Effective Rate Constants for the Surface Reaction between Solid Methanol and Deuterium Atoms at 10 K

Abstract

The surface reactions of CH3OH, CH2DOH, and CHD2OH with cold D atoms at 10 K were investigated using an atomic beam source and FTIR. Methyl-deuterated isotopologues CH2DOH, CHD2OH, and CD3OH were produced by exposure of amorphous solid CH3OH to D atoms at 10 K, and the pseudo-first-order rates for the reactions CH3OH + D --> CH2OH + HD, CH2DOH + D --> CHDOH + HD, and CHD2OH + D --> CD2OH + HD were estimated. The ratios of the reaction rates of the second and third reactions to the first reaction were 0.69 +/- 0.11 and 0.52 +/- 0.14, respectively. The difference in reaction rates is thought to be due to a secondary kinetic isotope effect on the H-abstraction reaction from the methyl side by D atoms.

Laboratory Simulation of Competition between Hydrogenation and Photolysis in the Chemical Evolution of H2O‐CO Ice Mixtures

“…The apparatus used in the present experiments was newly constructed, with a design similar to ASURA (Nagaoka et al 2005(Nagaoka et al , 2007Watanabe et al 2006) and LASSIE . Briefly, the apparatus consists of an atomic source and a main chamber.…”

“…In this experiment the temperature of the aluminum pipe is set to 100 K, and measured by an Au/Fe 0.07% chromel thermocouple clamped at the most downstream end of the aluminum pipe itself. The recombination of H atoms is suppressed on the aluminum surface at a low temperature (Koch & Steffens 1999) and, colliding many times with the inner walls of the pipe, they are finally thermalized at the pipe temperature (Nagaoka et al 2007). This simple arrangement can produce low-temperature atomic hydrogen (Walraven & Silvera 1982;Perry et al 2002).…”

WATER FORMATION THROUGH A QUANTUM TUNNELING SURFACE REACTION, OH + H2, AT 10 K

“…Previous studies have investigated the isotope effect on other astrophysically relevant chemical reactions around 10 K. Hidaka et al (2007) performed hydrogenation/deuteration of solid CO at 10-20 K and found that hydrogenation of CO proceeds faster than deuteration of CO by a factor of about 12. H-D isotope exchange reactions of methanol (CH 3 OH) were also demonstrated to occur through quantum tunneling (Nagaoka et al 2007), which would contribute to the observed abundance of deuterated methanol, such as CHD 2 OH.…”

Hydrogen isotope exchanges between water and methanol in interstellar ices

“…Among possible processes on the grain surfaces, the most efficient mechanism in molecular clouds, seems to be the stepwise hydrogenation of solid CO by the highly mobile H atoms (see Watanabe et al 2007, and references therein). The deuteration of methanol can also proceed through the addition of atomic deuterium on CO, or alternatively via H/D substitution reactions on CH 3 OH, as demonstrated experimentally by Nagaoka et al (2005Nagaoka et al ( , 2007. Observationally, the high deuterium enhancements observed for CH 2 DOH, CHD 2 OH, and CD 3 OH are consistent with the statistical addition of H and D atoms to CO, provided that the atomic [D]/[H] ratio in the gas-phase is ∼0.1-0.2 during methanol formation (Parise et al 2006).…”

“…Second, laboratory experiments have shown that H/D exchange reactions in solid state can produce a low CH 3 OD abundance. Thus, in the experiments of Nagaoka et al (2005Nagaoka et al ( , 2007, H/D substitution reactions with atomic deuterium were shown to lead to the formation of methyl-deuterated methanol isotopologues, but no hydroxyl-deuterated isotopologues. Thermally activated H/D exchanges between water and methanol were also studied by Souda et al (2003), using secondary ion mass spectrometry, and by Ratajczak et al (2009), using Fourier-transform infrared (FTIR) spectroscopy.…”