Reactions of Atomic Hydrogen with Formic Acid and Carbon Monoxide in Solid Parahydrogen I: Anomalous Effect of Temperature

Abstract

Low-temperature condensed phase reactions of atomic hydrogen with closed-shell molecules have been studied in rare gas matrices as a way to generate unstable chemical intermediates and to study tunneling-driven chemistry. Although parahydrogen (pH2) matrix isolation spectroscopy allows these reactions to be studied equally well, little is known about the analogous reactions conducted in a pH2 matrix host. In this study, we present Fourier transform infrared (FTIR) spectroscopic studies of the 193 nm photoinduced chemistry of formic acid (HCOOH) isolated in a pH2 matrix over the 1.7 to 4.3 K temperature range. Upon short-term irradiation the HCOOH readily undergoes photolysis to yield CO, CO2, HOCO, HCO and H atoms. Furthermore, after photolysis at 1.9 K tunneling reactions between migrating H atoms and trapped HCOOH and CO continue to produce HOCO and HCO, respectively. A series of postphotolysis kinetic experiments at 1.9 K with varying photolysis conditions and initial HCOOH concentrations show the growth of HOCO consistently follows single exponential (k = 4.9(7)x10(-3) min(-1)) growth kinetics. The HCO growth kinetics is more complex displaying single exponential growth under certain conditions, but also biexponential growth at elevated CO concentrations and longer photolysis exposures. By varying the temperature after photolysis, we show the H atom reaction kinetics qualitatively change at ∼2.7 K; the reaction that produces HOCO stops at higher temperatures and is only observed at low temperature. We rationalize these results using a kinetic mechanism that involves formation of an H···HCOOH prereactive complex. This study clearly identifies anomalous temperature effects in the reaction kinetics of H atoms with HCOOH and CO in solid pH2 that deserve further study and await full quantitative theoretical modeling.

Matrix-Isolation Studies on the Radiation-Induced Chemistry in H2O/CO2 Systems: Reactions of Oxygen Atoms and Formation of HOCO Radical

“…50,78,79 In particular, the formation of carboxyl radical by local reaction of hot OH radicals with CO molecules was observed in argon matrices by Mielke et al 79 Very recently, Paulson et al reported spectroscopic characterization of this radical resulting from reaction of hydrogen atoms with formic acid in a solid para-hydrogen matrix. 80 On the other hand, such a reaction was not found in a solid krypton matrix. 81 Meanwhile, here we present the first observation of formation of the HOCO radical as a result of radiation-induced evolution of the CO 2 −H 2 O system in condensed media.…”

Hydrogen Abstraction of Acetic Acid by Hydrogen Atom to Form Carboxymethyl Radical •CH2C(O)OH in Solid para-Hydrogen and Its Implication in Astrochemistry

“…Similar H atom tunneling reactions have been observed for the reactions of H atoms with HONO (∼21 kJ mol –1 ), CH 3 OH (∼35 kJ mol –1 ), , H 2 NC­(O)H (∼26 kJ mol –1 ), C 5 H 5 N (∼8 kJ mol –1 ), HC­(O)­OCH 3 (∼41 and 46 kJ mol –1 for the formation of HC­(O)­CH 2 and C­(O)­OCH 3 radicals, respectively), and CH 3 CONH 2 (∼41 kJ mol –1 ); predicted barriers are provided in parentheses. Paulson et al photolyzed HC­(O)­OH in solid p -H 2 at 1.7 K with laser light at 193 nm and observed tunneling reaction H + HC­(O)­OH because some H atoms were produced from photolysis; the reaction proceeded even after photolysis was terminated, resulting in the formation of HOCO from abstraction of the hydrogen on the carbon atom rather than the hydroxyl hydrogen. Similarly, in this work the H + CH 3 C­(O)­OH reaction proceeds through H-abstraction of the alkyl (CH 3 ) group that leads to the formation of the •CH 2 C­(O)­OH radical.…”

Quantum Diffusion-Controlled Chemistry: Reactions of Atomic Hydrogen with Nitric Oxide in Solid Parahydrogen

“…Therefore, we decided to reinvestigate this system at two different temperatures, 1.8 and 4.3 K, respectively, to measure the effect of temperature on the reaction rate. Another reason to revisit this system is the anomalous results we have recently measured − for other H atom reactions in solid pH 2 . For example, in the case of the H + N 2 O → cis -HNNO reaction we observed a very strange temperature dependence .…”