Hot hydrogen clue to chemicals in space "Hot hydrogen atoms'' may provide the clue to the mechanism for the formation of chemical compounds in interstellar space. Such hydrogen atoms, considered "hot" because of their high kinetic energies, could cause reactions to occur at temperatures thought too low for most chemical reactivity, according to Dr. Ralph F. Becker of the University of Houston. Reactions involving hot hydrogen atoms, performed in the laboratory of Dr. Becker and his associates Dr. Kong-yi Hong and Dr. Jane Huey Hong, verified that many molecules— including those found in interstellar space, as well as protein and nonprotein amino acids—can be made in this way. The chemists propose a mechanism of hydrogen abstraction to give a free radical, with subsequent reactions leading to other free radicals and products. The proposed mechanism involving hot hydrogen atoms overcomes what have been considered limitations to reactions in space—low density (concentration) of molecules and low temperatures. Hot atoms have energy— kinetic or translational—in excess of that which such atoms could be expected to have if they were in thermal equilibrium with their surroundings. The energy associated with hot hydrogen atoms can provide the energy needed to initiate reactions, or in effect overcome the activation energy to begin a process such as abstraction of a hydrogen atom from a molecule. The product free radicals then can react with each other to give new compounds. To date, 27 molecular species have been discovered in space. Among basic building blocks discovered are ammonia, methane, and water. And 16 organic compounds have been found, including, for example, methanol, formaldehyde, carbon monoxide, and formic acid. In addition, amino acids have been found in meteorites, suggesting that possible conditions exist in protoplanetary gas and dust clouds for synthesis of these basic materials of proteins. Such information stimulated the
University of Houston group to try to reproduce the reactions, which ultimately could lead to additional knowledge about the origins of life. Hot hydrogen atoms, with their high kinetic energies, obviously must have much higher temperatures than surrounding species, Dr. Becker explains. Such atoms could be generated by star sources or by the action of light on simple molecules, breaking them apart. Because of the near vacuum conditions of space, hot hydrogen atoms stay hot for a long time, perhaps as long as 100 years. If they do collide with other compounds, they can cause reactions to occur. Fragments from these reactions can further react and generate more complex molecules. Several other scientists have noted reactions of hot hydrogen atoms, such as abstraction of hydrogen atoms, substitution, addition, and fragmentation. These reactions can involve molecules as simple as those found in space but also can involve more complex organic compounds. Of these reactions, by far the most likely to occur, according to the work of Dr. Becker and his associates, is hydrogen abstraction, because the kinetic energies of hot hydrogen atoms are relatively low, ranging from 17 to 32 kcal. per mole. (The upper range of energy for which all the reactions can occur is about 1150 kcal. per mole.) In its work, the Houston group irradiated hydrogen sulfide or methyl mercaptan with light from a mercury or xenon lamp. The light passed through a filter system to give a band of 220 or 280 nm., with a maximum transmission of 54% at 252 nm. The hot hydrogen atoms were allowed to react with mixtures of compounds, such as methane, ethane, water, ammonia, ethanol, and ethylamine, in a conventional heat convection circulation system with a total volume of 6 liters. Pressure of each gas reactant was measured with a Bourdon gage; no mercury-containing devices were used, thus eliminating the possibility of mercury-catalyzed reactions. The reactants were allowed to mix
Typical hot-hydrogen experiment yields amino acids Reactants and pressure
Hydrogen sulfide (250 mm. Hg) Ammonia (250 mm. Hg) Methane (250 mm. Hg) Water (90 mm. Hg)
Lowest temperature 50° C.
