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.

Infrared Spectral Intensities of Amine Ices, Precursors to Amino Acids.

Here, we address the paucity of infrared (IR) spectral data needed to quantify low-temperature experiments with amine ices, such as the formation of amino acids, by reporting new IR results on solid phases of methylamine (CH3NH2) and ethylamine (CH3CH2NH2), precursors to glycine and alanine, respectively. Mid-IR band strengths and absorption coefficients for CH3NH2 and CH3CH2NH2, in both amorphous and crystalline forms, are presented, along with measurements of a density and refractive index (670 nm) for each. For these same compounds, we also have calculated IR optical constants, and they are being made available in electronic form. Some applications of our new results are described along with proposals for future investigations. Suggestions are made related to the methods employed in such work, and particularly to the application of Beer's Law to the IR study of compounds of astrobiological interest. Comments are also included on the methods used, and the results presented in a recently published work on amino-acid IR intensities.

Radiolytic Destruction of Uracil in Interstellar and Solar System Ices.

Uracil is one of the four RNA nucleobases and a component of meteoritic organics. If delivered to the early Earth, uracil could have been involved in the origins of the first RNA-based life, and so this molecule could be a biomarker on other worlds. Therefore, it is important to understand uracil's survival to ionizing radiation in extraterrestrial environments. Here we present a study of the radiolytic destruction kinetics of uracil and mixtures of uracil diluted in H2O or CO2 ice. All samples were irradiated by protons with an energy of 0.9 MeV, and experiments were performed at 20 and 150 K to determine destruction rate constants at temperatures relevant to interstellar and Solar System environments. We show that uracil is destroyed much faster when H2O ice or CO2 ice is present than when these two ices are absent. Moreover, destruction is faster for CO2-dominated ices than for H2O-dominated ones and, to a lesser extent, at 150 K compared with 20 K. Extrapolation of our laboratory results to astronomical timescales shows that uracil will be preserved in ices with half-lives of up to ∼107 years on cold planetary bodies such as comets or Pluto. An important implication of our results is that for extraterrestrial environments, the application of laboratory data measured for the radiation-induced destruction of pure (neat) uracil samples can greatly underestimate the molecule's rate of destruction and significantly overestimate its lifetime, which can lead to errors of over 1000%.

Laboratory Studies of Astronomical Ices: Reaction Chemistry and Spectroscopy.

ConspectusScientists have had evidence for molecules in both comets and interstellar space since the 19th and early 20th centuries. Since then, extraterrestrial molecules ranging from simple diatomics to C70 to amino acids have been detected and identified through remote spectroscopy, spacecraft, and sample return missions. These achievements have been made through the efforts of astronomers and laboratory chemists collaborating to identify molecules in a myriad of exotic environments. It is now understood that even in the coldest depths of dense molecular clouds there is a wealth of chemistry to explore, much of it driven by exposure to radiation. As molecular clouds condense to protostellar disks and eventually form new planetary systems, chemical processes continue and evolve. An understanding of these processes is paramount for explaining the compositions of different bodies in our Solar System and may provide insight into the origins of life.In this Account, we describe the work of the Cosmic Ice Laboratory at NASA's Goddard Space Flight Center to characterize the composition of and understand the chemistry occurring in icy bodies in the Solar System and beyond. Our work has touched on a wide range of extraterrestrial environments, including icy interstellar grains, small bodies such as comets and asteroids, and planets and moons. We are especially interested in the chemical and physical changes that occur in ices as a result of thermal changes or exposure to radiation. To this end, we conduct experiments designed to simulate cold extraterrestrial environments and measure physical properties of single- and multicomponent ices. We expose ices to radiation (e.g., MeV protons or keV-MeV electrons) or high-energy (e.g., UV) photons to initiate physical and chemical changes. We conduct experiments using cryo-vacuum chambers equipped with analytical tools and radiation sources to make most of our measurements, including the collection of all spectroscopic data, in situ. When possible and appropriate, we also collect reaction products for further ex situ analysis. The work of the Cosmic Ice Lab provides critical data to astrochemists and others seeking to understand observations, make predictions, and plan future space missions.

