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Gas phase recoil phosphorus reactions—IV Effect of moderators on the abstraction reactions
I. ino~. Rmcl. Clu,m. Vol. 41, pp. 785-789
Pergamon Press Lid., [979. Pnnted in Great Britain
GAS PHASE RECOIL PHOSPHORUS REACTIONS--IV EFFECT OF MODERATORS ON THE ABSTRACTION REACTIONS O. F. ZECK, R. A. FERRIERI, C. A. COPP, G. P. GENNARO and Y.-N. TANG Department of Chemistry, Texas A&M University,College Station,TX 77843, U.S.A.
(Received 25 October 1977; receivedfor publication 15 November 1978) Abetnct--The use of helium, neon and argon as moderators in recoil np systems has shown that the F-abstraction from PF3 to give 32PF3is not diminishedby moderation while the H-abstraction from PH3 to give 32PH3decreases to as low as 27% of the unmoderated value with the addition of 98% neon. These results are consistent with the supposition that the H-abstraction from PH3 by 32p atoms has a reaction cross section curve covering a wide energy range with a rather low threshold. The corresponding F-abstraction appears to involve thermal reactions of 32p with a much narrower energy range for its reaction cross section curve.
INTRODUCTION Inert gases are often employed as moderators in hot atom systems to thermalize the energetic species and therefore to distinguish the higher energy processes from the lower energy ones[l]. A classical example is the observation that all the hot products can be eliminated by the addition of a large excess of various moderators in a well-scavenged recoil tritium-methane system[2]. These results eventually led to the formulation of the Estrup--Wolfgang kinetic theory of hot atom reactions[l, 2]. Similar moderator effects have also been observed for other univalent hot atoms such as *'F and ~C113, 4]. For multivalent hot atoms, the effect of moderators on recoil *~C reactions with hydrocarbons is to increase the yield of the expected thermal products such as I'CO, and to decrease the yields of hot products such as acetyleneI'C and ethylene-'*C[l , 5, 6]. Similarly Gaspar and coworkers have observed in recoil 3'Si reactions with Sill4 that the yields of hot products such as 3~SiH4 decrease while the yields of thermal products such as 3~SiSiH6 increase with neon moderation[7]. However, for the recoil "C systems where the product yields are decreased but not eliminated at high moderator concentrations, Woifgang explained this observation by the supposition that the product formation reactions can occur with thermal as well as hot HC atoms[I]. As demonstrated in some recent studies, recoil 32p atoms abstract H and F atoms very efficiently from PH3 and PF3 molecules[8--12]. Stewart and Hower[8] have studied the effect of Ne on the product yields in a 93% PF3-7% PH3 mixture and have found that the 32pF3 yield stayed constant at about 2%, while the 32PH3 yield decreased from 30% in the unmoderated binary system to 12% in the essentially 100% moderated system. They concluded that 32PF3 is formed by a thermal reaction whereas 60% of 32PH3 is formed by a hot reaction and 40% by a thermal reaction. Other recoil 32p moderator studies include the decrease of product yields with argon moderation in the 32p reaction with trimethyiphosphine as observed by Halmann[13]. The use of argon as a moderator in the 32p reaction with PCI3 systems has also been studied[14, 15]. In the present work we have carried out a detailed systematic moderator study for two types of 32p abstraction reactions: (1) F atoms from PF3; and (2) H atoms from PH3. The difference in moderating efficiency
was also tested with the use of helium, neon and argon for different studies. The ultimate goal of this work was to obtain some pertinent information about the reaction cross section curves of these two abstraction reactions with respect to their relative energy ranges and their qualitative characteristics. EXPERIMENTAL Sample preparation. The general procedure used in this study was the same as that used in other nuclear recoil experiments[12]. Phosphorous trifluoride and phosphine each were sealed in Pyrex 1720 ampoules together with the desired moderator using standard high vacuum techniques. Pyrex 1720 instead of ordinary Pyrex was chosen because it gave reproducible results. A typical set of samples consisted of two with pure PF3, and eight samples of varying compositionwith the addition of 15-98% moderator, mostly in the high moderation range. The total pressure of each sample was maintained at 800 ton'. For moderated PH~ studies, a pure PH3 sample was also included in the set of ten. Irradiation. Phosphorus-32 from the 3*P(n,~) 32p nuclear transformation was formed using thermal neutrons from the Texas A&M Unive~ity Nuclear Science Center Reactor. Irradiations typically lasted for 30-40rain while exposing the outside of the rotisserie to a neutron flux of approx. 5 × 10'° n/(cm2sec). The longer irradiation time in this study compared to those in the previous studies arises from the fact that many of the samples here were highly moderated which therefore implieda lower PF3 or PH3 concentration and a concomitant lower 32p activity. By raising the irradiation time, 32p activities were obtained which were statistically valid even at high moderator concentrations. Sample analysis. After irradiation, a special procedure was used to transfer the nPF3 or 3zPH3to the injection loop because the noble gases which were used as moderators inhibited the condensation and trapping of these products at T/°K. For this reason, a vacuum was used to slowly pull the non-condensable gases mixed with labeled products through a trap after first attempting to condense ~ sample using ordinary trapping techniqnes for 5 rain. The trapped sample was then analyzed on a 0.25 in. × 8ft polyethylene column of Porapak Q, 50-80 mesh. PF3 and PH3 had retention times of 7 and 22 rain, respectively, when the column was operated at 25°(: with a helium flow of 28 mi/min. Ckemicais. Phosphorous trifluoride (>97%) was obtained from PCR, Inc. and purified by Ixdb-to-bulh distillation. Phosphine (99.596), argon (>99.9%), helium (>99.9%) and neon (>99.9%) were all obtained from Matheson. Phosphine was
de~ four times prior to usage but the noble gases were used without further purilkafion.
