E. K. C. LEE, Y. N. TANG, AND F. S. ROWLAND
3 18
The Gas Phase Reactions of Recoil Sulfur Atoms with Carbon Monoxide and Carbon Dioxide1
by Edward K. C. Lee, Y. N. Tang, and F. S. Rowland Department of Chemistry, University of K a n s a s , Lawrence, K a n s a s
(Received September 8, 1968)
Recoil S3jatoms from the nuclear reaction Cla5(n,p)S35 have been produced in CO-C2F4C12 and CO2-CzF4C12 gaseous mixtures. Carbonyl sulfide (OCSa5) has been isolated from each system by radio-gas chromatography. The reaction with C 0 2 is probably initiated by an energetic species and accounts for about 2% of the total Sa&. The reaction with CO is probably a scavenger reaction for thermal S85atoms and accounts for as much as an estimated 85% of the total Ss5production. The yield is greatly reduced in the simultaneous presence of 02 and CO. The very high yield of OCSa5from CO systems indicates a convenient synthetic method of preparation for chis labeled molecule.
Introduction The sulfur atom created by the nuclear reaction ‘2136( ~ , P ) has S ~ a~ recoil energy of about 31 kev.,2 easily sufficient to break the atom loose from the original bonding environment of the C135. This reaction is thus a good potential source for study of the atomic reactions of sulfur, complementing the studies carcied out by other methods, e.g., the photolysis of carbonyl sulfide (O=C=S.) . 3 - 5 Comparisons of experimental results obtained with diffe’rent sources of sulfur atoms should greatly facilitate the detailed understanding of the chemical reactions involved. The nuclear recoil atoms have the particular asset that they possess high kinetic energies and can perhaps undergo chemical reactions unavailable to atoms of lesser energy. Recoil C135(n,p)S36 atoms have previously been studied in various solid phases, but gas phase experiments have been only briefly mentioned.2 The S34(n,y)S35 nuclear reaction has been used in gas phase experiments with su!fur-containing molecules.e We have observed a large, anomalous radioactivity peak in the study of recoil tritium *reactions with certain halocarbons, especially CF9C1,and have identified this activity as OCS36. Since the radioactivity found as this compound represented an appreciable fraction of the total estimated S3jyield, it was apparenb that a rather specific chemical reaction was involved. Furthermore, the freon-type molecules are easy to handle i n vacuo and make good targets for neutron irT h e Journal of Physical Chemistry
radiation, permitting advantageous gas phase experiments with recoil S35atoms. The study of gas phase high energy chemical reactions through nuclear recoil nuclides has been carried out extensively with several radioactive species, especially C11, C14, T, and the h a l ~ g e n s . ~ - lThe ~ current studies of Sa5 reactions are following the pattern established in these earlier hot atom experiments. In order to take advantage of the precision in analysis offered by radio-gas chromatography, these experiments have involved the study of the formation of OCSa6from Sa5 This research was supported by U. S. Atomic Energy Commission Contract No. AT-(ll-)-407, and by fellowship support from the Pan American Petroleum Foundation (E. K. C. L.). R. B. Herber, “Chemical Effects of Nuclear Transform4tions,” Vol. 2, International Atomic Energy Agency, Vienna, 1961, p. 201. 0 . P. Strausz and H. E. Gunning, J . Am. Chem. Soc., 84, 4080 (1962). A. A. Knight, 0. P. Strausz, and H. E. Gunning, ibid., 8 5 , 1207 (1962). A. R. Knight, 0. P. Strausz, and H. E. Gunning, ibid., 8 5 , 2349 (1963). M. L. Hyder and 8 . S. Markowitz, UCRL-10,360-Revised’ 1962. M.Henchman, D. Urch, and R. Wolfgang, “Chemical Effects of Nuclear Transformations,” International Atomic Energy Agency, Vienna, 1961, gp. 83, 99; F. S. Rowland, J. K. Lee, B. Musgrave, a i d R . M.White, ibid., Vol. 2, p. 67. A. P. Wolf i h i d . , Vol. 2, p. 3; C . Maokay, M . Pandow, P. Polak, and It. Wolfgang, ihid., Vol. 2, p. 17. J. Willard. ibid.. Vol. 1. a. 218. (10) E. Rack and A. Gordus, J . Chem. Phys., 34, 1855 (1961); J . P h y s . Chem., 6 5 , 945 (1961).