The Radiation Stability of Thymine in Solid H2O.

Nucleobases are of significant importance to all known organisms, may be an important building block of life, and could be important biosignatures of current or past life. Given their potential significance to the field of astrobiology, it is important to understand the survival of these molecules when subjected to ionizing radiation as is present in a range of extraterrestrial environments. In this work, we present data on the kinetics of the radiolytic destruction of pure thymine and water + thymine ice mixtures at temperatures from 13 to 150 K. Rate constants were measured using in situ infrared spectroscopy, and radiolytic half-lives for thymine were computed for different planetary and interstellar environments. Our results demonstrate that the survival of thymine decreases as the dilution of thymine in water increases. Additionally, we find that thymine survival increases with ice temperature and that this decrease may be related to structure of the ice matrix.

Infrared band strengths and other properties of amorphous and crystalline dimethyl ether.

Laboratory astrochemists have generated infrared (IR) data for nearly all common classes of organic compounds, but ethers in the solid phase continue to be neglected despite detections of ethers in the interstellar medium by radio astronomers and uncertainty in how extraterrestrial ethers are formed. To address this paucity of data, here we present new mid-IR spectra of amorphous and crystalline dimethyl ether, (CH3)2O, the simplest member of its class. Spectral positions are tabulated and compared to previous results, but more importantly we also report IR band strengths and absorption coefficients, which we have not found in the literature and on which quantitative IR studies depend. Optical constants of amorphous and crystalline dimethyl ether also have been calculated. Some applications are described.

Mid-infrared spectra of dipropargyl ether ices revisited.

The infrared (IR) spectrum of dipropargyl ether, (HC≡C-CH2)2O, has been reinvestigated for the compound's liquid, amorphous, and crystalline forms. The IR baseline changes and bandshape distortions seen in literature spectra have been considerably reduced by a different choice of conditions for preparing the crystalline solid, leading to the discovery of two crystalline phases of the ether. A spectrum of the liquid phase has been recorded and compared to that of the amorphous ether to check for possible procedural artifacts. To facilitate cross-laboratory comparisons, estimates are made for absorption coefficients of three IR peaks of the amorphous solid's spectrum. An interpretation is discussed for changes reported in spectral baselines and bandshapes on warming amorphous dipropargyl ether, and tests and predictions are described. The suggestion that the results from dipropargyl ether warming experiments might pose problems in applying Beer's Law to astronomical observations is addressed.

Testing Densities and Refractive Indices of Extraterrestrial Ice Components Using Molecular Structures - Organic Compounds and Molar Refractions.

The use of infrared spectra to determine molecular abundances of icy astronomical objects and to study their chemistry requires laboratory measurements of reference spectra and related quantities, such as the index of refraction (n) and density (ρ) of candidate ices. Here we present new n and ρ measurements on ices involving over thirty C-, H-, and O-containing compounds, both acyclic and cyclic, representing seven chemical families. We examine the results in a way that is rare in the astrochemical literature, namely one in which data from an ice formed from molecules of a particular chemical family are compared to measurements on another member of the same family, such as of a homologous series or a pair of isomers. Apart from the intrinsic usefulness of the n and ρ data, a structure-based comparison can help establish trends and identify possibly spurious results. As liquid-phase data sometimes are used in low-temperature astrochemical work in the absence of solid-phase measurements, we compare our new ice results to those for the corresponding room-temperature liquids. We emphasize the use of our n and ρ data to compute the molar refraction (R M ) for each of our ices, and how the resulting R M values compare to those expected from molecular structures. The use of calculated RM values and measured n values to calculate ice densities, in the absence of direct measurements, also is addressed.

Quantifying acetaldehyde in astronomical ices and laboratory analogues: IR spectra, intensities, 13C shifts, and radiation chemistry.

Acetaldehyde is of interest to astrochemists for its relevance to both interstellar and cometary chemistry, but little infrared (IR) spectral data have been published for the solid phases of this compound. Here we present IR spectra of three forms of solid acetaldehyde, with spectra for one form being published for the first time. Direct measurements of band strengths and absorption coefficients also are reported for the first time for amorphous aldehyde, the form of greatest interest for astrochemical work. An acetaldehyde band strength at ~1350 cm-1 that has been used as a reference for about 20 years is seen to be in error by about 80% when compared to the direct measurements presented here. Spectra and peak positions also are presented for H13C(O)13CH3, and then used for the first identification of ketene as a radiation product of solid acetaldehyde.

