Diffusion of molecules in the bulk of a low density amorphous ice from molecular dynamics simulations.

The diffusion of molecules in interstellar ice is a fundamental phenomenon to take into account while studying the formation of complex molecules in this ice. This work presents a theoretical study on the diffusion of H2O, NH3, CO2, CO, and H2CO in the bulk of a low density amorphous (LDA) ice, while taking into account the physical conditions prevailing in space, i.e. temperatures below 150 K and extremely low pressure. This study was undertaken by means of molecular dynamics simulations. For CO2 for which no experimental data were available we conducted our own experiments. From our calculations we show that, at low temperatures, the diffusion of molecules in the bulk of a LDA ice is driven by the self-diffusion of water molecules in the ice. With this study we demonstrate that molecular dynamics allows the calculation of diffusion coefficients for small molecules in LDA ice that are convincingly comparable to experimentally measured diffusion coefficients. We also provide diffusion coefficients for a series of molecules of astrochemical interest.

Formaldehyde chemistry in cometary ices: the case of HOCH2OH formation.

Laboratory experiments devoted to simulate the chemistry occurring in interstellar and cometary ice analogues are of paramount importance to understand the formation of complex organic molecules that are detected throughout the universe. These laboratory simulations provide relevant hints on the fundamental physical and chemical steps associated with the increase of the molecular complexity in space and, moreover, give benchmark results for dedicated space missions. In the present work, we study the thermally promoted reactivity of H2O-dominated and D2O-dominated cometary ice analogues that contain various amounts of H2CO and NH3 by means of Fourier-transform infrared spectroscopy (FTIR), mass spectrometry and DFT calculations. Experimental measurements show that methyleneglycol (HOCH2OH) and D2-methyleneglycol (DOCH2OD, the corresponding isotopologue) are formed from the H2O- and D2O-dominated ices, respectively, only if ammonia is present. We also reported for the first time the mass spectrum of methyleneglycol and D2-methyleneglycol. B3LYP calculations have also been used to characterize the potential energy surface of the mechanistic steps associated with the formation of HOCH2OH as well as to simulate the IR spectrum of this compound. The fruitful interplay between theory and experiment has allowed us to elucidate the exact role of ammonia during the warming, which essentially stands for the formation and stabilization of the NH4(+)/OH(-) ion pair, thus enabling the OH(-) species to react with formaldehyde. The present results reproduce the heating of circumstellar ices in star formation regions and can be applied to the late thermal evolution of comets. In addition, the mass spectrum of methyleneglycol represents a benchmark for the analysis of the data coming from the ROSINA on-board instrument of the Rosetta mission.

Kinetics of the NH3 and CO2 solid-state reaction at low temperature.

Ammonia and carbon dioxide play an important role in both atmospheric and interstellar ice chemistries. This work presents a theoretical and experimental study of the kinetics of the low-temperature NH3 and CO2 solid-state reaction in ice films, the product of which is ammonium carbamate (NH4(+)NH2COO(-)). It is a first-order reaction with respect to CO2, with a temperature-dependent rate constant fitted to the Arrhenius law in the temperature range 70 K to 90 K, with an activation energy of 5.1 ± 1.6 kJ mol(-1) and a pre-exponential factor of 0.09-0.08(+1.1) s(-1). This work helps to determine the rate of removal of CO2 and NH3, via their conversion into ammonium carbamate, from atmospheric and interstellar ices. We also measure first-order desorption energies of 69.0 ± 0.2 kJ mol(-1) and 76.1 ± 0.1 kJ mol(-1), assuming a pre-exponential factor of 10(13) s(-1), for ammonium carbamate and carbamic acid, respectively.

Acetaldehyde solid state reactivity at low temperature: formation of the acetaldehyde ammonia trimer.

We focus on low temperature reactivity from 25 to 300 K, in ice containing acetaldehyde, ammonia, and formic acid. We show that the warming of this ice mixture forms the acetaldehyde ammonia trimer (2,4,6-trimethyl-1,3,5-hexahydrotriazine, C(6)H(15)N(3)) after five steps. The reaction is monitored by FTIR spectroscopy and mass spectrometry. We propose a mechanism for its formation that differs from the one proposed in the liquid phase. The reaction intermediates, α-aminoethanol (from 80 K) and ethanimine (formed at 180 K), have been identified by a mechanistic approach: each step of the reaction has been treated separately. The chemical implications and the astrophysical relevance of the study are also discussed.

Chemistry induced by low-energy electrons in condensed multilayers of ammonia and carbon dioxide.

We have investigated by means of HREEL spectroscopy electron induced reactivity in a binary CO2 : NH3 ice mixture. It was shown that the interaction of low energy electrons (9-20 eV) with such mixtures induces the synthesis of neutral carbamic acid NH2COOH and that flashing the sample at 140 K induces the formation of ammonium carbamate. The products have been assigned by FTIR spectroscopy of a CO2 : NH3 mixture heated from 10 K to 240 K. A mechanism involving dissociation of NH3 molecules into NH2* and H* radicals is proposed to explain the product formation.

UV and IR photoisomerization of acetylacetone trapped in a nitrogen matrix.

UV- and IR-induced photoisomerization of acetylacetone trapped in a nitrogen matrix at 4.3 K have been carried out using a tunable optical parametric oscillator type laser, or a mercury vapor lamp, coupled with Fourier Transform IR and UV spectroscopies. After deposition, the main form present in the cryogenic matrix is that chelated (enol). Upon UV irradiation, the intramolecular H bond is broken leading to nonchelated isomers among seven possible open forms. These forms have then been irradiated by resonant pi* <-- pi UV irradiation, or by resonant nuOH irradiation. The selective UV irradiation allows us to suggest a first vibrational assignment while the nuOH irradiation leads us to observe interconversions between the nonchelated isomers. In order to support our vibrational assignment, we have carried out theoretical calculations at the B3LYP/6-311++G(2d,2p) level of theory. This study shows that only five isomers are observed among eight postulated.