GASPHASE REACTIOZS OF RECOIL SULFUR ATOMS
reactions with CO and COZ. Freon-114 (lJ2-dichlorotetrafluoroethane) has been used as the chlorinecontaining target for the thermal neutron reaction.
Experimental The S3j production rate can 8 3 5 Production Rate. be calculated as about 270 d.p.m. of S35/min./cm.3 STP of target molecule for irradiation under our typical conditions,ll--l3 approximately 20 times the rate observed for the S34(n,y)reaction for sulfur of natural isotopic abundance. Integral neutron doses of the order of 3 X 1014n./cm.z produce conveniently measurable amounts of activity. Chemicals. Freon-1 14, C2F4C12 (Rlatheson Co.), was used without additional purification, except for degassing in vacuo a t - 296’. Carbon dioxide, carbon monoxide, and oxygen were used directly from the tanks. The stated purity of each gas is: CO, C.P. grade, 99.5% minimum purity; COa, Bone Dry grade, 99.8% minimum purity; C2F4C12,95.0% minimum purity; oxygen (Airco), 99% minimum purity. Carbonyl sulfide was prepared for gas chromatographic calibration by treating KSCN with dilute sulfuric acid.14 Sample Preparation and Irradiations. All samples were prepared by the techniques developed for recoil tritium irradiations. The irradiation bulbs, made from 1720 Pyrex glass, had volumes of 10-15 ml. and were equipped with break-seals. The samples were irradiated in the Lazy Susan facility of the TRIGA reactor of the Omaha VA Hospital a t an ambient temperature of 20’. The nominal flux of 1.0 x 10” n./cm.2/sec. was reduced to 5.5 x lo1@n./cm.2/sec. by the Bl0 content of the Pyrex 1720 glass. l5 Radio-gas Chromatography. The irradiated samples were stored for 3 weeks to allow decay of possible short-lived species and then were analyzed by radiogas chromatography. Two gas chromatographic columns were used for separation of the various components. The activity peak emerged only shortly before the large macroscopic peak of CnF4C12on each column (DAIS column: OCS a t 42 min., C2F4C12at 44 min.; Safrole column: OCS a t 58 min., C2F4C12 a t 62 min.). As with many halocarbons,16 C2F4CI2acts as a quenching agent when macroscopic amounts are present in the counting volume. The tail of the OCS35 peak was partially quenched in the DAIS runs because of incomplete separation, and all quantitative measurements were made with the Safrole column. The characteristics of these two g.1.c. columns are: (a) 50-ft. Safrole column, 30% by weight on firebrick,
319
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O’, helium flow rate 0.48 ml./sec., (He C3H8) flow rate, 1.43ml./sec. ; (b) 50 -ft. dimethylsulfolane column, 30y0 by weight on firebrick, 2 4 O , helium flow rate 0.43 ml./sec., (He C3H8)flow rate, 1.05 ml./sec. The irradiated sample bulbs were broken open within a vacuum line, and an aliquot (43%) of the contents was transferred to the gas chromatographic injection loop. After g.1.c. separation of the molecular components, propane was added to the helium flow gas to give a good proportional counting gas.16 The &activity of S35(E,,,, = 0.167 Mev.) was measured by internal, flow, proportional counting, in the same counting arrangement used for tritium assay. For purposes of estimation of the per cent of total activity found as OCS3j, the counting efficiency was assumed to be 100% within the 85-m1. activity volume of the counter. Corrections were made for S35decay after irradiation; the loss of activity by recoil to the walls was assumed to be negligible for this recoil energy and the gas pressures involved. Identification of Radioactivity as X35. Although nlo other long-lived active isotope was expected in such yield from nuclear reactions with the known components, thecounting systemis indiscriminateand furnishes no check information on the identity of the radioactivity. Carrier carbonyl sulfide was added to one sample, and both it and its accompanying radioactivity were trapped from the g.1.c. flow stream