785 JINC VO4.41, NO. 6- A
O. F. ZECK et al.
786 RESULTS
In this work, the normalized ~2pF3 and ~'PH~ specific activities, NSA(~'PF~) and NSA(~2pH~), were calculated as described previously[12]. In Fig. 1, NSA(32pFg values have been plotted as a function of moderator concentration. It is apparent from this figure that no matter what type of concentration of moderator is used, nearly constant (or slightly increasing) NSA('2PFg values are observed. The NSA(~=PH~) values for different moderator systems are presented in Fig. 2. The curves in this figure were fitted by a least-squares calculation. A general feature of each system is that as the moderator concentration is increased, the NSA(WZPH~) values decrease. The order of decreasing moderator
140
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0
120
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0
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0
0 0
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Neon
12C
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~
O
O
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O O O
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efficiency is: Ne > At> He. The least-squares lines indicate that the NSA(32pH3)value at approximately 98% neon is 27, while that of 98% argon is 56, and that of 98% helium is 80. DISCUSSION
A dilemma in recoil 3:p abstraction reactions Although the reactions of recoil 32p atoms with PF3 to give 32PF3 and PH3 to give ~2PH3 have been thoroughly studied, there are some seemingly contradictory observations. In the first place the absolute yield of nPF3 from pure PF3 systems is recorded as 38% while that of 32pH3 from pure PH3 systems is 78% [8]. This seems to mean that PH3 is about twice as reactive as PF3 towards recoil 32P atoms. However, in a binary mixture of these two subslrates as indicated by both Stewart and Hower's and our studies[8, 11, 12], PH3 is more reactive than PF, by a factor far greater than two. In fact, in a 7% PH3 and 93% PF3 system the 32PH3 yield is about twenty times higher than that of 32PF318]. This implies that PH3 is some 200 times more reactive than PF3. This dilemma cannot be resolved unless the two abstraction reactions involved occur at very different energy ranges. A shadowing effect must be in oEeration here where the higher energy process removes " P atoms from the reaction pool such that there are not many reactant atoms left for the lower energy process. If the higher energy process is efficient enough to capture most of the reactant atoms, it should show an apparent predomination even if both reactions have similar integrated areas for their reaction cross section curves. Threshold energies for the F.abstractions by 3:p atoms
O
The fact that there is no product-diminishing effect by either He, Ne or Ar as moderators on the 32PF3 yields 0 indicates the F-abstraction by ~'P atoms must have been 8O a threshold energy in the thermal range. When the energetic 321) atoms are thermalized by collisions with the t) 2o ~o 4o ~ go /o so so ~oo inert gases, they are still capable of F-abstraction from PF3 due to such a low threshold. The fact that the Modt~or Mole Frocllcn Effects of inert gases on the normalized nPF3 specific absolute yield of 32PF3 is always 38% and is independent of moderator concentration indicates that a fixed fraction activities. 0
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£=1
o
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| 2o 10 o
.~
~
~
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~
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.;o
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Moderator Mole Fraction rig. 2. Effects of inert igaseson the normalized ~Ph3 specific activities: (O) He; (U) Ne; ( x ) At.
Gas phase recoil phosphorusreactions--IV (62%) of 32p atoms are involved in some non-abstractive processes. Since previous studies have shown that the absolute yield of 3~PF3 is independent of pressure[10] in the range of 50-800 torr, the loss of 32p atoms, ~2PF or 32PF2 radicals to the wall must be minimal. This means that whatever resulted from the other processes is either not detectable by our analytical methods or is lost to the walls of the reaction vessel. In some previous studies ~2p activities have actually been recovered from washing of the reaction vessel with suitable solvents[16]. We have rinsed several of our reaction vessels with acetone and water, but the amount of radioactivity recovered was small.
Threshold energy for the H-abstraction reaction by 32p atoms It is generally believed that in a nuclear recoil system if a product yield is decreased but not eliminated at high moderator concentrations, the product formation reactions can occur with thermal as well as hot atoms. This belief is not necessarily true, and is likely to be subject to certain restrictions. If a certain reaction has a cross section curve extending from the hot region into the thermal reaction range and therefore is capable of undergoing thermal processes, all those species capable of reacting but failing to do so in the hifher energy ranges during thek energy cascade should eventually react as thermal species. As a result, the total number of reacting events should remain the same and not be diminished by moderation. This is exactly the case which we have observed above for the PF3 system. On the other hand, any process exhibiting a pronounced moderating effect may or may not possess a threshold for its reaction cross section curve in the thermal energy range. There are two possible cases corresponding to this kind of experimental observation which are described in detail below. For any system exhibiting a pronounced moderating effect, one possibility is that the threshold for its reaction cross section curve is above the thermal energy range. Consequently, all the hot atoms which are moderated directly from above the chemical reaction range into the thermal range without colliding or interacting with the reactant molecules become victims of moderation and fail to give successful reactions. However, if the same reaction can also take place in the thermal range, such victimized thermal atoms would have a chance to eventually react to give the same product, and a pronounced moderating effect is not expected to be observed. What this means is that whenever a product yield is sigai[tcaally decreased at high moderator concentrations, it is possible that the process is a hot reaction which does not have a thermal counterpart. However, for the PH3 system, the threshold cutoff may not be very much above the thermal range judging from the readiness of H-abstraction of 32p atoms. Another possible explunation[17] for a pronounced moderating effect is that there is an additional competing process in the thermal range with a reaction probability which is much higber than that of the reaction under consideration. This means the studied reaction will become less competitive or even insi~ificant in the thermal energy range, and therefore under hi~ moderating conditions, which shift the energies of the reacting species from the hot range into the thermal range, the observed product yield of the reaction will be significantly decreased. In other words, the PH3 system
787
is effectively self-scavanged by this non-abstractive competing process. In the PHr-PF3 mixture system, the fact that a minor amount of PH3 causes a tremendous drop in the 32PF3 yield could also be explained by the presence of such a highly efficient thermal reaction. With the above reasoning, the fact that the Hemoderated 32pH3 yield, as shown in Fig. 1, could be extrapolated to 27% of the normalized 32PH3 specific activity of a pure PH3 system, indicates that its reaction cross section curve has a threshold which is either slightly above the thermal energy range, or extended somewhat into the thermal range. However, if the latter case is true, the H-abstraction reaction is likely to be internally suppressed by some other highly efficient thermal processes.
