Theoretical study of the thermal decomposition of N,N-diacyl-N,N-dialkoxyhydrazines:: A comparison of HF, MP2, and DFT
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abstract
The thermal decomposition of N,N-diacyl-N,N-dialkoxyhydrazines, (N(COR1)(OR2))2, to the corresponding ester and dinitrogen was investigated with high-level ab initio and density functional calculations. The results suggest that the decomposition proceeds via a two step 1,1-elimination. Coupled cluster calculations on several specified model systems at the density functional geometry (CCSD(T)// B3P86) with zero-point energy and thermal corrections at the B3P86 level show that the barrier 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 only 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 concerted elimination is shown to be a second-order saddle point on the potential energy surface at the HF and density functional levels of theory with energies of 139.2 and 88.4 kcal/mol above the reactant, respectively. An unexpected result of this study was the failure of Hartree-Fock (HF) and Moller-Plesset second order perturbation (MP2) to properly describe every aspect of this seemingly simple system.