with a liquid Xz trap. The contents were chemically converted to BaSO4,I7 mounted on an aluminum planchet, and counted with a, Sharp Low Beta counter. The subsequent decay curve confirmed the radioactivity as S3j, in a yield corresponding to that observed in the radio-gas chromatography. Radiation Damage. Radiolysis will be caused in these samples by the general background level of irradiation and by the (n,p) reaction itself. S o precise measurements are available about over-all per cent decomposition, but this level is less than 0.1% as measured by the appearance of minor peaks. The radiolysis problem is not completely negligible, however, for
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Sab: half-life 86.4 days’%; thermal neutron cross section for 0.01 barn’?; neutron flux, 5 X 10’0 C136(n,p)S36 = 0.30 n./cm.S/sec.; assuming two C1 atoms per molecule. (12) R. D. Cooper and E. S. Cotton, Science, 129, 1360 (1959). (13) H . Berthet and J. Rossel, Helu. Phys. Acta, 2 8 , 265 (1955); the value 0.17 =k 0.04 is frequently quoted. (14) See J. W. Mellor, “A Comprehensive Treatise on Inorganic; and Theoretical Chemistrv,” Vol. 5 , Longmans, Green and Co., New York, N. Y , 1924, p. 972. (15) E. K. C. Lee and F. S. Rowland, J . A m . Chem. Soc., 85, 897
(11)
*
(1963). (16) J. K. Lee, et al., Ana7. Chem., 34, 741 (1962). (17) See F. T. Treadwell and W. T. Hall, “Analytical Chemistry,” Vol. 2, John Wiley and Sons, New York, N. Y., 1951, p. 690,
for the procedure.
Volume 68, hTumber
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the presence of O2 apparently facilitates radiolytic formation of C02 from CO and removal of C02. These radiation chemistry conclusions are necessarily qualitative, since the experiments were not specifically designed for such studies. The per cent radiolytic effect on CO and C02 may be 10-15%, through preferential reactions of some radiolytic reactive species. The recoil tritium studies in which OCS3j was first identified were carried out with CFsC1, CHFZC1, CH2FC1, and CH2Cl2. The amount of OCS35 found in each sample varied widely, but corresponded to >75% of the S3j produced in CF3C1, and about 50% with CHF2C1. Kone of these samples contained more than 0.1% CO or COz, indicating that the scavenger action of very minor components must be quite substantial.
Results A typical irradiation in these experiments created about lo5 d.p.m. of S3j activity, which should be subsequently found distributed among various sulfurcontaining species. Since the number of possible compounds is relatively large, and the gas chromatographic separations are not yet worked out (P502, S3GC12,S3jcontaining halocarbons, etc.), the initial studies have been limited to the measurement of OCEY5. Other active compounds of comparable volatility could have been observed, but none was found by our present experimental techniques. The observed yields of OCS35 from CO and C02 mixtures with C2FbC12 are listed in Tables I and 11.
Table I : O=C=S36 Mixtures
Sample -Pressures, no.
CaFaClr
318 319 320 321 322 323
67.0 69.6 76.7 62.9 72.2 77.9
Yields from Recoil Sa6 in CO-C2F4Cls
cm.--CO
Oa
10.8 0 7.6 0
Observed activity," counts
35,420 29,270 17,360 1.1 0 9920 15.8 1.3 2210 6.5 1.3 1290 1.6 1.3
activityb
Yo yieldb
f 200 2280 & 20 85 f 180 1680 f 10 63 f 140 866 f 8 32 i. 100
=!= 60 f 50
583 f 6 103 f 3 64 f 3
22
3.8 2.4
a Actual number of counts observed in the 85-ml. counter Relative activity, during the passage of the O=C=S36 peak. defined as lo3 times d.p.m./min. of irradiation a t 5.5 X 1O1O n./cm.z/sec., per ~ m . ~ / c m of. C2F4C1, pressure. If all of tjhe calculated S36 activity appeared as 0=C=S36, the relative activity would be 2680. The relative activity has been obtained from the observed number of counts by corrections for: pressure of C2F4C12,volume of bulb (11.1-13.3 ml.), irradiation time (59-63 min.), size of first aliquot (42.5-43.4%), and decay of Sa6 (22 to 29 days).
