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

Abstract

Although never spectroscopically identified in the laboratory, hydrogenated nitrogen (HN(2)) is thought to be an important species in combustion chemistry. The classical barrier height (10.6+/-0.2 kcal mol(-1)) and exothermicity (3.6+/-0.2 kcal mol(-1)) for the HN(2)-->N(2)+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+/-0.2 kcal mol(-1) and the reaction exothermicity to be 8.8+/-0.2 kcal mol(-1). The importance of these parameters for the thermal NO(x) decomposition (De-NO(x)) process is discussed. The unimolecular rate constant for dissociation of the HN(2) molecule and its lifetime are estimated by canonical transition-state theory and Rice-Ramsperger-Kassel-Marcus theory. The lifetime of the HN(2) molecule is here estimated to be 2.8x10(-10) s at room temperature. Our result is in marginal agreement with the latest experimental kinetic modeling studies (tau=1.5x10(-8) s), albeit consistent with the very rough experimental upper limit (tau<0.5 mus). For the dissociation reaction, kinetic isotope effects are investigated. Our analysis demonstrates that the DN(2) molecule has a longer lifetime than the HN(2) molecule. Thus, DN(2) might be more readily identified experimentally. The ionization potential of the HN(2) molecule is determined by analogous high level ab initio methods and focal point analysis. The adiabatic IP of HN(2) is predicted to be 8.19+/-0.05 eV, in only fair agreement with the experimental upper limit of 7.92 eV deduced from sychrothon-radiation-based photoionization mass spectrometry.