Solid-State Isomerization and Infrared Band Strengths of Two Conformational Isomers of Cyclopropanecarboxaldehyde, A Candidate Interstellar Molecule.

At least a dozen of the known interstellar molecules possess a formyl group (HCO), suggesting that other such species exist and await discovery in the interstellar medium. Here we examine the mid-infrared (mid-IR) spectrum and selected physical properties of one such candidate, cyclopropanecarboxaldehyde, in amorphous ices. Mid-IR transmission spectra of solid cyclopropanecarboxaldehyde are presented for the first time and used to determine the cis-to-trans ratio of conformational isomers present in amorphous samples. The measured ratio is compared to one from an electron-diffraction study of the gas-phase compound. The cis-to-trans isomerization in the amorphous compound is followed and the activation energy is determined. The first IR band strengths for solid cyclopropanecarboxaldehyde are reported. Also presented are refractive indices and densities at 15 K for amorphous forms of two related compounds, cyclopropane and cyclopropanemethanol. Two low-temperature reactions for the interstellar formation of cyclopropanecarboxaldehyde are briefly described.

Infrared intensities and molar refraction of amorphous dimethyl carbonate - comparisons to four interstellar molecules.

The first measurements of infrared (IR) band intensities of solid dimethyl carbonate are presented along with measurements of this compound's refractive index and density near 15 K, neither of which has been reported. Molar refractions are used to compare these results to other new data from ices made of methyl acetate, acetone, acetic acid, and acetaldehyde, four molecules known to exist in the interstellar medium. Comparisons are made to IR intensities taken from the literature on amorphous ices. The value and importance of comparisons based on molecular structures, to predict and test laboratory results, are highlighted.

Infrared band strengths for amorphous and crystalline methyl propionate, a candidate interstellar molecule.

Mid-infrared spectra of amorphous and crystalline methyl propionate, CH3CH2COOCH3, are presented for the first time from a single laboratory, along with measurements of the refractive index of each solid form. Density estimates are made and IR band strengths and absorption coefficients are calculated. Vapor pressures of crystalline methyl propionate at 140-150 K are reported along with an enthalpy of sublimation. Spectroscopic results are compared to a recent study of this compound, and the phase of methyl propionate in that work is identified. Several applications are described.

IR spectra and properties of solid acetone, an interstellar and cometary molecule.

Mid-infrared spectra of amorphous and crystalline acetone are presented along with measurements of the refractive index and density for both forms of the compound. Infrared band strengths are reported for the first time for amorphous and crystalline acetone, along with IR optical constants. Vapor pressures and a sublimation enthalpy for crystalline acetone also are reported. Positions of 13C-labeled acetone are measured. Band strengths are compared to gas-phase values and to the results of a density-functional calculation. A 73% error in previous work is identified and corrected.

An IR investigation of solid amorphous ethanol - Spectra, properties, and phase changes.

Mid- and far-infrared spectra of condensed ethanol (CH3CH2OH) at 10-160K are presented, with a special focus on amorphous ethanol, the form of greatest astrochemical interest, and with special attention given to changes at 155-160K. Infrared spectra of amorphous and crystalline forms are shown. The refractive index at 670nm of amorphous ethanol at 16K is reported, along with three IR band strengths and a density. A comparison is made to recent work on the isoelectronic compound ethanethiol (CH3CH2SH), and several astrochemical applications are suggested for future study.

Descent without Modification? The Thermal Chemistry of H2O2 on Europa and Other Icy Worlds.

