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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Equilibrium carbon isotope fractionation factors of hydrocarbons: Semi-empirical force-field method

Here, we have calculated the reduced partition function ratios for carbon isotopes (β-factor) of 67 hydrocarbons (alkanes, alkenes, alkynes, cycloalkanes, and aromatics), including their 267 single-substituted isotopomers. The calculations were performed using the harmonic oscillator – rigid rotator model and the Urey / Bigeleisen-Mayer approach. Normal frequencies of molecular vibrations of the isotopologues were calculated from the molecular structures data and valence field force constants stored in the Light-handled Elucidation of Vibrations (LEV) database elaborated by Dr. Gribov and his colleagues in Russia. The LEV database is constructed from experimental spectroscopic and structural data by solving inverse problems of molecular vibrations. The LEV is internally consistent and the most comprehensive database available to date. The β-factor were calculated in the temperature range of 200–800 K with a 10 K step. Our calculations predict that the β-factors increase with an increasing number of C atoms within the same groups of hydrocarbons (e.g., alkanes). Our calculations also show a general descending order of 13 C enrichments among the different groups of hydrocarbons: cycloalkanes, aromatics, alkenes (double bonds) and isoalkanes, alkanes, alkynes (triple bonds). Position-specific, intramolecular isotope effects within hydrocarbons are determined by the β-factors of C in the different functional groups in the order: quaternary (C), tertiary (methine, CH), secondary (methylene, CH 2 ), primary (methyl, CH 3 ) and double bond (C=C), saturated bond (C - C), triple bond (HC≡). Our calculations on bulk and position-specific carbon isotope β-factors of the hydrocarbons, which are generally consistent with very limited ab initio calculations in the literature, are internally consistent and the most comprehensive to date for future applications to position-specific isotope geochemistry of hydrocarbons.

58 GEOSCIENCES↗
DOE OSTI2023

Polypropylene Degradation Catalyzed by Tantalum Hydrides Supported on Sulfated Alumina

Ta–H + sites supported on sulfated aluminum oxide (SAO), [Ta(H) 2 (O–) 2 ][SAO] (1), catalyze the hydrogenolysis of isotactic polypropylene (iPP, M n = 13.3 kDa; D = 2.4; mmmm = 94%) to form low-molecular-weight branched alkanes (C 11 –C 30 ) in good yields (70%). Here, the alkanes formed lose stereochemical information originating from iPP, but residual iPP remains highly isotactic. In the presence of D 2 , similar mixtures of alkanes are formed containing –CH 3–x D x , –CHD, and –CD–. Residual iPP maintains high tacticity and incorporates deuterium primarily into –CH 3 groups of the polymer (–CH 3–x D x /–CHD–/–CD– ~10:1:0). 1 reacts with pinacolborane to form [TaH(κ 2 -H 2 BPin)(O–) 2 ][SAO] (2). At 200 °C in an iPP melt, 2 reacts to form products indicative of C–H bond activation at –CH 3 groups and internal –CH 2 – groups in iPP in a ~1:3 ratio, indicating a slight kinetic preference for C–H bond activation at internal –CH 2 – groups. 1 is more reactive toward iPP than high-density polyethylene in hydrogenolysis reactions

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Accessing metal-specific orbital interactions in C–H activation with resonant inelastic X-ray scattering

Photochemically prepared transition-metal complexes are known to be effective at cleaving the strong C–H bonds of organic molecules in room temperature solutions. There is also ample theoretical evidence that the two-way, metal to ligand (MLCT) and ligand to metal (LMCT), charge-transfer between an incoming alkane C–H group and the transition metal is the decisive interaction in the C–H activation reaction. What is missing, however, are experimental methods to directly probe these interactions in order to reveal what determines reactivity of intermediates and the rate of the reaction. Here, using quantum chemical simulations we predict and propose future time-resolved valence-to-core resonant inelastic X-ray scattering (VtC-RIXS) experiments at the transition metal L-edge as a method to provide a full account of the evolution of metal–alkane interactions during transition-metal mediated C–H activation reactions. For the model system cyclopentadienyl rhodium dicarbonyl (CpRh(CO) 2 ), we demonstrate, by simulating the VtC-RIXS signatures of key intermediates in the C–H activation pathway, how the Rh-centered valence-excited states accessible through VtC-RIXS directly reflect changes in donation and back-donation between the alkane C–H group and the transition metal as the reaction proceeds via those intermediates. We benchmark and validate our quantum chemical simulations against experimental steady-state measurements of CpRh(CO) 2 and Rh(acac)(CO)2 (where acac is acetylacetonate). Our study constitutes the first step towards establishing VtC-RIXS as a new experimental observable for probing reactivity of C–H activation reactions. More generally, the study further motivates the use of time-resolved VtC-RIXS to follow the valence electronic structure evolution along photochemical, photoinitiated and photocatalytic reactions with transition metal complexes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Novel, active, and uncultured hydrocarbon-degrading microbes in the ocean