Some mechanistic considerations The abstraction reactions of 32p atoms may proceed via either a single step mechanism as illustrated in (1), or a multiple step mechanism as illustrated in (2)-(4). 32p + px3 ~ 32px3 + p
(I)
32p + PX3 ~ 32PX+ PH2
(2)
;2PX + PX3 ~ 32PX2+ PH2
(3)
32PX2+ PX3-~ 32PX3+ PH2
(4)
From the general considerations, the multiple step abstraction mechanism is definitely more likely. But no information concerning the possible existence of the single step process can be deduced from the results of the present moderator studies. If the multiple step mechanism prevails, it is probable that the abstraction is really the net consequence of an insertion-decomposition process as proposed for the recoil HC systems[18]. In the present case, the insertion of 32p atoms into PH3 will give 32pH-PH2 radicals which may decompose by the cleavage of the P-P bonds to give 32PH. The resulting 32pH, which is a carbene analog, may insert into another PH3 molecule to give the molecule 32pH~-PH2 which may possess enough excitation to cleave its P-P bond yielding 3ZPH2 radical. The subsequent H-abstraction by the a2PH2 radical will give 32PH3 as the final product. The series of reactions involved in this possible insertion-decomposition mechanism are summarized below. 32p, + PH3 ~ 3"pH - PH2*
(5)
32PH - PH2* --, 32PH* + PH2
(6)
32pH* + PH~-~ 32PH2- PH2
(7)
32pH2- PH2* " 32pH2+ PH2
(8)
3"PH2+ PH3-* 32pH3'+PH,
(9)
It should be noted that for the above mechanism to operate the resultant edduct should always possess enough excitation energy to cleave the P-P bond. Therefore it is possible that the additional competing processes in the thermal energy range with a high reaction probability as mentioned in the previous section, could be processes such as thermal counterparts of reactions (5) and (7). If the 32P-containing species
788
O.F. ZECK et al.
derived from such reactions lack energy for the decomposition, they may undergo some other non-abstractive process to give products other than 32PH3. If such reactions are highly efficient in the thermal energy range, they essentially act as scavangers for the thermal 32p_ containing species in such nuclear recoil systems. Of course, the thermal counterparts of reactions (5) and (7)~ are not necessarily the operating scavanger reactions in these systems. Other processes may he actually respon- ' sible for the observed changes.
any such contributions, it is obvious that the hot yield is above 73%. Even if the "hot" contribution in the highly moderated systems is zero, the above reasoning employed by Stewart and Hower still does not hold. In such a case, all the 32p atoms will be thermalized before reaction. The observed 32PH3 yield will then represent a direct competition between the thermal H-abstraction reaction and the highly efficient thermal "scavanging" reactions as discussed in the previous section. The 27% value is definitely not derived from the net result of any direct competition between the hot and thermal H-abReaction cross section curve for H-abstraction by 32p straction processes. On the quantitative side the absolute yield of 32PH3 in atoms Judging from the information obtained in the present the pure system is 78% while that in the highly work, the reaction cross section curve for H-abstraction moderated system is only 21% (calculated from 27% and by 32p atoms should be high and should cover a wide 78%). However, in the pure PH3 system the number of energy range regardless of whether the reaction posses- 32p atoms available for thermal reactions is limited, and ses a threshold which is either above or in the thermal is much less than the number available in a highly moderated system. As a result, the thermal contribution energy range. In the first place let us assume that the H-abstraction of the 32PH3 yield in a pure PH3 system is expected to be possesses an above-thermal threshold energy. With this much less than 21% of the absolute yield, which means assumption, the fact that even with 98% neon modera- that the "hot" contribution to the ~PH3 absolute yield is tion a yield of 27% of 32PH3 is still observed is consistent likely to be 60% or above. This amount of absolute yield with the supposition that the reaction cross section curve is definitely very high among the hot reactions of various for H-abstraction by 32p atoms covers an extremely wide recoil atoms, especially when we take into conenergy range. In fact, the energy range should be so wide sideration that the "total" H-abstraction cross section is that for a batch of 3,p atoms with energies above the likely to be the product of the three individual ones for chemical reaction range, the energy degradation by 30 or reactions (2)-(4). Such high absolute yields should cor50 collisions with neon will leave about 30% of them with respond to a reaction cross section curve which is high in an above-thermal energy. At the high energy end such a magnitude and broad in its energy range. curve may extend to as high as 50 eV or above. The magnitude of the reaction cross section should be fairly Reaction cross section curve for F-abstraction by 32p high judging from the 78% 32pH3 absolute yield observed atoms It has been previously maintained that F-abstraction in the pure system and the relatively low collision density per energy interval expected for the energy by 32p atoms has a threshold in the thermal range. From degradation for such a reaction in the higher energy the various experimental observations, it can he deduced that the reaction cross section of this F-abstraction is