Chlorofluoroamines: ab initio and DFT studies on their structure, enthalpies of formation, and unimolecular reaction pathways

Abstract

Ab initio and density functional theory calculations are reported for the chlorofluoroamines HNXY (X/Y = F/Cl) and all possible unimolecular reaction products from their ground state. Reliable enthalpies of formation for these molecules and reaction products have been calculated using the G2 model. The large discrepancy between theory and experiment over the ΔH°_f(NCl) has been resolved by reevaluating the old experimental data. Optimized structures of all the species have been obtained at various levels up to MP2/6-311++G^∗∗ and B3LYP/6-311++G^∗∗. The triplet-singlet energy gaps have been estimated for NH, NF and NCl at various levels. Enthalpies of various reactions have been calculated at advanced levels including PMP4, CBS-Q, G1, G2, and G2-MP2. Transition states (TS) for the three-centered HX elimination reactions have been characterized. Vibrational frequencies for all the reactants, products, and transition states have been calculated using HF, MP2 and DFT methods with 6-311++G^∗∗ basis set. Threshold energies for the bond dissociation reactions and the HX (X = F/Cl) elimination reactions have been calculated. RRKM calculations have been carried out with these results to determine the branching ratio for the various possible reactions. The barrier for HCl elimination from HNFCl is 18 kcal mol^-1 higher than that of HF elimination. Still, according to DFT results, it is found that the HCl elimination is an important channel because of entropy factors, possibly explaining the experimental observations. However, the MP2 and G2 results predict the HF elimination to be more important for HNFCl. Moreover, the N-Cl bond energies in HNFCl and HNCl₂ are less than the HCl elimination barriers. Hence, the as yet unsuspected N-Cl bond dissociation may be a dominant decomposition pathway for HNFCl and HNCl₂.