ABSTRACT Given the vast quantity of oil and gas input to the marine environment annually, hydrocarbon degradation by marine microorganisms is an essential ecosystem service. Linkages between taxonomy and hydrocarbon degradation capabilities are largely based on cultivation studies, leaving a knowledge gap regarding the intrinsic ability of uncultured marine microbes to degrade hydrocarbons. To address this knowledge gap, metagenomic sequence data from the Deepwater Horizon (DWH) oil spill deep-sea plume was assembled to which metagenomic and metatranscriptomic reads were mapped. Assembly and binning produced new DWH metagenome-assembled genomes that were evaluated along with their close relatives, all of which are from the marine environment (38 total). These analyses revealed globally distributed hydrocarbon-degrading microbes with clade-specific substrate degradation potentials that have not been reported previously. For example, methane oxidation capabilities were identified in all Cycloclasticus . Furthermore, all Bermanella encoded and expressed genes for non-gaseous n -alkane degradation; however, DWH Bermanella encoded alkane hydroxylase, not alkane 1-monooxygenase. All but one previously unrecognized DWH plume member in the SAR324 and UBA11654 have the capacity for aromatic hydrocarbon degradation. In contrast, Colwellia were diverse in the hydrocarbon substrates they could degrade. All clades encoded nutrient acquisition strategies and response to cold temperatures, while sensory and acquisition capabilities were clade specific. These novel insights regarding hydrocarbon degradation by uncultured planktonic microbes provides missing data, allowing for better prediction of the fate of oil and gas when hydrocarbons are input to the ocean, leading to a greater understanding of the ecological consequences to the marine environment. IMPORTANCE Microbial degradation of hydrocarbons is a critically important process promoting ecosystem health, yet much of what is known about this process is based on physiological experiments with a few hydrocarbon substrates and cultured microbes. Thus, the ability to degrade the diversity of hydrocarbons that comprise oil and gas by microbes in the environment, particularly in the ocean, is not well characterized. Therefore, this study aimed to utilize non-cultivation-based ‘omics data to explore novel genomes of uncultured marine microbes involved in degradation of oil and gas. Analyses of newly assembled metagenomic data and previously existing genomes from other marine data sets, with metagenomic and metatranscriptomic read recruitment, revealed globally distributed hydrocarbon-degrading marine microbes with clade-specific substrate degradation potentials that have not been previously reported. This new understanding of oil and gas degradation by uncultured marine microbes suggested that the global ocean harbors a diversity of hydrocarbon-degrading bacteria, which can act as primary agents regulating ecosystem health.

Howe, Kathryn L.↗
DOE OSTI2021

Higher Energy-Content Jet Blending Components Derived from Ethanol (CRADA 467)

This project aims to develop a new sustainable aviation fuel (SAF) that comprises favorable properties, such as high energy density, excellent thermal stability, and favorable cold fold properties. Through catalyst development, this work will provide a route to control the cycloalkane/n-alkane/iso alkane content of a next-generation fuel with minimal or no aromatic content. Combining n-alkane and iso-alkane streams (high specific energy, MJ/kg) with cycloalkanes (higher energy density, MJ/L), is expected to enable at least a 4% net increase in combined (specific (MJ/kg) and volumetric (MJ/L)) energy content without impacting ‘drop-in’ fuel requirements, such as seal swelling. PNNL and LanzaTech have already demonstrated a sustainable, non-petroleum, route to isoalkanes. However, economically attractive cycloalkane production from waste and biomass is challenged by large hydrogen requirements, preferential selectivity to aromatic compounds and low yields to jet fuel range components. Many gaps in understanding cycloalkane properties and performance in complex jet fuel mixtures remain. The close tie and integration of Purdue’s fuel property analysis, with PNNL’s process development, can lead to an economically attractive process. The fuel analysis and testing by Purdue will enable a robust understanding of the properties and behavior of the cycloalkanes produced to inform process development. Additionally, seal-swelling analysis will quantify the ability of fuel blends with zero or minimal aromatics content to satisfy the seal swell requirement of O rings. Lastly, Purdue’s system-level analysis will lead to the development of a roadmap for deployment in key regions that considers system pressures such as hydrogen, water, energy efficiency, and ease of infrastructure access.