ranges. Strictly speaking, the extrapolation of the moderator likely to he low and narrow. For a reaction which can curves to pure (100%) moderator systems with a positive extend into the thermal energy range where the collision intercept has no physical si~ificance, Theoretically all density per unit energy interval is extremely high, a 38% the moderator curves should drop to 0% yields at infinite absolute yield can only transcribe into a fairly low reacdilution. The lines in Fig. 2 are meaningful only as far as tion cross section per collision. The narrowness of the there are data points. A 27% normalized 32PH~ specific F-abstraction cross section curve can be deduced from activity at 98% neon moderation means that even with the observation as shown in Fig. 3 where we have plotted such a large excess of inert gas only 73% of the reacting Stewart and Hower's 32PH3 yields from the PF3-PH3 sap atoms or 32p-containing radicals are deactivated system by treating PF3 as an additive to the PH3 directly from an energy above the H-abstraction reaction system[8]. The PH3 moderator curves [from Fig. 2) by cross section curve to an energy below its threshold He, Ne, and Ar are also presented here for comparison. without reacting with PH3 to give ~PH3. The remaining It is observed that PF3 behaves exactly like neon except 27%, although partially deactivated by moderator col- in the nearly pure PF3 systems. This means that in the lisions, fail to jump over such an extensive energy range presence of PH3, the PFs molecules behave in the same without interacting with PH3. Since the reaction prob- way as other non-reacting moderators. The inertness of ability is not likely to be unity in the entire energy range PF3 in the presence of PH3 is dramatically demonstrated it covers, there actually should be much more than 27% by the fact that even if 93% of the mixture is PF3 the of the ~P species failing to skip over the entire H- observed absolute yield of 32PF3 is only 1.5% in comabstraction cross section curve--the ones which succeed parison with a 38% yield in a pure PF3 system[8]. The in colliding and reacting with PH3 to give the eventual best way to explain this result is that H-abstraction by 32p occurs at a higher energy range than its F-abstraction product, ~PH3, is 27% of the total. In the second place, let us assume the H-abstraction counterpart. The recoil ~2p atoms, mostly being energetic possesses a threshold in the thermal energy range. The to begin with, undergo H-abstraction reactions and are fact that the 32PH3 yield at high moderation could be removed from the reaction pool before their energy is extrapolated to 27% of the normalized ~PH3 specific low enough to interact with PF~. The tremendous activity of a pure PH3 system definitely does not mean decrease in the ~2PF~ yield by minute amounts of PH3 that the reaction is 73% hot and 27% thermal as impliM also indicates that the overlap of their reaction cross by Stewart and Hower to explain results of similar section curves is not very extensive such that the Fsystems [8]. At 98% moderation the observed yield may abstraction cannot become competitive with the H-abor may not contain any "hot" contribution. If there were straction process. Since the H-abstraction cross section
Gas phase recoil phosphorus reactions--IV
789
100 9o 80 .~ 70; 6o
4G .N
"6 30
Q
E z~ 2o
ib
.io
.6 .PF3
.6
6b
.7'0
sb
9'0
ibo
Mole Froetion
Fig. 3. Effect of PF~ as a moderator on the normalized 32PH~specific activities (from Ref. [8]). curve is likely to be broad in its energy range and extends to a fairly low energy, the F-abstraction cross section curve therefore is likely to be much narrower. Acknowledgements---Generous support by the U.S. Department of Energy under Contract No. EY-76-S-05-3898 is gratefully acknowledged. The authors also wish to express their appreciation to the personnel at the Texas A&M Nuclear Science Center.
REFERENCES I. R. Wolfgang, Prog. React. Kin. 3, 97 (1%5): Ann. Rev. Phys. Chem. 16, 15 (1%5). 2. P. J. Estrup and R. Wolfgang, J. Am. Chem. Soc. 82, 2661; 2665 (1960). 3. I. F. J. Todd, N. Colebourne and R. Wolfgang, J. Phys. Chem. 71, 2875 (1967). 4. C. M. Wai and F. S. Rowland, J. Am. Chem. Soc. 90, 3638 (1%8). 5. A. P. Wolf, presented to the Panel on Theory of Hot Atom Chemistry, International Atomic Energy Agency, Vienna, May 1974. 6..I. Dubrin, H. Roseoberg, R. Wolfgang and C. Mackay, Chemical E~ects of Nuclear Transformations, Vol. 1, p. 133. International Atomic Energy Agency, Vienna (1965).
7. P. P. Gaspar, S. A. Bock and W. C. Eckelman, J. Am. Chem. Soc. 9e, 6914 (1968). 8. G. W. Stewart and C. O. Hower, J. lnorg. Nucl. Chem. 34, 39 (1972). 9. G. P. Gennaro and Y.-N. Tang, J. Inorg. Nucl. Chem. 35, (1973). I0. G. P. Gennaro and Y.-N. Tang, J. Inorg.Nucl. Chem. 36, 259 (1974). II. O. F. Zeck, G. P. Gennaro and Y.-N. Tang, J. Chem. Soc. (Chem. Commun.) 52 (1974). 12. O. F. Zeck, G. P. Gennaro and Y.-N. Tang, J. Am. Chem. Soc. 97, 4498 (1975). 13. M. Halmann, Chemical Egects o[ Nuclear Trnns/ormations, Vol. I, p. 185. InternationalAtomic Energy Agency, Vienna (1961). 14. H. Drawe and A. Henglein, Z. Naturlorsch.17b, 486 0%2). 15. A. Henglein, H. Drawe and D. Perner, Radiochim. Acta 2, 19 (1%3). 16. M. Halmann and L. Kugel, J. Inorg. Nucl. Chem. 25, 1343 (1963). 17. The possibilityof a highly efficientcompetitive thermal reaction was firstsuggested by a reviewer of this article.We appreciate his suggestion. 18. H. Ache, K. Taylor and A. P. Wolf, presented at the 8th International Hot Atom Chemistry Symposium, Spa, Belgium, May 1975.