The Journal of Physical Chemistry
LEE, Y. N. TANG, AND F. S. ROWLAND
Table I1 : O=C=S36 Mixtures
Yields from Recoil
Sample ---Pressures,
Observed activity,a
om.---
no.
CaF4Cla
COa
315 316 317 324 325 326
63.4 70.1 76.1 67.5 74.9 76.5
10.5 6.0
1.1 12.4 4.5 2.2
S36
0 2
0 0 0 1.3 1.3 1.3
counts
1280 2500 2870 1020 890 1010
in C02-C2FaCh
Relative O=C=SSn
activityb
f 50 86 f 4 f 60 146 f 2 f 70 152 f 3 f 50 53 & 3 f 50 43 f 2 f 50 47 f 3
yo yieldb 3.2 5,5 5.7 2.0 1.6 1.8
See footnotes a and 6 of Table I.
The ratio of CO or COz to CzF4C12was varied in three samples each for an indication of relative efficiencies of reaction. A second set of three samples each with CO and with COz was run in the presence of Oz,on the plausible assumption that molecular oxygen might be a good scavenging agent for thermal Sa5 atoms. The yields of OCS35are quite high from CO-containing ampoules and have been estimated in the last column to account for as much as 85% of the total S35 production. These percentages have considerable uncertainty attached, because 100% is a calculated figure without independent experimental check. The neutron flux in the TRIGA reactor contains a large fast neutron component, for which the C135(n,p)S35 cross section approaches the thermal but no correction has been made. Accurate percentage calculations will require calibration of the S35production rate for the neutron spectrum actually available in this reactor.
Discussion Chemical Nature of the Reacting Sulfur Species.
Relative O=C=SX5
E(. C.
While the radioactivity identifies with certainty the particular nuclide formed in the nuclear reaction, very little information is directly available about its precise properties a t the time of chemical reaction. The charge state, electronic state, and kinetic energy are not definitely known a priori, but must be deduced from the experiments themselves. The extensive measurements made with recoil tritium atoms have assumed that the observed chemical reactions are those of a neutral, ground-state atom with excess kinetic energy.'~~However, the especially favorable circumstances for tritium (ionization potential > I.P. of most target compounds; first excited (18) The cross section is given as 125 mbarns for 14-Mev. neutrons in D. L. Allan, Nucl. Phys., 24, 274 (1961).
GASPHASE REACTIONS OF RECOIL SULFUR ATOMS
electronic state > 10 e.v.) are not found in the case of atomic sulfur. The initial charge state of the S35 formed by this (n,p) reaction is not completely certain. Ionization of a moving atom by virtue of its own high velocity can be crudely estimated from the relative velocities of the atom and its bound electron^.'^ This calculation is often summarized by the rough rule-of-thumb that ionization begins when the recoil energy in kilovolts equals the mass in atomic mass units- the S35atom is in the borderline region for which ionization cannot automatically be either excluded or assumed, although most of the atoms probably do not ionize.Ig The ionization potential of the free S atom is 10.36 e.v.Zo and the neutralization of S+ by reaction with other molecules will be endothermic in many materials, including all of those used in the present experiments.21 The ground state of the neutral sulfur atom is 3Pz, with the 3P1and 3P, states about 1130 and 1640 kcal./ mole higher.20 Two more electronic states also need to be considered, ID2 a t 26,390 kcal./mole and ‘So at 63,370 kcal./mole above the ground state. Some of the reactions of photolytically produced sulfur atoms (see below) have chemical behavior characteristic of singlet reactants and are assumed to involve ID atom^.^!^ The available evidence from spectroscopy and mass spectrometry thus suggests that the neutral species (”, ID, IS) and S+ (perhaps in various electronic states) cannot be readily eliminated from consideration as at least partial contributors to the observed reactions of recoil S35. Thermochemistry of OCS Reactions. The over-all reactions leading to the formation of OCS35in these systems are reactions 1 and 2 if the initiating species is neutral. 535
+ co2+ocs35 + o + co +ocs35
s35
(1) (2)
The values of AH for these reactions are 67.2 kcal./ mole endothermic for (1) and 59.6 kcal./mole exothermic for ( 2 ) ,with all species in the respective ground states.22 The ionic equation corresponding to ( l ) , as given in eq. 3, is even more endothermic (142 kcal./ mole) because of the differences in ionization potential.