The strong oxidant H2O2 is known to exist in solid form on Europa and is suspected to exist on several other Solar System worlds at temperatures below 200 K. However, little is known of the thermal chemistry that H2O2 might induce under these conditions. Here, we report new laboratory results on the reactivity of solid H2O2 with eight different compounds in H2O-rich ices. Using infrared spectroscopy, we monitored compositional changes in ice mixtures during warming. The compounds CH4 (methane), C3H4 (propyne), CH3OH (methanol), and CH3CN (acetonitrile) were unaltered by the presence of H2O2 in ices, showing that exposure to either solid H2O2 or frozen H2O+H2O2 at cryogenic temperatures will not oxidize these organics, much less convert them to CO2. This contrasts strongly with the much greater reactivity of organics with H2O2 at higher temperatures, and particularly in the liquid and gas phases. Of the four inorganic compounds studied, CO, H2S, NH3, and SO2, only the last two reacted in ices containing H2O2, NH3 making NH4+ and SO2 making SO(4)2- by H+ and e- transfer, respectively. An important astrobiological conclusion is that formation of surface H2O2 on Europa and that molecule's downward movement with H2O-ice do not necessarily mean that all organics encountered in icy subsurface regions will be destroyed by H2O2 oxidation.

Glycine's radiolytic destruction in ices: first in situ laboratory measurements for Mars.

We report new laboratory studies of the radiation-induced destruction of glycine-containing ices for a range of temperatures and compositions that allow extrapolation to martian conditions. In situ infrared spectroscopy was used to study glycine decay rates as a function of temperature (from 15 to 280 K) and initial glycine concentrations in six mixtures whose compositions ranged from dry glycine to H2O+glycine (300:1). Results are presented in several systems of units, with cautions concerning their use. The half-life of glycine under the surface of Mars is estimated as an extrapolation of this data set to martian conditions, and trends in decay rates are described as are applications to Mars' near-surface chemistry.

Amino acids from ion-irradiated nitrile-containing ices.

Solid CH(3)CN and solid H(2)O + CH(3)CN were ion irradiated near 10 K to initiate chemical reactions thought to occur in extraterrestrial ices. The infrared spectra of these samples after irradiation revealed the synthesis of new molecules. After the irradiated ices were warmed to remove volatiles, the resulting residual material was extracted and analyzed. Both unhydrolyzed and acid-hydrolyzed residues were examined by both liquid and gas chromatographic-mass spectral methods and found to contain a rich mixture of products. The unhydrolyzed samples showed HCN, NH(3), acetaldehyde (formed by reaction with background and atmospheric H(2)O), alkyamines, and numerous other compounds, but no amino acids. However, reaction products in hydrolyzed residues contained a suite of amino acids that included some found in carbonaceous chondrite meteorites. Equal amounts of D- and L-enantiomers were found for each chiral amino acid detected. Extensive use was made of (13)C-labeled CH(3)CN to confirm amino acid identifications and discriminate against possible terrestrial contaminants. The results reported here show that ices exposed to cosmic rays can yield products that, after hydrolysis, form a set of primary amino acids equal in richness to those made by other methods, such as photochemistry.

Infrared detection of HO2 and HO3 radicals in water ice.

Infrared spectroscopy has been used to detect HO(2) and HO(3) radicals in H(2)O + O(2) ice mixtures irradiated with 0.8 MeV protons. In these experiments, HO(2) was formed by the addition of an H atom to O(2) and HO(3) was formed by a similar addition of H to O(3). The band positions observed for HO(2) and HO(3) in H(2)O-ice are 1142 and 1259 cm(-1), respectively, and these assignments were confirmed with (18)O(2). HO(2) and HO(3) were also observed in irradiated H(2)O + O(3) ice mixtures, as well as in irradiated H(2)O(2) ice. The astronomical relevance of these laboratory measurements is discussed.

Infrared spectra and radiation stability of H2O2 ices relevant to Europa.

In this paper we present spectra of H2O2-containing ices in the near- and mid-infrared (IR) regions. Spectral changes on warming are shown, as is a comparison of near-IR bands of H2O and H2O2-containing ices. An estimate of the A-value (absolute intensity) for the largest near- IR feature of H2O2 is given. Radiation-decay half-lives are reported for 19 K and 80 K, and are related to the surface radiation doses on Europa. The radiation data show that H2O2 destruction is slower at 80 K than 19 K, and are consistent with the claim that icy material in the outermost micrometer of Europa's surface has been heavily processed by radiation.