^ Flakes of solid nitrogen falling off glass surface at temperatures in the neighborhood of 4 0 ° K.
Fre A Hi£ KEY role which free radicals play in determining the course of chemical reactions has long b e e n recognized by chemists and physicists. Those w h o h a v e been concerned, for example, with problems of chemical kinetics, of t h e mechanisms of combustion reactions, a n d of the details of electric discharge p h e n o m e n a h a v e felt t h e need for more information directed specifically toward t h e properties of t h e short-lived reactive species—the free radicals—which a r e observed in various ways as intermediates in these systems. Knowledge of free radicals goes back more than half a century. Their existence was d e m o n s t r a t e d first by Prof. Moses G o m b e r g more than 50 years ago. Hut it was not until t h e experiments of Prof. Fritz Paneth and Dr. \Y. Hofeditz nearly 30 years later, followed b y the intensive pioneering researches of Prof. Francis O. Rice and others that t h e reluctance to accept free radicals as real entities was oxercome. Since that time much of the interest and attention devoted to free radicals has been directed toward the short-lived species, generally of low molecular weight, p r o d u c e d whenever molecules are broken u p by some energetic process such as a chemical reaction, heat, an electric discharge, or t h e attack of ultraviolet or g a m m a radiation. T h e species formed in this way,
ψ
Dr. Maurice Peyron of the University of Lyon looks at emission from trapped nitrogen atoms ( upper left ) condensed at 4.2° K. from a microwave discharge ( glare at lower left)
FEATURE
Three year research program has just scratched the surface of knowledge on . . .
adica! Chemistry by Dr. James W . Moyer, General Electric C o . , Santa Barbara, Dr. Arnold M. Bass, National
a l t h o u g h electrically neutral, are usually characterized hy having odd n u m b e r s of electrons. It is the u n p a i r e d elec tron which accounts, in part, for the short life a n d highly reactive nature of these radicals. W o r k on the t r a p p i n g of radicals has been carried on during t h e past 30 years. liowever, it is only in tlie past five years that there has been wide spread, concentrated research on stabi lizing a n d storing the reactive species. T h e recent upsurge of interest in the field h a s its origin in a combination of factors, namely the general availability of powerful new research tools (e.g. electron spin resonance spectroscopy, liquid helium, and liquid hydrogen ) and recognition of possible commercial and military uses of t r a p p e d radicals. T h e s e proposed practical applica tions a r e based, to a large extent, on the fact that the trapping process makes it possible, in principle, to store a p preciable quantities of energy in the form of the potential reactivity of the t r a p p e d species. For example, the energy released in t h e recombination of nitrogen atoms is 225 kilocalories per mole. Clearly, if some method could b e developed, which would per mit the preparation a n d storage of large concentrations of such reactive species, this would represent a potent source of stored energy.
Bureau of Standards,
Some experiments, performed nearly five years ago by Dr. Herbert P. Broida and his associates at the National Bureau of Standards, demonstrated that nitrogen atoms could be produced in an electric discharge and subse quently trapped in a solid condensed at the t e m p e r a t u r e of liquid helium (4.2 Γ Κ. ). Initial evidence was in the form of bright green glows and flashes observed at the condensate surface. Later calorimetry experiments showed that considerable heat was evolved as the condensate was warmed. At the time this was attributed to atom-atom recombination. And electron spin res onance studies showed the existence of nitrogen atoms in the condensates. Observations such as these led to rudimentary calculations for the effects of free radical fuels, in various degrees of dilution, upon propulsion character istics of rockets. For light species, par ticularly hydrogen, feverish interest would develop if concentrations of KK'r or more atoms could be trapped. This would lead to expected increases in specific impulse by a factor of four or five over current "good" chemical fuels. Considerations of this kind are of great importance and interest in an energyoriented economy. It is not surprising that pressures to investigate and de velop this promising potential arose soon after the early experiments.
Calif., and
Washington,
D. C.
Upon examination of available data it was clear that certain basic informa tion was lacking. Three years ago we lacked answers to such questions as what is the maximum concentration of radicals t h a t c a n b e produced and stored, h o w does one make accurate measurements of concentrations at 4.2° K., what is the most effective diluent, a n d what is t h e most efficient way of m a k i n g stored radicals? In 1956 the D e p a r t m e n t of Defense decided t h a t it would b e wiser to invest in a basic research program before starting a larger development project that might not b e successful because of the readily identifiable difficulties. In keeping with this view, D O D initi ated a national program to speed up the rate of learning on the formation and properties of trapped radicals and assigned management responsibility to the Department of t h e Army. This large-scale program was directed by a D e p a r t m e n t of t h e Army steering com mittee w i t h members drawn from in dustrial labs as well as the three mili tary services, and under the chairman ship of Norman L. Klein of the Office of the Chief of Ordnance. T h e chief ingredient of this national program was the establishment of a central research effort at National Bureau of Standards for a three-year period under the direc tion of Dr. Broida. AUG.
2 4.
