Ab Initio Density Functional Calculations of Deuterium Kinetic Isotope Effects for Decomposition of Dimethylnitramine

Ab initio harmonic force fields were computed using the Becke3−Lee−Yang−Parr hybrid density functional with the standard 6-31G* basis set for dimethylnitramine, for dimethylamino radical, and for the transition structure for five-center elimination of HONO from dimethylnitramine. These force fields were used to calculate the primary deuterium kinetic isotope effect for the HONO elimination, and the secondary isotope effect for the N−N bond homolysis of dimethylnitramine. The computed primary effect is k H / k D6 = 4.21, and the computed secondary effect is k H / k D6 = 1.40, both at 240 °C. Comparison with the experimentally observed isotope effect of 1.57 for decomposition of dimethylnitramine- d 6 in solution at 240 °C suggests a significant part of the observed effect is due to the secondary effect on N−N bond homolysis. Similar β secondary deuterium isotope effects are expected for the N−NO 2 cleavages that initiate the decompositions of the nitramine explosives HMX and RDX.