Theoretical study of the thermal decomposition of N,N '-diacyl-N,N '-dialkoxyhydrazines: A comparison of HF, MP2, and DFT

The thermal decomposition of N,N'-diacyl-N,N'-dialkoxyhydrazines, (N(COR1)( OR2))(2), to the corresponding ester and dinitrogen was investigated with h igh-level ab initio and density functional calculations. The results sugges t that the decomposition proceeds via a two step 1,1-elimination. Coupled c luster calculations on several model systems (R-1=R-2=H; R-1=CH3, R-2=H; R- 1=R-2=CH3) at the density functional geometry (CCSD(T)// B3P86) with zero-p oint energy and thermal corrections at the B3P86 level show that the barrie r for the first elimination, the rate-determining step, is 24-34 kcal/mol, while the barrier for the second step in the two step 1,1-elimination is on ly 1-3 kcal/mol, a value that is much smaller than the exothermicity of the first step. This result explains why experimentalists have been unable to trap the intermediate nitrene. The critical point associated with the conce rted elimination is shown to be a second-order saddle point on the potentia l energy surface at the HF and density functional levels of theory with ene rgies of 139.2 and 88.4 kcal/mol above the reactant, respectively. An unexp ected result of this study was the failure of Hartree-Fock (HF) and Moller- Plesset second order perturbation (MP2) to properly describe every aspect o f this seemingly simple system.