s++ c02 +OCS + o+
(3)
Reactions of Energetic Atoms. The direct reaction of 535 atoms with COZ, as given in eq. 1 is sufficiently endothermic to eliminate it from consideration except as a reaction initiated by energetic atoms. The two chief possibilities are that (a) the reacting S35 species is in the lS electronic state, thereby almost entirely
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accounting for the energy deficit through the electronic excitation; or (b) the sulfur atom possesses extra kinetic energy sufficient to balance the energetics. Current interpretations of recoil tritium experiments suggest that even larger amounts of kinetic energy (5 e.v.) are possessed by some of the tritium atoms as they enter the bond-forming reaction.16 Experimentally, the OCS35yields obtained from COPC2F4Cl2 mixtures suggest the existence of such an energetic atom reaction, especially since the yields are relatively little affected by the introduction of O2 into the system. Seither, however, is the OCS35 yield appreciably affected by the concentration of COz in the gas mixture. The latter observation is compatible with the existence in the system of an energetic sulfur atom species, which is basically unreactive toward CZF4Cl2,but which reacts readily with COz according to eq. 1. The distinction between electronic excitation (IS) and excess kinetic energy (3P or ID) cannot readily be made from the present experiments. Kinetically excited S atoms would be expected t o lose energy rather rapidly in collisions with C21~4C12,and hence not be ((unreactive”in the sense required above. Electronically excited S atoms, however, might also be expected to be converted rapidly to 3P or ID by collision, and hence also not be “unreactive.” If the atom in either case is able to maintain its excess energy through about 10 or 20 collisions, then the observed experimental results can be rationalized. The chemical characteristics of S atoms in the IS state, for which eq. 1 is only 3.8 kcal./mole endothermic, are totally unknown, so that it is not possible a t present to ascertain independently its reactivity toward c?P‘4C11. The approximately 2% yield of OCS35may arise either from a specific reaction of a 2% fraction of atoms in this excited state, or as one of the products of reaction of a larger component of the total S3jatom flux. The increase in OCS35with decreasing COZ concentration in 02-free samples may include partial contributions from the reactions of S35with small amounts of radiolytic C0.23 The qualitative side observations (19) See ref. 2 ; the calculated electron velocity is about 20% of the atomic velocity. (20) Ionization potentials: 0, 13.61 e.v.; S, 10.36 e.v.; data from “Atomic Energy Levels,” N.B.S. Circular 467, 1’01. 1, 1949. (21) Ionization potentials: CO, 14.01 e.v.; COI, 13.79. e.v.; 02, 12.2 e.v.; CZaClz, e&. 12 e.v., from CClzFz, 11.8 + 0.5 e.v., and CClFa, 12.8 + 0.2 e.v.; data from F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press, Xew Y o r k , N. Y., 1957. (22) A H f values obtained from “Selecte& Values of Chemical Thermodynamic Properties,” N.B.S. Circular 500, 1952: S, 53.25 kca1,imole; 0, 59.16; CO, -26.42; COm, -94.05; O C S , -32.80,
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E. K. C. LEE, Y. N.TARTG, ASD F. S.ROWLAND
in the recoil tritium systems suggest that the scavenging reaction is quite efficient with trace components and hence difficult to regulate. The O2 concentration s ocs ----f sz CO (5) in the other samples should be sufficient to divert the S35atoms away from any CO impurity. Reaction 5 is not possible in our experiments since the Scavenging Reaction of CO. The data of Table I is present only in carrier-free tracer levels. The show that recoil S3j is very efficiently converted to successful observation of a scavenger-type reaction in OCS35in the presence of CO, presumably by reaction our system implies that competitive reactions similar 2. The very high yields of labeled carbonyl sulfide to (5) must be nearly absent. indicate that the