The barrier height, unimolecular rate constant, and lifetime for the dissociation of $HN2$

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen $(HN2)$ is thought to be an important species in combustion chemistry. The classical barrier height $(10.6±0.2kcalmol−1)$ and exothermicity $(3.6±0.2kcalmol−1)$ for the $HN2→N2+H$ reaction are predicted by high level ab initio quantum mechanical methods [up to CCSDT(Q)]. Total energies are extrapolated to the complete basis set limit applying the focal point analysis. Zero-point vibrational energies are computed using fundamental (anharmonic) frequencies obtained from a quartic force field. Relativistic and diagonal Born–Oppenheimer corrections are also taken into account. The quantum mechanical barrier with these corrections is predicted to be $6.4±0.2kcalmol−1$ and the reaction exothermicity to be $8.8±0.2kcalmol−1$⁠. The importance of these parameters for the thermal $NOx$ decomposition $(De-NOx)$ process is discussed. The unimolecular rate constant for dissociation of the $HN2$ molecule and its lifetime are estimated by canonical transition-state theory and Rice–Ramsperger–Kassel–Marcus theory. The lifetime of the $HN2$ molecule is here estimated to be $2.8×10−10s$ at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies $(τ=1.5×10−8s)$⁠, albeit consistent with the very rough experimental upper limit $(τ<0.5μs)$⁠. For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the $DN2$ molecule has a longer lifetime than the $HN2$ molecule. Thus, $DN2$ might be more readily identified experimentally. The ionization potential of the $HN2$ molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of $HN2$ is predicted to be $8.19±0.05eV$⁠, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry.