolidine and 1,3-dinitroimidazolidine have puckered geometries, while the most stable conformers of the piperazines are chairs. For both piperazine and 1,4-dinitropiperazine, there are three chair conformers that are very close in energy, one of which has a calculated non-zero dipole moment. Our results are therefore consistent with the experimentally determined non-zero dipole moments for these two molecules. Imidazolidine and piperazine are found to have an added degree of stability relative to the analogous cycloalkanes, which may be due to σ-conjugation of the amine lone pairs. The introduction of nitro groups results in a greater degree of ring flattening in 1,3-initroimidazolidine than in 1,4-dinitropiperazine. The large negative regions in the electrostatic potentials of imidazolidine and piperazine, attributed to the ring nitrogen lone pairs and indicative of attractive sites for electrophiles, are greatly diminished in strength and size by substitution of nitro groups for the amine hydrogens. This effect increases as the C-N(NO 2)-C portion of the molecule approaches planarity. The ring nitrogens in the dinitro derivatives are thus predicted to be much less attractive as sites for electrophilic attack and less basic than in the parent molecules.