1959
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(;oals of the central effort at XBS were to pursue research ideas with maximum speed and effectiveness and to disseminate widely the resulting in formation and techniques. The basic research project was organized to per mit rapid follow-up as soon as utility in some direction was indicated by data obtained. The technical course of the program developed in three main categories: production and determination of con centrations of trapped species, nature of the solid containing the trapped species, and reactions of the trapped species in the1 solid. Within these categories, and from the large variety of results obtained, the following are especially significant. Production a n d Concentrations Almost all conceivable methods of formation were explored. These in cluded electrical discbarge, thermal dissociation, ultraviolet photolysis, chemical reactions, electron and proton
bombardment, gamma- and x-irradiation, and thermochemical explosions. Free radicals were either formed in the gas phase and subsequently condensed on a cold surface, or formed directly in the condensed solid phase. Of the gas phase methods, none seemed more efficient than the micro wave discharge which, according to Thomas M. Shaw of the (General elec tric Microwave Laboratory, is capable of dissociating very large fractions of diatomic molecules such as hydrogen. That large concentrations are not real ized subsequently in the condensed phase1 is apparently due to limitations in the solid itself. Mr. Shaw has also shown that frequency of discharge is not particularly important, but that use of high power discharges will permit high production rates through higher gas flows. This microwave discharge was the most common source of free radicals on the XBS project. Whether the radicals are first formed in the gas phase and then condensed to a solid or are made directly in the
Industrial Participation in t h e NBS Free Radicals Research P r o g r a m British O x y g e n Research and Development, Ltd. California Research C o r p . Callery Chemical Co. Columbia-Southern Chemical C o r p . Convair Dow Chemical of C a n a d a , Ltd. G e n e r a l Electric Co. Ethyl Corp. Grumman Aircraft Engineering Corp. Humble Oil and Refining Co. Koppers Co. Monsanto Chemical Co. Oiin-Mathieson Chemical C o r p . Shell Research, Ltd. Sinclair Research Laboratories Space Technology Laboratories S t a n d a r d O i l (Ind.) Union C a r b i d e Corp. U. S. Industrial Chemicals Co. W y a n d o t t e Chemicals C o r p .
Industrial Consultants Combustion and Explosives Research, inc G e n e r a l Electric Co. G e n e r a l Motors C o r p . Westinghouse Electric C o r p .
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1959
solid, the resulting stable configurations in the solid appear to be the same. Dr. Leo A. Wall, Daniel \V. Brown, and Dr. Roland E. Florin irradiated various con densed gases with gamma rays at liquid helium and liquid hydrogen tempera tures. The largest concentrations of trapped radicals that they measured were a few tenths of a percent. Mixtures of atoms and molecules may condense until certain critical condi tions in the condensate are reached, 1
.
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takes place. This was discovered by Dr. Heppino J. Fontana, XBS research guest from California Research Corpo ration, by making careful measurements of the temperature of the condensed layers and of the magnetic susceptibility at the same time. Evidences of the change are sudden increases in tem perature, disappearance of susceptibil ity, and Hashes of light. All these observations point to abrupt atom recombination. The critical con ditions depend on the atom-molecule ratio in the gas phase. If the discharge is adjusted to produce not over 0.5 mole vr Ν atoms in XL>, 0.2 to 0.5 mole rA X atoms could be stabilized for brief time periods (five minutes at 6° K.). Under these conditions, ignition occurs apparently when a certain vol ume of condensate is reached. When more than I to 2 mole % nitrogen atoms condense, essentially instantaneous re combination occurs. Dr. Julius L. Jackson of the XBS staff predicted a chain reaction recom bination for concentrations above cer tain critical values on the basis of theo retical considerations. He arrived at a value of about 0.3% for nitrogen atoms, in fair agreement with Dr. Fontana's experiments. Other theoretical studies dealing with the statistics of the condensation proc ess were carried out by Prof. Sidney Golden of Brandeis University and by Dr. Elliot Monti oil c-i Maryland Uni versity, both consultants to the project. Making only statistical assumptions, the calculations place an upper limit on the order of \i)('A atoms in a stable con figuration. Attempts to introduce mechanisms into the calculation all act to lower the values of the upper limit, and it may well be that experimental values on the order of 0 . 1 % are as high as can be achieved. Thus all indications point to concen trations of trapped radicals which are significant and useful from the point of view of investigating the properties of solids. On the other hand, the con-
eventrations achieved are far below the values desired for propulsion applica tions, at least for systems based solely on recombination effects.
Nature of the Solid Considerable effort was devoted to studying properties and structures of solids containing trapped radicals. A large number of techniques were ap plied, some for the first time to systems of condensed gases. In particular, the x-ray and electron diffraction groups made very impressive strides in apply ing these research techniques to sys tems of simple gases at very low tem peratures. The x-ray studies showed that the degree of crystallinity of the condensates is very sensitive to the way in which gases arc deposited. H. Steffen Peiser, Floyd A. Mauer, and Leonard A. Bolz of the XBS staff studied low temperature (4.2 K.) con densates of a number of substances in cluding Ar, N·., HNV IL.O, NIL,, O,, 0 : { , alcohols, and B-_»H