atomic sulfur species involved canThe recent observation of characteristic singlet not react very efficiently with C2F4C12, the major conreactions (insertion in cyclopropane and several stituent of each sample. A similar statement can be alkanes to form mercaptans) have been convincingly made about CF3C1from our experiments, and has been interpreted as the reactions of photolytic sulfur atoms made about SFe for S3j atoms formed through the in the ID ~ t a t e . ~Comparable ,~ experiments are now S34(n,y)nuclear reaction.6 Certainly, there is no inbeing carried out with recoil S35as an aid to identificasertion reaction into C-F and C-C1 bonds in these extion of the reactive S forms present in our systems. periments that can approach the efficiency of ID sulfur Applications to Other Experiments: Synthesis OJ” atom insertion into C-H b o n d ~ . ~ J OCS”5, Fast Separations in Nuclear Chemistry. The Reaction 2 is quite exothermic and requires eventually very high yields of carrier-free OCS3j obtained in CO a third body collision for stabilization of the product. scavenged chlorocarbons suggest that useful syntheses If extra kinetic or electronic energy were available, of this labeled molecule can be simply performed. the molecule would be still more excited, and a faster None of these experiments has been carried out in decomposition rate could be anticipated, leading to the high flux to determine the possible limits to the total back reaction of eq. 2 . Very probably, reaction 2 activity available by such a technique; high specific occurs primarily with thermal S35atoms and illustrates activity seems assured, however, for both of the an efficient method of scavenging these atoms to form molecules required should be obtainable free from sulfura readily measured product. containing impurities to a very high degree. An estimate of relative reaction efficiencies can be The high total yield of S35activity in a simple, readily made by comparison of the percentage OCS35yields separable gaseous compound implies that these revs. 0 2 and CO concentrations. The OCS35 yield is actions can be used for very rapid isolation of sulfur rapidly diverted or suppressed by the 0 2 , even when its radioactivities in nuclear chemistry problems. Carbon concentration is much lower than that of CO. If this monoxide scavenging of tellurium and selenium isotopes change is entirely attributed to the greater efficiency seems quite likely to give OCTe and OCSe and could of reaction per O2 collision than in CO collisions, a be useful for fast separations of short-lived fission factor of about 10-20 is indicated, favoring reaction products. with 02. In view of the possible complexities in these systems (e.g., radiation effects), other factors are Acknowledgment. The neutron irradiations were likely also to be affecting these percentage yields. carried out with the kind cooperation of the reactor Comparison with Photolytic S Atoms. Sulfur atoms staff of the Omaha Veterans Administration Hospital. have been produced from the photolysis of gaseous OCS by several groups of e x p e r i m e n t e r ~ , ~ - ~ , ~ ~ ’ ~ j (23) Radiolysis of COS leads t o CO formation. See, for example, and possibly analogous reactions have been known for P. Harteck and S. Dondes, J . Chem. Phys., 23, 902 (1955): nearly a century.2G The scavenging reaction of eq. 2 26, 1727 (1957). was found to be negligible following photolysis of OCS (24) G . S. Forbes and J. E. Cline, J . Am. Cham. Soc., 61, 151 (1939). at 228 mH, under the experimental conditions involved.24 ( 2 5 ) V. Kondratiev, Acta Physicochim. U R S S , 16, 272 (1942). IIowever, the mechanism for this photolysis reaction (26) G . Chevrier, Compt. rend., 69, 136 (1869). A mixture of sulfur has been suggested to include reactions 4 and 5 as the vapor and carbon monoxide was sparked, with carbonyl sulfide reported as a product. first two steps.25
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T h e Journal of Phusical Chemistry
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