Quantum Mechanical Studies on Molecular Structure of Hindered Nitro Aromatics

Abstract

The molecular structures of 2-nitro- and 3-nitrobenzaldehydes, 2-(2‘-nitrophenyl)- and 2-(3‘-nitrophenyl)-1,3-dioxolanes, and trans-2,2‘-dinitrostilbene were studied theoretically by means of ab initio quantum mechanical calculations at the RHF/6-31G*//RHF/6-31G* and MP2/6-311++G(2d,2p)//RHF/6-31G* levels. A noncoplanar conformer was found for 2-nitrobenzaldehyde, where the carbonyl and nitro groups are twisted with respect to the phenyl ring. Different intramolecular interactions were studied by the analysis of the charge density topology, finding that the aldehyde H atom forms a intramolecular H bond with one oxygen of the nitro group. This result clarifies the classical controversy on the nature of the interactions between the nitro and aldehyde groups in the 2-nitrobenzaldehyde. Similar twisted conformation and (Hα···ON) intramolecular hydrogen bonds were found in 2-(2‘-nitrophenyl)-1,3-dioxolane. On the contrary, both carbonyl and nitro groups are coplanar with the phenyl ring in 3-nitrobenzaldehyde, and similar behavior was found in 2-(3‘-nitrophenyl)-1,3-dioxolane. No hydrogen bond was found in these meta isomers. The calculated dipole moments of the nitrobenzaldehydes reproduce the experimental values. A noncoplanar s-trans-gauche/s-trans-gauche (NO2arylCCarylNO2) conformation was also found as a minimal conformer in trans-2,2‘-dinitrostilbene, where the aromatic groups are twisted with respect to the central double bond and the nitro moieties are also twisted out of the aromatic rings. This structure is consistent with several experimental physical−chemical properties of this molecule. Intramolecular (Hα···ON) hydrogen bonds with low stability were found in this last compound.