Electron impact dissociation of amorphous cis-methyl acetate ice analogs

Surface science studies of the coverage dependent adsorption of methyl acetate and methyl propanoate on graphite

Complex organic molecules (COMs) have been detected in a wide range of astrophysical environments, including the interstellar medium, comets and proto-planetary disks. The icy mantles that form on dust grains in these environments are thought to be the chemical nurseries that allow the formation of many of the COMs that have been identified. As such, the adsorption, thermal processing and desorption of COMs from dust grain surfaces are important in understanding the astrochemical networks as a whole. To study these processes, surface science techniques (temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS)) have been used to investigate ices of the simple esters, methyl acetate and methyl propanoate, adsorbed on a graphitic dust grain analogue surface (highly oriented pyrolytic graphite, HOPG) at 28 K. From the TPD experiments, kinetic parameters have been determined for the desorption of the esters from graphite. The data show a clear coverage dependence for the desorption energies and pre-exponential factors in the sub-monolayer regime. For methyl acetate, the desorption energies and pre-exponential factors range from 57.1 ± 0.4 to 47.2 ± 0.3 kJ mol-1 and 3.1 × 1019±0.2 to 1.6 × 1019±0.1 s-1 respectively. For methyl propanoate the same parameters range from 57.0 ± 0.1 to 51.0 ± 0.1 kJ mol-1 and 7.7 × 1019±0.1 to 4.4 × 1019±0.1 s-1. As expected, neither ester shows coverage dependent values for multilayer ices. The determined desorption energies and pre-exponential values for the multilayer ices are 43.5 ± 0.9 kJ mol-1 and 4.2 × 1032±0.4 molecules cm-2 s-1 for methyl acetate and 45.7 ± 0.9 kJ mol- 1 and 8.7 × 1029±0.4 molecules cm-2 s-1 for methyl propanoate. Experimental RAIRS data were also recorded, showing that the ices undergo an irreversible phase change from an amorphous to a crystalline structure when thermally processed. This study provides fundamental data for use in astrochemical models as well as the basis for a future investigation of methyl acetate and methyl propanoate adsorbed in mixed ice environments with water ice.

Mechanistical study on the formation of hydroxyacetone (CH3COCH2OH), methyl acetate (CH3COOCH3), and 3-hydroxypropanal (HCOCH2CH2OH) along with their enol tautomers (prop-1-ene-1,2-diol (CH3C(OH)CHOH), prop-2-ene-1,2-diol (CH2C(OH)CH2OH), 1-methoxyethen-1-ol (CH3OC(OH)CH2) and prop-1-ene-1,3-diol (HOCH2CHCHOH)) in interstellar ice analogs

We unravel, for the very first time, the formation pathways of hydroxyacetone (CH₃COCH₂OH), methyl acetate (CH₃COOCH₃), and 3-hydroxypropanal (HCOCH₂CH₂OH), as well as their enol tautomers within mixed ices of methanol (CH₃OH) and acetaldehyde (CH₃CHO) analogous to interstellar ices in the ISM exposed to ionizing radiation at ultralow temperatures of 5 K. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labeled ices, the reaction products were selectively photoionized allowing for isomer discrimination during the temperature-programmed desorption phase. Based on the distinct mass-to-charge ratios and ionization energies of the identified species, we reveal the formation pathways of hydroxyacetone (CH₃COCH₂OH), methyl acetate (CH₃COOCH₃), and 3-hydroxypropanal (HCOCH₂CH₂OH) via radical-radical recombination reactions and of their enol tautomers (prop-1-ene-1,2-diol (CH₃C(OH)CHOH), prop-2-ene-1,2-diol (CH₂C(OH)CH₂OH), 1-methoxyethen-1-ol (CH₃OC(OH)CH₂) and prop-1-ene-1,3-diol (HOCH₂CHCHOH)) via keto-enol tautomerization. To the best of our knowledge, 1-methoxyethen-1-ol (CH₃OC(OH)CH₂) and prop-1-ene-1,3-diol (HOCH₂CHCHOH) are experimentally identified for the first time. Our findings help to constrain the formation mechanism of hydroxyacetone and methyl acetate detected within star-forming regions and suggest that the hitherto astronomically unobserved isomer 3-hydroxypropanal and its enol tautomers represent promising candidates for future astronomical searches. These enol tautomers may contribute to the molecular synthesis of biologically relevant molecules in deep space due to their nucleophilic character and high reactivity.

Infrared intensities of methyl acetate, an interstellar compound - comparisons of three organic esters

The mid-infrared (IR) spectra of the simplest aliphatic esters have been studied in the past in the solid, liquid, and gas phases with an emphasis on vibrational frequencies and peak assignments. However, relatively little has been published on the IR intensities of the amorphous forms of these compounds. These IR intensities are of particular interest to the astrochemical community as they are needed to help quantify laboratory measurements of the formation and destruction of extraterrestrial molecules, including esters. Here we report and compare IR intensities of three organic esters: methyl formate, methyl acetate, and methyl propionate, all studied with the same equipment and procedures. Of these three esters, our main interest is with methyl acetate, for which little quantitative IR work is available. For each ester, we report apparent absorption coefficients and band strengths, and compare them to earlier work. We also have calculated the first IR optical constants for both amorphous and crystalline methyl acetate. We use our new results to measure vapor pressures and a sublimation enthalpy for methyl acetate and to comment on a radiation-chemical experiment with methyl acetate and how it can be better quantified.

Radiation chemistry of solid acetone in the interstellar medium – a new dimension to an old problem

A laboratory investigation of acetone, an interstellar and cometary molecule, has produced new results concerning its decomposition in a radiation environment.