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-Yan g-Parr hybrid density functional with the standard 6-31G basis set fo r 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 kinet ic isotope effect for the HONO elimination, and the secondary isotope effect for the N-N bond homolysis of dimethylnitramine. The computed p rimary effect is k(H)/k(D6) = 4.21, and the computed secondary effect is k(H)/k(D6) = 1.40, both at 240 degrees C. Comparison with the exper imentally observed isotope effect of 1.57 for decomposition of dimethy lnitramine-d(6) in solution at 240 degrees C suggests a significant pa rt of the observed effect is due to the secondary effect on N-N bond h omolysis. Similar beta secondary deuterium isotope effects are expecte d for the N-NO2 cleavages that initiate the decompositions of the nitr amine explosives HMX and RDX.