Reactivity of CON-NO2 Bonds in Nitramides: A Bond Disproportionation Approach

The homolytic N-NO2 bond dissociation, both in nitramines and nitramides, represents the primary fission process of these energetic materials under thermal, impact, shock and electric spark initiation stimuli. The N-NO2 bond fission in nitramides is mostly characterized by homolytic bond dissociation energy BDE(N-NO2). Recently the alternative method was suggested: the bond disproportionation approach which describes the bond fission as an isodesmic reaction RCON-NO2 + SCON-H -> RCON-H + SCON-NO2 where SCON-NO2 is a standard nitramide (1-nitropiperidin-2-one, NPO). This type of virtual symmetrical chemical equilibria inherently cancels the electron correlation effects accompanying the theoretical calculations of homolytic bond dissociation energies. In this paper the bond disproportionation energies DISP(N-NO2) and bond dissociation energies BDE(N-NO2) were calculated for 17 nitramides by DFT B3LYP/6-311+G(d, p) method and simultaneously the corresponding total electronic charges of nitro groups Q(NO2) were derived from DFT B3LYP/6-31G(d, p) calculations. The BDE and DISP energies were correlated with calculated detonation velocities D, heats of detonation Q(det) and electronic charges of nitro groups Q(NO2). It is shown that DISP approach can be used. The BDE energies correlate more tightly with detonation velocities D of nitramides studied but DISP energies fit more precisely the relationships with heats of detonation Q(det) and electronic charges Q(NO2). The analysis of the relationships for DISP(N-NO2) leads to finding that the resulting correlations with D-2 and Q(det) somewhat remind analogous relationships between the rate and Hammett constants (detonation is taken as a "zero order" reaction in this case).