10 SYNTHETIC FUELS↗
DOE OSTI2023

Thermal Scattering Law Data Development for Paraffin Wax

Paraffin wax is often used as a nuclear moderator to slow down the fast neutrons in experimental critical assemblies [1]. It is a colorless and soft solid material that consists primarily of straight-chain alkanes (n-alkanes), which are hydrocarbons with the general formula CnH2n+2 [2-3]. The length of the hydrocarbon chain ranges from C20 to C30 and higher [2]. It is distinguished by its solid state at room temperature and begins to melt above approximately 310 K [4]. Paraffin wax is a commonly employed substance in the manufacture of shielding. One of its noteworthy characteristics is its ability to effectively absorb the neutrons. Also, it possesses a high macroscopic cross section, which enables it to efficiently moderate neutrons. As a result, paraffin wax is extensively utilized in various applications where moderation and shielding of neutrons are needed. For simulations, it is necessary to evaluate its thermal scattering law (TSL) and cross sections. Computationally, classical molecular dynamics (CMD) simulations provide the capability of simulating atomic details. For example, several unary, binary, and few multi component mixtures have been investigated of the paraffin model by using molecular dynamics simulations [5-12]. An assessment of thermal neutron scattering in a heavy paraffinic oil treated both as a solid and a viscous fluid containing 25% linear branched paraffin (C30H62), 35% one ring cycloalkane (C30H60), 15% two rings cycloalkane (C30H58), and 25% aromatic (C30H60) chains has been studied using CMD simulations for producing TSL data [13]. Nevertheless, there is lack of TSL and cross section data for paraffin wax as most of the reported analyses focus on the unary and binary mixture of n-alkanes, which is not consistent with actual paraffin wax [2]. In this work, we applied the equilibrium CMD simulations technique to explore the structure and dynamical properties of wax, which are fundamental input to calculate the TSL. A paraffin wax system was modeled using the CMD code LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) [14-15] with the semi-empirical COMPASS [16] force field. The density of state (DOS) was calculated from the normalized velocity autocorrelation function (VACF), which is the Fourier transform of the normalized VACF. The DOS was used for the calculation of the TSL and thermal scattering cross sections. The paraffin wax atomic system was constructed by using the MedeA material design platform [17], and was benchmarked using available properties (i.e., density, bond lengths, angles, diffusivity, and viscosity).

Nuclear Criticality Safety Program (NCSP)↗
DOE OSTI2022

Overview of Diverse Methyl/Alkyl-Coenzyme M Reductases and Considerations for Their Potential Heterologous Expression

Methyl-coenzyme M reductase (MCR) is an archaeal enzyme that catalyzes the final step of methanogenesis and the first step in the anaerobic oxidation of methane, the energy metabolisms of methanogens and anaerobic methanotrophs (ANME), respectively. Variants of MCR, known as alkyl-coenzyme M reductases, are involved in the anaerobic oxidation of short-chain alkanes including ethane, propane, and butane as well as the catabolism of long-chain alkanes from oil reservoirs. MCR is a dimer of heterotrimers (encoded by mcrABG) and requires the nickel-containing tetrapyrrole prosthetic group known as coenzyme F430. MCR houses a series of unusual post-translational modifications within its active site whose identities vary depending on the organism and whose functions remain unclear. Methanogenic MCRs are encoded in a highly conserved mcrBDCGA gene cluster, which encodes two accessory proteins, McrD and McrC, that are believed to be involved in the assembly and activation of MCR, respectively. The requirement of a unique and complex coenzyme, various unusual post-translational modifications, and many remaining questions surrounding assembly and activation of MCR largely limit in vitro experiments to native enzymes with recombinant methods only recently appearing. Production of MCRs in a heterologous host is an important step toward developing optimized biocatalytic systems for methane production as well as for bioconversion of methane and other alkanes into value-added compounds. This review will first summarize MCR catalysis and structure, followed by a discussion of advances and challenges related to the production of diverse MCRs in a heterologous host.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1972

Origin of organic matter in early solar system. V - Further studies of meteoritic hydrocarbons and a discussion of their origin.