Pergamon Press Lid., [979. Pnnted in Great Britain
GAS PHASE RECOIL PHOSPHORUS REACTIONS--IV EFFECT OF MODERATORS ON THE ABSTRACTION REACTIONS O. F. ZECK, R. A. FERRIERI, C. A. COPP, G. P. GENNARO and Y.-N. TANG Department of Chemistry, Texas A&M University,College Station,TX 77843, U.S.A.
(Received 25 October 1977; receivedfor publication 15 November 1978) Abetnct--The use of helium, neon and argon as moderators in recoil np systems has shown that the F-abstraction from PF3 to give 32PF3is not diminishedby moderation while the H-abstraction from PH3 to give 32PH3decreases to as low as 27% of the unmoderated value with the addition of 98% neon. These results are consistent with the supposition that the H-abstraction from PH3 by 32p atoms has a reaction cross section curve covering a wide energy range with a rather low threshold. The corresponding F-abstraction appears to involve thermal reactions of 32p with a much narrower energy range for its reaction cross section curve.
INTRODUCTION Inert gases are often employed as moderators in hot atom systems to thermalize the energetic species and therefore to distinguish the higher energy processes from the lower energy ones[l]. A classical example is the observation that all the hot products can be eliminated by the addition of a large excess of various moderators in a well-scavenged recoil tritium-methane system[2]. These results eventually led to the formulation of the Estrup--Wolfgang kinetic theory of hot atom reactions[l, 2]. Similar moderator effects have also been observed for other univalent hot atoms such as *'F and ~C113, 4]. For multivalent hot atoms, the effect of moderators on recoil *~C reactions with hydrocarbons is to increase the yield of the expected thermal products such as I'CO, and to decrease the yields of hot products such as acetyleneI'C and ethylene-'*C[l , 5, 6]. Similarly Gaspar and coworkers have observed in recoil 3'Si reactions with Sill4 that the yields of hot products such as 3~SiH4 decrease while the yields of thermal products such as 3~SiSiH6 increase with neon moderation[7]. However, for the recoil "C systems where the product yields are decreased but not eliminated at high moderator concentrations, Woifgang explained this observation by the supposition that the product formation reactions can occur with thermal as well as hot HC atoms[I]. As demonstrated in some recent studies, recoil 32p atoms abstract H and F atoms very efficiently from PH3 and PF3 molecules[8--12]. Stewart and Hower[8] have studied the effect of Ne on the product yields in a 93% PF3-7% PH3 mixture and have found that the 32pF3 yield stayed constant at about 2%, while the 32PH3 yield decreased from 30% in the unmoderated binary system to 12% in the essentially 100% moderated system. They concluded that 32PF3 is formed by a thermal reaction whereas 60% of 32PH3 is formed by a hot reaction and 40% by a thermal reaction. Other recoil 32p moderator studies include the decrease of product yields with argon moderation in the 32p reaction with trimethyiphosphine as observed by Halmann[13]. The use of argon as a moderator in the 32p reaction with PCI3 systems has also been studied[14, 15]. In the present work we have carried out a detailed systematic moderator study for two types of 32p abstraction reactions: (1) F atoms from PF3; and (2) H atoms from PH3. The difference in moderating efficiency
was also tested with the use of helium, neon and argon for different studies. The ultimate goal of this work was to obtain some pertinent information about the reaction cross section curves of these two abstraction reactions with respect to their relative energy ranges and their qualitative characteristics. EXPERIMENTAL Sample preparation. The general procedure used in this study was the same as that used in other nuclear recoil experiments[12]. Phosphorous trifluoride and phosphine each were sealed in Pyrex 1720 ampoules together with the desired moderator using standard high vacuum techniques. Pyrex 1720 instead of ordinary Pyrex was chosen because it gave reproducible results. A typical set of samples consisted of two with pure PF3, and eight samples of varying compositionwith the addition of 15-98% moderator, mostly in the high moderation range. The total pressure of each sample was maintained at 800 ton'. For moderated PH~ studies, a pure PH3 sample was also included in the set of ten. Irradiation. Phosphorus-32 from the 3*P(n,~) 32p nuclear transformation was formed using thermal neutrons from the Texas A&M Unive~ity Nuclear Science Center Reactor. Irradiations typically lasted for 30-40rain while exposing the outside of the rotisserie to a neutron flux of approx. 5 × 10'° n/(cm2sec). The longer irradiation time in this study compared to those in the previous studies arises from the fact that many of the samples here were highly moderated which therefore implieda lower PF3 or PH3 concentration and a concomitant lower 32p activity. By raising the irradiation time, 32p activities were obtained which were statistically valid even at high moderator concentrations. Sample analysis. After irradiation, a special procedure was used to transfer the nPF3 or 3zPH3to the injection loop because the noble gases which were used as moderators inhibited the condensation and trapping of these products at T/°K. For this reason, a vacuum was used to slowly pull the non-condensable gases mixed with labeled products through a trap after first attempting to condense ~ sample using ordinary trapping techniqnes for 5 rain. The trapped sample was then analyzed on a 0.25 in. × 8ft polyethylene column of Porapak Q, 50-80 mesh. PF3 and PH3 had retention times of 7 and 22 rain, respectively, when the column was operated at 25°(: with a helium flow of 28 mi/min. Ckemicais. Phosphorous trifluoride (>97%) was obtained from PCR, Inc. and purified by Ixdb-to-bulh distillation. Phosphine (99.596), argon (>99.9%), helium (>99.9%) and neon (>99.9%) were all obtained from Matheson. Phosphine was
de~ four times prior to usage but the noble gases were used without further purilkafion.
