Mechanisms for the Breakdown of Halomethanes through Reactions with Ground‐State Cyano Radicals

Abstract

ARTICLE Mechanisms for Breakdown of Halomethanes through Reactions with Ground State Cyano RadicalsPooria Farahani, [a] Satoshi Maeda, [b] Joseph S. Francisco, [c] and Marcus Lundberg* , [a] Abstract: One route to break down halomethanes is through reactions with radical species. The capability of the Artificial Force Induced Reaction (AFIR) algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes CX3Y (X= H,F; Y=Cl,Br) and ground state ( 2 Σ + ) cyano radicals (CN). For CH3Cl + CN 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (10 8 s -1 M -1 at 298 K) is obtained for hydrogen abstraction. In CH3Br the rate constants for hydrogen and halogen abstraction are similar (10 9 s -1 M -1 ) while replacing hydrogen with fluorine eliminates the hydrogen abstraction route and decreases the rate constants for halogen abstraction by 2-3 orders of magnitude. The detailed mapping of stationary points opens up for accurate calculations of product distributions and the encouraging rate constants motivate future studies with other radicals.

Artificial Force Induced Reaction (AFIR) Method for Exploring Quantum Chemical Potential Energy Surfaces

“…The AFIR has been applied most extensively to organo and organometallic catalysis, in combination with quantum chemical calculations 52–64. The other applications are: gas‐phase reactions;65–67 enzyme catalysis;68 domino reactions;69 and metal‐cluster catalysis 70,71. It has also been applied to electronic excited states and geometries of the seam of crossing between two electronic states 72–75.…”

Reaction kinetics of the CN radical with methyl bromide

“…The primary experimental result in this study is the finding that hydrogen abstraction to form HCN + CH 2 Br, channel (1a), is only a minor product channel, with an estimated branching fraction of 0.12±0.02. It is useful to compare this result with recent ab initio calculations [33]. In It is also useful to compare our results to previous results for the CN + CH 3 Cl reaction [32].…”

“…This molecule is a trace component of the atmosphere, partly due to recent use as a fumigant, and plays a role in stratospheric ozone depletion and is a greenhouse gas. There is, however, a recent ab initio study of reactions of CN with several halomethanes, including CH 3 Cl and CH 3 Br [33]. In that study, reaction pathways were calculated at the G4 level of theory, using B3LYP/GTBas3 and/or QCISD/GTBas3 optimized structures.…”

“…The lowest energy pathway for the CN + CH 3 Cl reaction was the expected hydrogen abstraction to produce HCN + CH 2 Cl; this prediction is consistent with the experimental observation of a positive activation energy [32], typically observed for hydrogen abstraction reactions. For CN + CH 3 Br, however, the two reaction pathways were calculated to have nearly identical barrier heights: hydrogen abstraction (4.9 kcal/mole standard Gibbs energy of activation) to produce HCN + CH 2 Br, and attack at the bromine atom (5.0 kcal/mole standard Gibbs energy of activation) to produce BrCN + CH 3 , CH 2 CN + HBr, or CH 3 CN + Br [33]. This calculation therefore suggests the possibility that HCN is not necessarily the major product of this reaction.…”

“…The primary goal of the experimental study described here is to test that prediction. We report measurements of total rate constants and quantitative HCN product yields: The thermodynamic values are obtained from the ab initio study [33]; none was reported for channel (1d).…”