The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C8 are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C8, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH4/C2H6 ratio is greater than 30, possibly greater than 700. Isoprenoids from C17 to C20 occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments, and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigeneous. A comprehensive review of current evidence shows that Fischer-Tropsch-type reactions can account for most principal features of meteorite organic matter.

Studier, M. H.↗
NASA NTRS1978

Infrared band intensities of saturated hydrocarbons

Kramers-Kronig analysis is applied to measured values of spectral reflectance at near-normal incidence to determine the real and the imaginary parts of the complex index of refraction for methane, ethane, propane, n-butane, n-hexane, n-heptane, and n-decane in the liquid state. The results indicate that the strengths of the characteristic bands as measured by the integral of the imaginary part are roughly constant for all the liquid alkanes except for methane. The intensity of the CH valence vibration bands in the spectra of the alkanes except methane is directly proportional to the number of CH groups per unit volume. The relations for the intensity of the bands due to CH2 and CH3 deformations are examined. Characteristic band intensities of the type established for NH4(+) and SO4(2-) groups in solutions and crystals cannot be extended to the more closely coupled CH2 and CH3 groups in alkane molecules.

Pinkley, L. W.↗
NASA NTRS1980

The carbon isotope biogeochemistry of the individual hydrocarbons in bat guano and the ecology of insectivorous bats in the region of Carlsbad, New Mexico

The structures and C-13 contents of individual alkanes extracted from bat guano found in the Carlsbad region of New Mexico can be related to both the photosynthetic pathways of the local plants and the feeding habits of the insects that support the bats. Carbon isotopic analyses of the 62 most important plant species in the Pecos River Valley, the most significant feeding area for the Carlsbad bats, reveal the presence of 29 species with C3 photosynthesis and 33 species, mostly grasses, with C4 photosynthesis. Although the abundances of nonagricultural C3 and C4 plants are similar, alfalfa and cotton, both C3 plants, constitute over 95 per cent of the crop biomass. The molecular composition of the bat guano hydrocarbons is fully consistent with an insect origin. Two isotopically distinct groups of insect branched alkanes were discerned. These two groups of alkanes derived from two chemotaxonomically distinct populations of insects possessing distinctly different feeding habits. It is likely that one population grazes predominantly on crops whereas the other population prefers native vegetation. This and other isotopic evidence supports the notion that crop pests constitute a major percentage of the bats' diet.

Desmarais, D. J.↗
NASA NTRS1981

Development of urethane coating and potting material with improved hydrolytic and oxidative stability

A series of saturated hydrocarbon based urethanes was prepared and characterized for hydrolytic and oxidative stability. A series of ether based urethanes was used as a basis for comparison. The alkane base urethanes were found to be hydrolytically and oxidatively stable and had excellent electrical properties. The alkane based materials absorbed little or no water and were reversion resistant. There was little loss in hardness or weight when exposed to high temperature and humidity. Dielectric properties were excellent and suffered little adverse effects from the high temperature/humidity conditions. The alkane based urethanes were not degraded by ozone exposure.

Morris, D. E.↗
NASA NTRS1992

Indications of photochemical histories of Pacific air masses from measurements of atmospheric trace species at Point Arena, California

Measurements were made of a suite of photochemically active trace species (including light hydrocarbons, ozone, peroxyacetyl nitrate, HNO3, NO3(-), NO(x), and NO(y)) in marine air collected during a 10-day period in April and May 1985 at Point Arena (California), a coastal inflow site. It was found that the mixing ratios of the alkanes, ozone, peroxyacetyl nitrate, and HNO3 correlated with variations in the origins of calculated air parcel trajectories and with variations in the ratios of the light alkanes. The highest levels of alkanes and the photochemical products were found in parcels that had been rapidly transported across the North Pacific Ocean from near the 600-mbar level above the east Asian coast. It is suggested that production over the continents, transport to the marine areas, and parallel removal processes account for much of the observed correlation.

Parrish, D. D.↗
NASA NTRS1998

Fiber Supported Droplet Combustion-2 (FSDC-2)

