Hydrogen/deuterium exchange in interstellar ice analogs

Abstract

Context. For several reasons, methanol is believed to be formed on grain surfaces and, in warm environments, released in the gas phase. In the past, multiply deuterated isotopologues of methanol have been detected in gas phase around several low-mass protostars. In all these sources, there is significantly more CH 2 DOH than CH 3 OD. Various hypotheses have been suggested to explain this anomaly, but none is fully convincing. Aims. In this work, we test a new hypothesis experimentally: the spontaneous exchange between hydrogen and deuterium atoms in water ice as responsible for the deficiency of CH 3 OD with respect to CH 2 DOH. Methods. We follow the temperature dependence of the composition of interstellar ice analogs initially composed of CD 3 OD and H 2 O. To this aim, thin films of intimate H 2 O:CD 3 OD ice mixtures, condensed at low temperature (<110 K), are monitored by Fourier transform infrared (FTIR) spectroscopy up to the complete evaporation of CD 3 OD (∼170 K). Results. Rapid hydrogen/deuterium (H/D) exchange is observed, at 120 K and above, through the growth of the ν OD stretching mode of HDO at ∼2425 cm −1 . It is also shown that H/D exchange occurs i) on the hydroxyl functional group of methanol, i.e through hydrogen bonds, and ii) before the completion of crystallization.Conclusions. The present results suggest that the much lower abundance of CH 3 OD compared to CH 2 DOH in low-mass protostars could reflect H/D exchanges in water ice either prior to or definitely during the grain mantle sublimation. This solid-state depletion mechanism, so far neglected in the astronomical literature, might affect other deuterated molecules with hydrogen bonds.

The puzzling deuteration of methanol in low- to high-mass protostars

“…Thus, in the experiment of Nagaoka et al (2005), the deuterium enrichment was found to proceed via H/D substitution in solid CH 3 OH leading to the formation of methyldeuterated methanol isotopologues, but no CH 3 OD. An alternative solid-state depletion mechanism was proposed by Ratajczak et al (2009): in their experiment, rapid H/D exchanges between CD 3 OD and H 2 O ices were found to occur at T ∼ 120 K on the hydroxyl (-OD) functional group of methanol, resulting in CD 3 OH and HDO molecules.…”

“…To make the situation even more puzzling, the same [CH 2 DOH]/[CH 3 OD] ratio is close to 1 in the high-mass protostar Orion IRc2 (Jacq et al 1993). This difference between high-and low-mass protostars A&A 528, L13 (2011) might reflect different deuteration degrees and/or ice compositions, as suggested by Ratajczak et al (2009 …”

“…Deuteration is expected to be very different in the two cases and, specifically, Table A.1 is only available in electronic form at http://www.aanda.org much higher in the first case. Secondly, the alteration of the deuteration degree can occur on the grain surfaces via H/D atom exchanges both because of thermal activation (Ratajczak et al 2009) or, again, photolysis (Weber et al 2009). In addition, when solid methanol sublimates, it can undergo chemical reactions in the gas phase, which may again alter the original deuteration degree set on the grain surfaces (Charnley et al 1997;Osamura et al 2004).…”

The extended spectroscopic database for deuterated species of formamide up to 1 THz

“…However, this ratio has been observed to lie in the range 5 to 30 towards low-mass and intermediate protostars (Parise et al 2006;Ratajczak et al 2011) and close to or below one in high-mass protostars (Ratajczak et al 2011). Recent laboratory studies (Nagaoka et al 2005;Ratajczak et al 2009) have shown that a much lower abundance of CH 3 OD than for CH 2 DOH, as observed in low-and intermediate-mass protostars (Ratajczak et al 2011), can be qualitatively explained via H/D exchanges occurring on the grain surface following methanol formation. The results of Ratajczak et al (2009), in particular, suggest that such exchanges occur on the hydroxyl (-OD) or amine (-ND) functional groups, via hydrogen bonds with water ice.…”

“…Recent laboratory studies (Nagaoka et al 2005;Ratajczak et al 2009) have shown that a much lower abundance of CH 3 OD than for CH 2 DOH, as observed in low-and intermediate-mass protostars (Ratajczak et al 2011), can be qualitatively explained via H/D exchanges occurring on the grain surface following methanol formation. The results of Ratajczak et al (2009), in particular, suggest that such exchanges occur on the hydroxyl (-OD) or amine (-ND) functional groups, via hydrogen bonds with water ice. As a result, in the case of formamide, DCONH 2 species should be significantly more abundant than HCONDH and HCONHD ones, if solid H/D exchanges do happen, because these should take place on the amino group -ND 2 .…”