Stationary Points on the Ground-State Potential Energy Surface of Dimethyldiazene. Isomerization and Decomposition in Competition

Abstract

The singlet ground-state potential energy surface as related to the thermal unimolecular mechanisms of dimethyldiazene was investigated, in order to obtain activation energies for trans−cis isomerization, as well as for the isomerization leading to 1,1-dimethyldiazene and for the simple C−N bond cleavage in the formation of CH3N2• and CH3• radicals. We used approximate density functional theory (DFT) with B3LYP potential to locate minimum energies and transition-state structures, as well as to determine harmonic vibrational frequencies and resulting zero-point vibrational energies. To obtain more reliable results, some structures were reoptimized by the multiconfiguration SCF wave function. The energetics were determined using quadratic configuration interaction singles and doubles (QCISD), perturbation theory with UHF and CAS reference wave functions (MP2-MP4 and CASPT2), and the configuration interaction wave function, which includes single and double excitations from the selected set of reference configurations constructed with the CASSCF molecular orbitals (MR-CISD). In contrast to previous theoretical studies, our results show that the trans−cis isomerization of 1,2-dimethyldiazene, which proceeds by rotation around the NN bond, or semilinearization, requires an activation energy near to that of its decomposition. It was also confirmed that isomerization to 1,1-dimethyldiazene is less favored than the cis−trans isomerization. The following barrier heights have been estimated: 229 kJ/mol for decomposition, 207 kJ/mol for isomerization by rotation, 227 kJ/mol for isomerization by semilinearization, and 424 kJ/mol for rearrangement to 1,1-dimethyldiazene. Calculation of RRKM rate coefficients showed that in the temperature range from 450 to 600 K the trans−cis isomerization and a direct decomposition proceed in competition. However the more stable trans-1,2-dimethyldiazene preferentially decomposes directly, as a consequence of the fast reverse cis−trans isomerization rate. At the temperatures above 600 K a direct decomposition increasingly predominates over isomerization.