785 JINC VO4.41, NO. 6- A
O. F. ZECK et al.
786 RESULTS
In this work, the normalized ~2pF3 and ~'PH~ specific activities, NSA(~'PF~) and NSA(~2pH~), were calculated as described previously[12]. In Fig. 1, NSA(32pFg values have been plotted as a function of moderator concentration. It is apparent from this figure that no matter what type of concentration of moderator is used, nearly constant (or slightly increasing) NSA('2PFg values are observed. The NSA(~=PH~) values for different moderator systems are presented in Fig. 2. The curves in this figure were fitted by a least-squares calculation. A general feature of each system is that as the moderator concentration is increased, the NSA(WZPH~) values decrease. The order of decreasing moderator
140
Ar~
O0 0
0
0
120
O0
IOC
0
0
0
0 0
8G 60 40
Neon
12C
O O
~
O
O
O O
O O O
Oo
O
0
O O
O
efficiency is: Ne > At> He. The least-squares lines indicate that the NSA(32pH3)value at approximately 98% neon is 27, while that of 98% argon is 56, and that of 98% helium is 80. DISCUSSION
A dilemma in recoil 3:p abstraction reactions Although the reactions of recoil 32p atoms with PF3 to give 32PF3 and PH3 to give ~2PH3 have been thoroughly studied, there are some seemingly contradictory observations. In the first place the absolute yield of nPF3 from pure PF3 systems is recorded as 38% while that of 32pH3 from pure PH3 systems is 78% [8]. This seems to mean that PH3 is about twice as reactive as PF3 towards recoil 32P atoms. However, in a binary mixture of these two subslrates as indicated by both Stewart and Hower's and our studies[8, 11, 12], PH3 is more reactive than PF, by a factor far greater than two. In fact, in a 7% PH3 and 93% PF3 system the 32PH3 yield is about twenty times higher than that of 32PF318]. This implies that PH3 is some 200 times more reactive than PF3. This dilemma cannot be resolved unless the two abstraction reactions involved occur at very different energy ranges. A shadowing effect must be in oEeration here where the higher energy process removes " P atoms from the reaction pool such that there are not many reactant atoms left for the lower energy process. If the higher energy process is efficient enough to capture most of the reactant atoms, it should show an apparent predomination even if both reactions have similar integrated areas for their reaction cross section curves. Threshold energies for the F.abstractions by 3:p atoms
O
The fact that there is no product-diminishing effect by either He, Ne or Ar as moderators on the 32PF3 yields 0 indicates the F-abstraction by ~'P atoms must have been 8O a threshold energy in the thermal range. When the energetic 321) atoms are thermalized by collisions with the t) 2o ~o 4o ~ go /o so so ~oo inert gases, they are still capable of F-abstraction from PF3 due to such a low threshold. The fact that the Modt~or Mole Frocllcn Effects of inert gases on the normalized nPF3 specific absolute yield of 32PF3 is always 38% and is independent of moderator concentration indicates that a fixed fraction activities. 0
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Gas phase recoil phosphorusreactions--IV (62%) of 32p atoms are involved in some non-abstractive processes. Since previous studies have shown that the absolute yield of 3~PF3 is independent of pressure[10] in the range of 50-800 torr, the loss of 32p atoms, ~2PF or 32PF2 radicals to the wall must be minimal. This means that whatever resulted from the other processes is either not detectable by our analytical methods or is lost to the walls of the reaction vessel. In some previous studies ~2p activities have actually been recovered from washing of the reaction vessel with suitable solvents[16]. We have rinsed several of our reaction vessels with acetone and water, but the amount of radioactivity recovered was small.
Threshold energy for the H-abstraction reaction by 32p atoms It is generally believed that in a nuclear recoil system if a product yield is decreased but not eliminated at high moderator concentrations, the product formation reactions can occur with thermal as well as hot atoms. This belief is not necessarily true, and is likely to be subject to certain restrictions. If a certain reaction has a cross section curve extending from the hot region into the thermal reaction range and therefore is capable of undergoing thermal processes, all those species capable of reacting but failing to do so in the hifher energy ranges during thek energy cascade should eventually react as thermal species. As a result, the total number of reacting events should remain the same and not be diminished by moderation. This is exactly the case which we have observed above for the PF3 system. On the other hand, any process exhibiting a pronounced moderating effect may or may not possess a threshold for its reaction cross section curve in the thermal energy range. There are two possible cases corresponding to this kind of experimental observation which are described in detail below. For any system exhibiting a pronounced moderating effect, one possibility is that the threshold for its reaction cross section curve is above the thermal energy range. Consequently, all the hot atoms which are moderated directly from above the chemical reaction range into the thermal range without colliding or interacting with the reactant molecules become victims of moderation and fail to give successful reactions. However, if the same reaction can also take place in the thermal range, such victimized thermal atoms would have a chance to eventually react to give the same product, and a pronounced moderating effect is not expected to be observed. What this means is that whenever a product yield is sigai[tcaally decreased at high moderator concentrations, it is possible that the process is a hot reaction which does not have a thermal counterpart. However, for the PH3 system, the threshold cutoff may not be very much above the thermal range judging from the readiness of H-abstraction of 32p atoms. Another possible explunation[17] for a pronounced moderating effect is that there is an additional competing process in the thermal range with a reaction probability which is much higber than that of the reaction under consideration. This means the studied reaction will become less competitive or even insi~ificant in the thermal energy range, and therefore under hi~ moderating conditions, which shift the energies of the reacting species from the hot range into the thermal range, the observed product yield of the reaction will be significantly decreased. In other words, the PH3 system
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is effectively self-scavanged by this non-abstractive competing process. In the PHr-PF3 mixture system, the fact that a minor amount of PH3 causes a tremendous drop in the 32PF3 yield could also be explained by the presence of such a highly efficient thermal reaction. With the above reasoning, the fact that the Hemoderated 32pH3 yield, as shown in Fig. 1, could be extrapolated to 27% of the normalized 32PH3 specific activity of a pure PH3 system, indicates that its reaction cross section curve has a threshold which is either slightly above the thermal energy range, or extended somewhat into the thermal range. However, if the latter case is true, the H-abstraction reaction is likely to be internally suppressed by some other highly efficient thermal processes.
