Reactivity of N-NO2 bonds in nitramines:: Bond dissociation and bond disproportionation approach

The homolytic dissociation of N-NO2 bond represents the primary fission process of nitramines under thermal, impact, shock and electric spark initiation stimuli. The N-NO2 bond fission is mostly characterized by homolytic bond dissociation energies BDE(N-NO2). The theoretical calculations of BDE energies arc substantially influenced from inadequate treatment of electron correlation. Recently the alternative method was suggested to overcome this substantial drawback - the bond separation approach described by an isodesmic reaction RN-NO2 + SN-H -> RN-H + SN-NO2 where SN-NO2 is a standard nitramine (NPI). This type of virtual symmetrical chemical equilibria, characterized as bond disproportionation reactions, inherently cancel the electron correlation effects accompanying homolytic bond dissociation. The bond disproportionation energies DISP(N-NO2) and bond dissociation energies BDE(N-NO2) were evaluated for 20 nitramines at ab initio DFT B3LYP/6-311+G(d,p) level and correlated with their detonation velocities D. It is shown that DISP approach can be used seemingly with advantage over BDE energies for unambiguous description of molecular structural dependence of detonation. However, semilogarithmic analysis of these two relationships leads to finding that the resulting correlations of In D values with DISP(N-NO2) or BDE(N-NO2) values remind analogous LFER relationships between the rate and Hammett substituent constants.