Light source
500-watt highpressure mercury lamp
Irradiation time
Amino acids detected
Other products detected
113.5 hours
Aspartic acid Serine or threonine Glutamic acid Proline
Sulfur and possibly some amino compounds more basic and acidic than the protein amino acids
Glycine Alanine Valine Leucine Isoleucine
Dr. Ralph Becker (center), Dr. Jane Huey Hong (left), and Dr. Kong-yi Hong
overnight. Reaction times for experiments have ranged from 21 hours to two weeks. After photolysis, the products and reactants remaining were analyzed using gas chromatography-mass spectrometry. The mixtures are processed differently when amino acids are expected. Two methods are used. In one, a water solution is filtered, concentrated by vacuum evaporation, passed through an ion exchange resin, dried, and analyzed using a commercial automatic amino acid analyzer. In another method, the amino acids are converted to the Af-trifluoroacetyl isopropyl esters and analyzed using gas chromatography and mass spectrometry. Quantum yields of amino acids are in the range of about 2 to 4 X 10- 5 molecules, Dr. Becker says. This estimate is based on cold trapping of all the materials except hydrogen, using liquid nitrogen at 77° K. Hydrogen pressure is measured and the quantity of hydrogen calculated, assuming the ideal gas law valid. When hydrogen sulfide is used, the amount of hydrogen is equal to the number of photons absorbed by the hydrogen sulfide, since each photon gives a hot hydrogen atom which then abstracts another hydrogen atom from a substrate to give hydrogen molecules. One approximation necessary is that reactions involving a third body are not important. Such an approximation is valid, Dr. Becker points out, because the concentration of all the gases is much greater than that of radicals, HS~, and hydrogen atoms. Thus, three-body collisions are unlikely. June 3, 1974 C&EN
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The "hot hydrogen" concept makes the meaning of temperature important in the experiments and in interpretation of the results, Dr. Becker says. When a system is in thermal equilibrium, the molecules are colliding frequently, the molecules have an average velocity or kinetic energy, and the kinetic energy doesn't change. A temperature can be defined for the system. These unusual circumstances prevail for systems where the density of molecules is high, as on earth. However, in interstellar space where the density of molecules is very low, collisions of molecules occur rarely, Dr. Becker points out. If for some reason a molecule or atom attains a high velocity (or kinetic energy) it could remain To date, 27 molecular species have been discovered in space that way for a long time and not be in thermal equilibrium with its surroundings. ambient temperature and density con- reaction initiator also is expected to be Therefore, despite the low ambient ditions are low and not conducive to valid for reactions occurring in the atmospheres of planets, according to Dr. temperatures of the surroundings in reaction. space—other molecules and dust—the Various other scientists have noted Becker. temperature of the molecules with high that conditions for formation of moleIn typical experiments from which velocity would be considered very high. cules in space are far from thermody- amino acids are produced, the reactant ("Temperature" in this situation isn't namic equilibrium. Also, they note mixture contains substances such as used rigorously, for the word really that changes in temperature have rela- ammonia, water, methane or ethane, applies to a system in equilibrium, but tively little effect on average kinetic and hydrogen sulfide or methyl merit roughly describes the velocity or ki- energy and activation energy of reac- captan as sources of hot hydrogen. Dr. netic energy of the molecules.) In the tion. Since activation energy and reac- Becker points out that amino acids laboratory, the thermal nonequilibrium tion threshold energy are related, such as aspartic acid, serine or threocondition of space can be reproduced. threshold energies also are expected to nine, glutamic acid, proline, glycine, The reactions that occur could thus be practically temperature indepen- and others are detected regardless of duplicate those of space, where the dent. This basic idea of hot atoms as a whether methane or ethane (a one- or two-carbon substrate) is present. As many as 10 protein amino acids, including one containing sulfur, have been detected, and six protein and eight nonprotein amino acids have been verified with gas chromatography-mass spectrometry. Methane used in the experiments is a specially puriNow you can search over 3.5 million bibliographic references in 7 fied grade—99.97+% methane—to large scientific data bases without buying any equipment or investminimize possible contamination from ing valuable man-hours to learn a variety of command languages. ethane or other carbon sources. Our information chemists conduct your search on our equipment. Other products found vary with the You can wait for answers —or we can mail you a printout of the reaction mixture. Usually sulfur and search results. Now even the small laboratory, the growing company sulfur compounds are present. Other or the infrequent user can afford the benefits of searching the compounds such as methanol, ethanol, chemical literature by computers. acetic acid, propionic acid, and methylamine and ethylamine also have The files you can search been created in the reactions. The CAIN-475,000 records from 1970 Chemical Abstracts Condensates — presence of some of the last compounds about agriculture and related 1.2 million records from 1970. led to work utilizing these as additionsubjects. al substrates for amino acids. National Technical Information INSPEC-700,000 abstracts from Currently, the Houston group is Services—400,000 records of 1969 on physics, electrical and working with a very simple system of government research from 1963. electronic engineering, computers carbon monoxide, methane, and hydroEngineering Index—300,000 records and control engineering. gen sulfide. Products found so far inof engineering literature from 1970. PANDEX/TRANSDEX-520,000 clude carbon disulfide, carbonyl sulTOXLINE-325,000 records from records from 1970 published in fide, acetaldehyde, methyl mercaptan, 1965 on toxicity and health aspects 2,400 scientific, technical and and ethane. Two of these materials of chemicals and drugs. medical journals. have been identified in interstellar space and a third is a precursor to anSend for complete details other. Soon to begin is work using monoCall us when you need to know ( 6 1 4 ) 2 6 1 - 7 1 0 1 chromatic light. The aim of this effort is to limit activity of the hot hydrogen atoms. If the activity can be limited, then possibly more specific details of the reaction mechanisms can be obtained. Reactions might then be di3620 north high street/columbus,ohio 43214 rected toward making specific compounds.
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