Some mechanistic considerations The abstraction reactions of 32p atoms may proceed via either a single step mechanism as illustrated in (1), or a multiple step mechanism as illustrated in (2)-(4). 32p + px3 ~ 32px3 + p
(I)
32p + PX3 ~ 32PX+ PH2
(2)
;2PX + PX3 ~ 32PX2+ PH2
(3)
32PX2+ PX3-~ 32PX3+ PH2
(4)
From the general considerations, the multiple step abstraction mechanism is definitely more likely. But no information concerning the possible existence of the single step process can be deduced from the results of the present moderator studies. If the multiple step mechanism prevails, it is probable that the abstraction is really the net consequence of an insertion-decomposition process as proposed for the recoil HC systems[18]. In the present case, the insertion of 32p atoms into PH3 will give 32pH-PH2 radicals which may decompose by the cleavage of the P-P bonds to give 32PH. The resulting 32pH, which is a carbene analog, may insert into another PH3 molecule to give the molecule 32pH~-PH2 which may possess enough excitation to cleave its P-P bond yielding 3ZPH2 radical. The subsequent H-abstraction by the a2PH2 radical will give 32PH3 as the final product. The series of reactions involved in this possible insertion-decomposition mechanism are summarized below. 32p, + PH3 ~ 3"pH - PH2*
(5)
32PH - PH2* --, 32PH* + PH2
(6)
32pH* + PH~-~ 32PH2- PH2
(7)
32pH2- PH2* " 32pH2+ PH2
(8)
3"PH2+ PH3-* 32pH3'+PH,
(9)
It should be noted that for the above mechanism to operate the resultant edduct should always possess enough excitation energy to cleave the P-P bond. Therefore it is possible that the additional competing processes in the thermal energy range with a high reaction probability as mentioned in the previous section, could be processes such as thermal counterparts of reactions (5) and (7). If the 32P-containing species
788
O.F. ZECK et al.
derived from such reactions lack energy for the decomposition, they may undergo some other non-abstractive process to give products other than 32PH3. If such reactions are highly efficient in the thermal energy range, they essentially act as scavangers for the thermal 32p_ containing species in such nuclear recoil systems. Of course, the thermal counterparts of reactions (5) and (7)~ are not necessarily the operating scavanger reactions in these systems. Other processes may he actually respon- ' sible for the observed changes.
any such contributions, it is obvious that the hot yield is above 73%. Even if the "hot" contribution in the highly moderated systems is zero, the above reasoning employed by Stewart and Hower still does not hold. In such a case, all the 32p atoms will be thermalized before reaction. The observed 32PH3 yield will then represent a direct competition between the thermal H-abstraction reaction and the highly efficient thermal "scavanging" reactions as discussed in the previous section. The 27% value is definitely not derived from the net result of any direct competition between the hot and thermal H-abReaction cross section curve for H-abstraction by 32p straction processes. On the quantitative side the absolute yield of 32PH3 in atoms Judging from the information obtained in the present the pure system is 78% while that in the highly work, the reaction cross section curve for H-abstraction moderated system is only 21% (calculated from 27% and by 32p atoms should be high and should cover a wide 78%). However, in the pure PH3 system the number of energy range regardless of whether the reaction posses- 32p atoms available for thermal reactions is limited, and ses a threshold which is either above or in the thermal is much less than the number available in a highly moderated system. As a result, the thermal contribution energy range. In the first place let us assume that the H-abstraction of the 32PH3 yield in a pure PH3 system is expected to be possesses an above-thermal threshold energy. With this much less than 21% of the absolute yield, which means assumption, the fact that even with 98% neon modera- that the "hot" contribution to the ~PH3 absolute yield is tion a yield of 27% of 32PH3 is still observed is consistent likely to be 60% or above. This amount of absolute yield with the supposition that the reaction cross section curve is definitely very high among the hot reactions of various for H-abstraction by 32p atoms covers an extremely wide recoil atoms, especially when we take into conenergy range. In fact, the energy range should be so wide sideration that the "total" H-abstraction cross section is that for a batch of 3,p atoms with energies above the likely to be the product of the three individual ones for chemical reaction range, the energy degradation by 30 or reactions (2)-(4). Such high absolute yields should cor50 collisions with neon will leave about 30% of them with respond to a reaction cross section curve which is high in an above-thermal energy. At the high energy end such a magnitude and broad in its energy range. curve may extend to as high as 50 eV or above. The magnitude of the reaction cross section should be fairly Reaction cross section curve for F-abstraction by 32p high judging from the 78% 32pH3 absolute yield observed atoms It has been previously maintained that F-abstraction in the pure system and the relatively low collision density per energy interval expected for the energy by 32p atoms has a threshold in the thermal range. From degradation for such a reaction in the higher energy the various experimental observations, it can he deduced that the reaction cross section of this