Formation of Thioformic Acid (HCOSH)─The Simplest Thioacid─in Interstellar Ice Analogues

Abstract

Since the observation of the first sulfur-containing molecule, carbon monosulfide (CS), in the interstellar medium (ISM) half a century ago, sulfur-bearing species have attracted great attention from the astrochemistry, astrobiology, and planetary geology communities. Nevertheless, it is still not clear in which forms most of the sulfur resides in molecular clouds, an unsolved problem referred to as "sulfur depletion". Reported herein is the formation of thioformic acid (HCOSH)�the simplest thioacid�in interstellar ice analogues containing carbon monoxide (CO) and hydrogen sulfide (H 2 S) at 5 K. Utilizing single photoionization reflectron timeof-flight mass spectrometry and isotopically labeled molecules, thioformic acid molecules were selectively photoionized in the temperature-programmed desorption phase. These studies unravel a key reaction pathway to thioformic acid, an organic molecule recently detected toward the giant molecular cloud G+0.693−0.027 and the hot core G31.41+0.31, thus shedding light on interstellar sulfur chemistry.

Implementation of rare isotopologues into machine learning of the chemical inventory of the solar-type protostellar source IRAS 16293-2422

“…Hydroxyacetone (CH 3 COCH 2 OH), a structural isomer of 3-hydroxypropanal, was detected with a column density of 1.2 × 10 16 cm −2 toward IRAS 16293B. 60 Experiments by Wang et al 61 showed that 3-hydroxypropanal can be formed in methanol-acetaldehyde ices irradiated with energetic electrons at 5 K. They concluded that this molecule can be produced in interstellar ices of star-forming regions that have high abundances of methanol and acetaldehyde (which is the case in IRAS 16293B). This molecule would then be desorbed into the gas phase as the protostar is heated.…”

Reaction mechanism – explored with the unified reaction valley approach

“…It is noteworthy that both formic (HC(O)OH) and thioformic acid (HC(O)SH) have recently attracted attention because they were discovered in interstellar space. [174][175][176] According to DLPNO-CCSD(T)/aug-cc-pVQZ calculations, (R1) proceeds via a four-center TS with an activation enthalpy of 34.2 kcal mol À1 (see Fig. 4).…”

Synthesis of interstellar propen-2-ol (CH3C(OH)CH2) – the simplest enol tautomer of a ketone

“…Since the infrared-active fundamentals of organics in complex mixtures often overlap with complex mixtures of organics formed in the processed ice, it is challenging to identify individual molecules using FTIR spectroscopy alone. [53][54][55] Therefore, an alternative technique, which has the ability to identify individual molecules isomerselectively, is required.…”

“…9,10 Laboratory simulation experiments revealed a facile synthesis of the acetaldehyde (CH 3 CHO) -vinyl alcohol (CH 2 CHOH, +41 kJ mol À1 ), 7 ketene (H 2 CCO) -ethynol (HCCOH, +140 kJ mol À1 ), 16 pyruvic acid (CH 3 COCOOH) -2-hydroxyacrylic acid (CH 2 C(OH)COOH, +28 kJ mol À1 ), 8 acetic acid (CH 3 COOH) -1,1-ethenediol (H 2 CC(OH) 2 , +114 kJ mol À1 ), 10 and glycolaldehyde (HCOCH 2 OH)-1,2-ethenediol (H(HO)CC(OH)H, +39 kJ mol À1 ) 9 pairs with relative energies of the enols provided in parentheses (Scheme 1), suggesting that enols may be ubiquitous in deep space and readily available for an abiotic synthesis of complex organic molecules in these extreme environments. [8][9][10]17 However, as of today, only vinyl alcohol (CH 2 CHOH) and 1,2-ethenediol (H 2 CC(OH) 2 ) have been identified in deep space. 15,18 The enol of the simplest ketone acetone (1, CH 3 COCH 3 )-propen-2-ol (2, CH 3 C(OH)CH 2 ) -has remained elusive in the interstellar medium, but its identification could provide fundamental constraints of the chemical and physical conditions of cold molecular clouds and star-forming regions as their descendants.…”