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 ∆H°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 HNCl 2 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 HNCl 2 .

Unimolecular HCl Elimination from 1,2-Dichloroethane: A Single Pulse Shock Tube and ab Initio Study

“…Adding electron correlation at the MP2 level decreases the C−Cl distance to 2.415 Å, but the DFT calculation shows an increase to 2.599 Å. The C−Cl distance is directly related to the “looseness” of the TS and it is clear that DFT predicts a looser TS than MP2 as observed in our earlier work on HNFCl . It should be noted, however, that even for the ground state, the C−Cl distances change in a similar fashion at the three levels of theory for EC and 1,2-DCE.…”

“…This influences the low-frequency normal mode vibrations (CCCl deformation and CCl stretching) for the two conformers and the TST rate constants, vide infra. Extensive searches were made for the transition state structure for HCl elimination reactions as it was felt that most of the earlier work ,, found too long a C−Cl distance for the TS. The C−Cl distance was varied from that of the reactant itself.…”

Selected Ion Flow Tube Study of the Ion−Molecule Reactions of Monochloroethene, Trichloroethene, and Tetrachloroethene

“…The majority were taken from standard sources, 14,22 exceptions being the enthalpies of formation for CF 3 + (+ 406 kJ mol -1 ), 23 CClF (+ 31 kJ mol -1 ), 24 SF 5 + (+ 29 kJ mol -1 ), 25 SF 5 (-915 kJ mol -1 ), 26 SF 4 (-768 kJ mol -1 ), 27 SF 2 + (+ 693 kJ mol -1 ), 27 SF 2 (-295 kJ mol -1 ), 27 SF + (+ 998 kJ mol -1 ) 27 and NCl (+ 314 kJ mol -1 ). 28 The values for the parent neutrals were taken from Manion. 29 The enthalpies of formation for the parent ions formed from trichloroethene and tetrachloroethene were taken from the photoionisation study on these molecules, 1 Tables 1−3…”

Ab initio, DFT and transition state theory calculations on 1,2-HF, HCI and CIF elimination reactions from CH2F–CH2Cl

“…Our earlier work on HX elimination from chlorofluoroamines also showed that MP2 level calculations overestimated barrier for HF/HCl elimination reactions in comparison with advanced level calculations. 44 In order to gain more insight into this problem, CCSD(T) calculations were carried out for HF and HCl elimination reactions from CFE and they are discussed later in this section. Moreover, experimental rate constants are compared to theoretical ones directly for CCSD(T) results in that section (III.2.D).…”