Experimental results for the burning characteristics of fiber supported, liquid droplets in ambient Shuttle cabin air (21% oxygen, 1 bar pressure) were obtained from the Glove Box Facility aboard the STS-94/MSL-1 mission using the Fiber Supported Droplet Combustion - 2 (FSDC-2) apparatus. The combustion of individual droplets of methanol/water mixtures, ethanol, ethanol/water azeotrope, n-heptane, n-decane, and n-heptane/n-hexadecane mixtures were studied in quiescent air. The effects of low velocity, laminar gas phase forced convection on the combustion of individual droplets of n-heptane and n-decane were investigated and interactions of two droplet-arrays of n-heptane and n-decane droplets were also studied with and without gas phase convective flow. Initial diameters ranging from about 2mm to over 6mm were burned on 80-100 micron silicon fibers. In addition to phenomenological observations, quantitative data were obtained in the form of backlit images of the burning droplets, overall flame images, and radiometric combustion emission measurements as a function of the burning time in each experiment. In all, 124 of the 129 attempted experiments (or about twice the number of experiments originally planned for the STS-94/MSL-1 mission) were conducted successfully. The experimental results contribute new observations on the combustion properties of pure alkanes, binary alkane mixtures, and simple alcohols for droplet sizes not studied previously, including measurements on individual droplets and two-droplet arrays, inclusive of the effects of forced gas phase convection. New phenomena characterized experimentally for the first time include radiative extinction of droplet burning for alkanes and the "twin effect" which occurs as a result of interactions during the combustion of two-droplet arrays. Numerical modeling of isolated droplet combustion phenomenon has been conducted for methanol/water mixtures, n-heptane, and n-heptane/n-hexadecane mixtures, and results compare quantitatively with those found experimentally for methanol/water mixtures. Initial computational results qualitatively predict experimental results obtained for isolated n-heptane and n-heptane/n-hexadecane droplet combustion, although the effects of sooting are not yet included in the modeling work. Numerical modeling of ethanol and ethanol/water droplet burning is under development. Considerable data remain to be fully analyzed and will provide a large database for comparisons with further numerical and analytical modeling and development of future free droplet experiments aboard space platforms.

Colantonio, Renato↗
NASA NTRS1999

Pressure Effects on Combustion of Methanol and Methanol-Docecanol Droplets

The objective of this research is to improve understanding of the combustion of binary fuel mixtures in the vicinity of the critical point. Fiber-supported single droplets and two-droplet arrays of methanol and of mixtures of methanol and 1-dodecanol, initially 0.9 mm in diameter, were burned in room-temperature air at pressures from 0.1 MPa to 9.0 MPa in the NASA Lewis 2.2-second drop tower. The work is a continuation of a collaborative Japan-US research effort designed to increase knowledge of high-pressure combustion of fuel sprays, relevant to application in propulsive and power-production devices such as Diesel engines. Some previous publications from this cooperative program may be cited. All of the previous experiments concerned alkanes and alkane mixtures. The new research reported here addresses alcohols and alcohol mixtures, to ascertain the degree to which previous results for alkanes extend to alcohols. There have been many previous experimental studies of methanol droplet combustion and a few of alcohol mixtures, but not at the high pressures of interest here. There is some experimental information on methanol droplet combustion at elevated pressure but none on the alcohol mixtures extending to critical pressures, as in the present study.

Okai, K.↗
NASA NTRS2000

Experimental and Modeling Study of the Burning of an Ethanol Droplet in Microgravity

The microgravity ethanol droplet combustion experiments were performed aboard the STS-94/MSL-1 Shuttle mission within the Fiber-Supported Droplet Combustion-2 (FSDC-2) program. The burning histories and flame standoffs for pure ethanol and ethanol/water droplets were obtained from the images recorded with two 8 mm videocameras. The obtained results show that average gasification rate is related to the initial droplet size in a manner similar to n-alkanes and methanol and consistent with the results of Hara and Kumagai and the data taken recently in the NASA-Lewis 2.2 s droptower. A transient, moving finite-element chemically reacting flow model applied previously to sphero-symmetric combustion of methanol, methanol/water, n-alkane, and n-alkane binary mixture droplets was adopted for the problem of ethanol droplet combustion. The model includes detailed description of gas-phase reaction chemistry and transport, a simplified description of liquid phase transport, and non-luminous radiative heat transfer. Gas-phase chemistry was described with the detailed reaction mechanism of Norton and Dryer, which consists of 142 reversible elementary reactions of 33 species. Another recently published reaction mechanism of high-temperature ethanol oxidation was also considered. The model predictions were found to compare favorably with the experimental data. The model analysis also indicates that water condensation in the case of ethanol has smaller effect on average droplet gasification rate as compared with previously studied methanol cases. This effect is explained by non-ideal (azeotropic) behavior of binary ethanol-water mixtures. Further analysis of computational results and ethanol droplet radiative extinction behavior will be discussed.

