The photodissociation of s-tetrazine via a three-body fragmentation channel (C2N2H2→2HCN+N2) has been studied by ab initio direct classical trajectory calculations using Hartree–Fock and density functional methods with split valence and polarized basis sets [HF/3-21G, HF/6-31G(d) and B3LYP/6-31G(d)]. The calculated transition state is planar. At our most reliable method (CBS-APNO), the heat of reaction and barrier height are −53.0 kcal mol−1 and 41.1 kcal mol−1, respectively. To simulate the experimental photolysis of s-tetrazine, trajectories were started from a microcanonical ensemble at the transition state with 12 kcal mol−1 excess energy distributed among the vibrational modes and the transition vector. At all levels of theory, the HCN product has
a very broad rotational distribution, ranging up to J=64, and has extensive excitation of the bending vibration. By contrast, N2 is produced with low J and with only a small amount of vibrational excitation in agreement with experiments. In accord with the experiment, the relative translation motion of the products receives about 80–83% of the available energy.
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