Photochemistry of Hydrochlorofluorocarbons

The photochemistry of an HCFC is explained via ultrafast dynamics simulations.

Since the discovery of the harmful effect of chlorofluorocarbons (CFCs) to the ozone layer and global warming, these compounds have been prohibited by the Montreal Protocol, leading to the usage of hydrochlorofluorocarbons (HCFCs) as substitutes. Not the best solution though, as now we know that HCFCs are important greenhouse gases. For economic reasons, they will still be around until 2020.

Gessenildo Rodrigues (a grad student of the Brazilian program Science Without Borders), Elizete Ventura, Silmar do Monte (colleagues from the Federal University of Paraiba, Brazil), and I have been working on simulations of a series of such HCFC molecules.

The results of our first joint project have just appeared in the J. Phys. Chem. A. It is the investigation of the photochemistry of C₂H₂F₃Cl also known as HCFC-133a.

After exciting this molecule at the far-UV (10 eV), we observed that, although the excited state relaxation is ultrafast (137 fs), it occurs through several different reaction pathways and each of these pathways lead to different products. These products include atomic elimination (Cl, F, or H), multifragmentation mechanisms (Cl+F, Cl+H, or F+H), and CC bond-fission mechanisms (alone or with Cl or H elimination).

Many states, pathways, and products characterize the photochemistry of HCFC-133a. Dynamics simulations have helped to understand how this complex excited state relaxation takes place. (Click at the figure to enlarge.)

An example of simulated trajectory featuring Cl and F dissociation is shown in the movie below. This simulation was done with surface hopping using TDDFT excited states.

• G. P. Rodrigues E. Ventura, S. do Monte, M. Barbatti, Photochemical Deactivation Process of HCFC-133a (C₂H₂F₃Cl): A Nonadiabatic Dynamics Study; J. Phys. Chem. A, doi:10.1021/jp507681g (2014). doi:10.1021/jp507681g