Investigations of correlation between nitro group charges and C-nitro bond strength, and amino group effects on C-nitro bonds in planar conjugated molecules

The correlation between the nitro group charges (Q(Nitro)) and the C-Nitro bond length (RC-Nitro), and the positional effects of amino groups on C-nitro bond strength in planar conjugated molecules such as the amino-nitro derivatives of benzene, naphthalene, anthracene, phenanthrene, alkene, and minor heterocyclic nitrogen compounds are computationally investigated. All calculations are performed using density functional theory (DFT) and the general gradient approximation (GGA) method with the Beck-LYP hybrid functional and the DNP basis set in Acceryls' code Dmol(3). The results are: (1) Q(Nitro) can be used as a structural parameter to estimate the strength of the C-Nitro bond, and further the stability of the nitro compound for 15 fitted lines show there is a good linear Q(Nitro)R(C-Nitro) relationship in any group of derivatives. In addition, Q(Nitro) is more sensitive to structural changes and more effective to assess the C-Nitro bond strength than RC-Nitro (2) the nitrating reaction of aromatic hydrocarbon can be qualitatively predicted and compared by Q(Nitro): the more Q(Nitro), the easier the nitrating condition, and the higher occurrence ratio of the corresponding nitrating product and (3) the amino groups on even positions can significantly strengthen C-Nitro bonds, other than those on odd positions. Therefore, in synthesizing insensitive explosives, it should be recommendable to introduce amino groups on even positions, particularly on 2-position due to the combination of the hydrogen bond, the induction effect and the mesomeric effect, which can obviously increase the molecular stability. (c) 2005 Elsevier B.V. All rights reserved.