Kazakov, Andrei↗
NASA NTRS2000

An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact-generated gases

Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO2, and H2 below approximately 250-300 degrees C during rapid cooling of trapped magmatic or impact-generated gases. Depending on temperature, bulk composition, and oxidation-reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO2 ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01-0.001 together with high CO/CO2 ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO2 ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine-grain magnetite, formation of a CO-rich local gas phase, reduction of water vapor to H2, reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high-temperature quenching of CO-, H2-rich impact gases could have led to magnetite-catalyzed hydrocarbon synthesis.

Non-NASA Center↗
NASA NTRS2017

Analysis of Mixed Aryl/Alkyl Esters by Pyrolysis Gas Chromatography-Mass Spectrometry in the Presence of Perchlorate

Mars is an important target for Astrobiology. A key goal of the MSL mission was to determine whether Mars was habitable in the past, a que-tion that has now been definitely determined to be yes. Another key goal for Mars exploration is to understand the origin and distribution of organic material on Mars; this question is being addressed by the SAM instrument on MSL, and will also be informed by two upcoming Mars exploration missions, ExoMars and Mars 2020. These latter two missions have instrumentation capable of detecting and characterize organic molecules. Over the next decade, these missions will analyze organics in surface, near-surface and sub-surface samples. Each mission has the capability to analyze organics by different methods (pyrolysis gas chromatography-mass spectrometry [py-GC-MS]; laser desorption and thermal volatilization GC-MS; and Raman spectroscopy). Plausibly extraterrestrial organics were recently discovered by the Mars Science Laboratory (MSL), providing an important first step towards understanding the organic inventory on Mars [1]. The compounds detected were chlorobenzenes and chloroalkanes, but it was argued that chlorination of these compounds occurred during pyrolysis of samples containing unchlorinated organics in the presence of perchlorate. A recent report analyzed a suite of aromatic (benzene, toluene, benzoic acid, phthalic acid, and mellitic acid) and aliphatic (acetic acid, propane, propanol, and hexane) by pyrolysis under SAM-like conditions in the presence of perchlorate to attempt to constrain possible precursor molecules for the organic molecules detected on Mars. For aromatic compounds, the aromatic acids all readily produced SAM-relevant chlorobenzes, whereas benzene and toluene did not. This observation suggests that the chlorobenzene detected on Mars could have derived from compounds like mellitic acid, consistent with the previous hypothesis by Benner et al. [3]. Among the aliphatic molecules, it was shown that pyrolysis of alkanes and alcohols in the presence of perchlorates produced polychlorine containing chloro-alkanes similar to what was observed on Mars. Surpris-ingly, however, similar treatment of acetic acid pro-duced chloroketones, instead, and no chloroalkanes were reported. This suggests that the chloroalkanes detected in the Sheepbed mudstone were not derived from aliphatic carboxylic acids, but instead were from more reduced alcohols or even alkanes, or perhaps were degradation products of more complicated organic material. Because organics analyses on mars will rely heavily on py-GC-MS of perchlorate-containing samples over the next decade, it is important to understand the fate of organic molecules of biotic and abiotic origin under such conditions. In this work we begin a series of experiments to improve our understanding of products generated during py-GC-MS of increasingly complex organic molecules (esters, amides, peptides, nucleic acids, fatty acids) in the presence of perchlorate.

Burton, A. S.↗
DOE OSTI2024

Development of Thermal Scattering Law and Cross Sections of Hydrogen in Paraffin Wax

Paraffin wax is frequently used as a neutron moderator and shielding material. The main component of paraffin wax is straight-chain alkanes (n-alkanes). The deposition of paraffin wax is primarily attributed to the crystallization of n-alkanes. It is important to gain a deeper understanding of the mechanisms underlying the behavior of paraffin wax, which would impact the thermal scattering Law (TSL) and cross sections and affect the analysis of neutronic and critical systems. In this work, a classical molecular dynamics (CMD) simulation model was used in LAMMPS to create the TSL and cross sections at room temperature and pressure. To generate the required velocity autocorrelation functions (VACF), previously published data were used to validate the approach and models by comparing them with key model parameters. The phonon density of state (DOS) was calculated using Fourier transformation of the normalized VACF. This DOS was used as the primary input to estimate the TSL (S($a$, $β$)) and cross sections of hydrogen in paraffin wax. The TSL and cross sections of hydrogen were estimated using the Full Law Analysis Scattering System Hub (FLASSH) code. The cross section of hydrogen in paraffin wax is consistent with other hydrocarbon materials such as polyethylene with deviations due to structure in the lowest energy region.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