F/Cl+C2H2 reactions: Are the addition and hydrogen abstraction direct processes?

Initiation and Carbene Induced Radical Chain Reactions in CH2F2 Pyrolysis

High temperature dissociations of organic molecules typically involve a competition between radical and molecular processes. In this work, we use a modeling, experiment, theory (MET) framework to characterize the high temperature thermal dissociation of CH2F2, a flammable hydrofluorocarbon (HFC) that finds widespread use as a refrigerant. Initiation in CH2F2 proceeds via a molecular elimination channel; CH2F2→CHF+HF. Here we show that the subsequent self-reactions of the singlet carbene, CHF, are fast multichannel processes and a facile source of radicals that initiate rapid chain propagation reactions. These have a marked influence on the decomposition kinetics of CH2F2. The inclusion of these reactions brings the simulations into better agreement with the present and literature experiments. Additionally, flame simulations indicate that inclusion of the CHF+CHF multichannel reaction leads to a noticeable enhancement in predictions of laminar flame speeds, a key parameter that is used to determine the flammability of a refrigerant.

Association of Cl with C2H2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C₂H₂→C₂H₂Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C₂H₂Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C-H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well. We use these results to calculate the standard-state free energy and equilibrium constant of the reaction.

Matrix-isolation and computational study of H2CCCl and H2CCBr radicals

We report on two new radicals, H2CCCl and H2CCBr, prepared in low-temperature noble-gas matrices and characterized using infrared spectroscopy. These radicals are made by UV photolysis of HCCCl and HCCBr and subsequent thermal annealing to mobilize hydrogen atoms in the matrices and promote their reaction with the residual precursor molecules. Three characteristic infrared bands are observed for each radical. The assignments are supported by quantum chemical calculations at the B3LYP and CCSD(T) levels of theory with the def2-TZVPPD basis set.

Theoretical study of HCN(+) + C2H2 reaction

A detailed theoretical investigation for the ion-molecule reaction of HCN (+) with C 2H 2 is performed at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. Possible energetically allowed reaction pathways leading to various low-lying dissociation products are probed. It is shown that eight dissociation products P 1 (H 2C 3N (+)+H), P 2 (CN+C 2H 3 (+)), P 3 (HC 3N (+)+H 2), P 4 (HCCCNH (+)+H), P 5 (H 2NCCC (+)+H), P 6 (HCNCCH (+)+H), P 7 (C 2H 2 (+)+HCN), and P 8 (C 2H 2 (+)+HNC) are both thermodynamically and kinetically accessible. Among the eight dissociation products, P 1 is the most abundant product. P 7 and P 3 are the second and third feasible products but much less competitive than P 1 , followed by the almost negligible product P 2 . Other products, P 4 (HCCCNH (+)+H), P 5 (HCNCCH (+)+H), P 6 (H 2NCCC (+)+H), and P 8 (C 2H 2 (+)+HNC) may become feasible at high temperatures. Because the intermediates and transition states involved in the reaction HCN (+) + C 2H 2 are all lower than the reactant in energy, the title reaction is expected to be rapid, as is consistent with the measured large rate constant at room temperature. The present calculation results may provide a useful guide for understanding the mechanism of HCN (+) toward other pi-bonded molecules.