Competing Molecular and Radical Pathways in the Dissociation of Halons via Direct Chemical Dynamics Simulations

Abstract

A great deal of attention has been given to the decomposition chemistry of halons (halomethanes) due to their role in stratospheric ozone depletion. Knowledge of certain aspects of dissociation of halons such as the competition between radical and molecular pathways and their mechanistic details is limited. Halon molecules can isomerize to an iso form containing a halogen–halogen bond and such iso-halon forms have been identified as intermediates in condensed phase chemistry. Recently, a quantum chemistry study of role of iso-halons in the gas phase decomposition of halomethanes has been reported. In the present work, we have investigated the ground state dissociation chemistry of select halon molecules - CF2Cl2, CF2Br2, CHBr3, and CH2BrCl using electronic structure theory calculations and direct chemical dynamics simulations. Classical trajectories were generated on-the-fly using density functional PBE0/6-31G* level of theory at a fixed total energy. Simulation results showed that molecular products, in general, were dominant for all the four molecules at the chosen energy. A variety of mechanisms such as direct dissociation via multicenter transition states, decomposition via isomerization, radical recombinations, and roaming pathways contributed to the formation of molecular products. Atomic level mechanisms are presented, and the role of iso-halons in the gas phase chemistry of halomethanes is clearly established.

The fire-extinguishing performance and mechanism of fluorinated cyclobutane through experimental measurement and numerical calculation

“…According to studies of halon 1,2,4,23 and HFCs, 6,7,13,24 in terms of environmental protection, the molecular structure of agents must not be too stable to be degraded in the atmospheric environment, as is the case with high ALT and GWP values. At the same time, it is best to avoid Br and Cl atoms in the molecular structure to prevent severe ozone destruction.…”

VUV Photofragmentation of Chloroiodomethane: The Iso-CH2I–Cl and Iso-CH2Cl–I Radical Cation Formation

“…It is therefore fundamental to understand the link between ion and neutral chemistry. As for the halo-compounds and halomethanes, dihalomethanes XCH 2 X or XCH 2 Y (where X and Y= I, Br, Cl and F) have been largely studied, − because for example the daytime destruction of O 3 over coastal areas is mainly due to I atoms, while Br and Cl atoms are mainly involved in the reaction with VOCs . The iodine-containing dihalomethanes are mainly emitted by oceans and consist of volatile organic iodine compounds (VOICs): CH 2 I 2 , ClCH 2 I and BrCH 2 I, CH 3 I with lifetimes in the range from minutes (CH 2 I 2 ) to days (CH 3 I).…”