Kinetic studies of the infrared-induced reaction between atomic chlorine and solid parahydrogen

• Published in: Journal of molecular spectroscopy 2015-04, Vol.310, p.72-83
• Main Authors: Raston, Paul L., Kettwich, Sharon C., Anderson, David T.
• Format: Article
• Language: English
• Subjects: Cooperative transitions; Double transitions; Fourier-transform infrared spectroscopy; Matrix isolation spectroscopy; Para-hydrogen; Radicals; Reaction kinetics
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/j.jms.2015.01.012

[Display omitted] •Monitor the kinetics of both the Cl reagent and the HCl product.•IR-induced Cl+H2 reaction mechanism depends on spectral profile of IR radiation.•Can distinguish between two proposed mechanisms using measured kinetics.•The Cl spin–orbit+Q1(0) H2 cooperative transition has a high efficiency to induce reaction.•Presence of Br atoms effectively quench Cl+H2(ν=1) reaction. We present Fourier-transform infrared (FTIR) spectroscopic studies of the IR-induced Cl+H2(v=1)→HCl+H reaction in a parahydrogen (pH2) matrix aimed at distinguishing between two proposed reactions mechanisms; direct-IR and vibron-mediated. The Cl atom reactants are produced via 355nm in situ photolysis of a Cl2 doped pH2 matrix. After photolysis is complete, a long-pass IR filter in the FTIR beam is removed and we measure the ensuing IR-induced reaction kinetics using rapid scan FTIR spectroscopy. We follow both the decay of the Cl atom reactant and growth of the HCl product using the Cl spin–orbit (SO)+Q1(0) and HCl R1(0) transitions, respectively. We show the IR-induced reaction mechanism depends on the spectral profile of the IR radiation; for IR spectral profiles that have significant IR intensities between 4000 and 5000cm−1 we observe first-order kinetics that are assigned to a vibron-mediated mechanism and for spectral profiles that have significant IR intensities that include the Cl SO+Q1(0) transition near 5094cm−1 we observe bi-exponential kinetics that are dominated by the direct-IR mechanism at early reaction times. We can distinguish between the two mechanisms using the observed kinetics. We investigate the reaction kinetics for different FTIR optical setups, for a range of sample conditions, and start and stop the IR-induced reaction to investigate the importance of secondary H atom reactions. We also study the IR-induced reaction in Br/Cl co-doped pH2 samples and show the presence of the Br atom quenches the vibron-mediated reaction kinetics presumably because the Br-atoms serve as efficient vibron traps. This paper indicates that in a highly enriched pH2 matrix the H atoms that are produced by the IR-induced Cl atom reaction likely do not play a significant role in the measured reaction kinetics which implies these secondary H atom reactions are highly selective.