F-abstraction is ranges. Strictly speaking, the extrapolation of the moderator likely to he low and narrow. For a reaction which can curves to pure (100%) moderator systems with a positive extend into the thermal energy range where the collision intercept has no physical si~ificance, Theoretically all density per unit energy interval is extremely high, a 38% the moderator curves should drop to 0% yields at infinite absolute yield can only transcribe into a fairly low reacdilution. The lines in Fig. 2 are meaningful only as far as tion cross section per collision. The narrowness of the there are data points. A 27% normalized 32PH~ specific F-abstraction cross section curve can be deduced from activity at 98% neon moderation means that even with the observation as shown in Fig. 3 where we have plotted such a large excess of inert gas only 73% of the reacting Stewart and Hower's 32PH3 yields from the PF3-PH3 sap atoms or 32p-containing radicals are deactivated system by treating PF3 as an additive to the PH3 directly from an energy above the H-abstraction reaction system[8]. The PH3 moderator curves [from Fig. 2) by cross section curve to an energy below its threshold He, Ne, and Ar are also presented here for comparison. without reacting with PH3 to give ~PH3. The remaining It is observed that PF3 behaves exactly like neon except 27%, although partially deactivated by moderator col- in the nearly pure PF3 systems. This means that in the lisions, fail to jump over such an extensive energy range presence of PH3, the PFs molecules behave in the same without interacting with PH3. Since the reaction prob- way as other non-reacting moderators. The inertness of ability is not likely to be unity in the entire energy range PF3 in the presence of PH3 is dramatically demonstrated it covers, there actually should be much more than 27% by the fact that even if 93% of the mixture is PF3 the of the ~P species failing to skip over the entire H- observed absolute yield of 32PF3 is only 1.5% in comabstraction cross section curve--the ones which succeed parison with a 38% yield in a pure PF3 system[8]. The in colliding and reacting with PH3 to give the eventual best way to explain this result is that H-abstraction by 32p occurs at a higher energy range than its F-abstraction product, ~PH3, is 27% of the total. In the second place, let us assume the H-abstraction counterpart. The recoil ~2p atoms, mostly being energetic possesses a threshold in the thermal energy range. The to begin with, undergo H-abstraction reactions and are fact that the 32PH3 yield at high moderation could be removed from the reaction pool before their energy is extrapolated to 27% of the normalized ~PH3 specific low enough to interact with PF~. The tremendous activity of a pure PH3 system definitely does not mean decrease in the ~2PF~ yield by minute amounts of PH3 that the reaction is 73% hot and 27% thermal as impliM also indicates that the overlap of their reaction cross by Stewart and Hower to explain results of similar section curves is not very extensive such that the Fsystems [8]. At 98% moderation the observed yield may abstraction cannot become competitive with the H-abor may not contain any "hot" contribution. If there were straction process. Since the H-abstraction cross section
Gas phase recoil phosphorus reactions--IV
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Fig. 3. Effect of PF~ as a moderator on the normalized 32PH~specific activities (from Ref. [8]). curve is likely to be broad in its energy range and extends to a fairly low energy, the F-abstraction cross section curve therefore is likely to be much narrower. Acknowledgements---Generous support by the U.S. Department of Energy under Contract No. EY-76-S-05-3898 is gratefully acknowledged. The authors also wish to express their appreciation to the personnel at the Texas A&M Nuclear Science Center.
REFERENCES I. R. Wolfgang, Prog. React. Kin. 3, 97 (1%5): Ann. Rev. Phys. Chem. 16, 15 (1%5). 2. P. J. Estrup and R. Wolfgang, J. Am. Chem. Soc. 82, 2661; 2665 (1960). 3. I. F. J. Todd, N. Colebourne and R. Wolfgang, J. Phys. Chem. 71, 2875 (1967). 4. C. M. Wai and F. S. Rowland, J. Am. Chem. Soc. 90, 3638 (1%8). 5. A. P. Wolf, presented to the Panel on Theory of Hot Atom Chemistry, International Atomic Energy Agency, Vienna, May 1974. 6..I. Dubrin, H. Roseoberg, R. Wolfgang and C. Mackay, Chemical E~ects of Nuclear Transformations, Vol. 1, p. 133. International Atomic Energy Agency, Vienna (1965).
7. P. P. Gaspar, S. A. Bock and W. C. Eckelman, J. Am. Chem. Soc. 9e, 6914 (1968). 8. G. W. Stewart and C. O. Hower, J. lnorg. Nucl. Chem. 34, 39 (1972). 9. G. P. Gennaro and Y.-N. Tang, J. Inorg. Nucl. Chem. 35, (1973). I0. G. P. Gennaro and Y.-N. Tang, J. Inorg.Nucl. Chem. 36, 259 (1974). II. O. F. Zeck, G. P. Gennaro and Y.-N. Tang, J. Chem. Soc. (Chem. Commun.) 52 (1974). 12. O. F. Zeck, G. P. Gennaro and Y.-N. Tang, J. Am. Chem. Soc. 97, 4498 (1975). 13. M. Halmann, Chemical Egects o[ Nuclear Trnns/ormations, Vol. I, p. 185. InternationalAtomic Energy Agency, Vienna (1961). 14. H. Drawe and A. Henglein, Z. Naturlorsch.17b, 486 0%2). 15. A. Henglein, H. Drawe and D. Perner, Radiochim. Acta 2, 19 (1%3). 16. M. Halmann and L. Kugel, J. Inorg. Nucl. Chem. 25, 1343 (1963). 17. The possibilityof a highly efficientcompetitive thermal reaction was firstsuggested by a reviewer of this article.We appreciate his suggestion. 18. H. Ache, K. Taylor and A. P. Wolf, presented at the 8th International Hot Atom Chemistry Symposium, Spa, Belgium, May 1975.