3418
J . Phys. Chem. 1986, 90, 3418-3422
Chemical Effects of Nuclear Transformations In Mixed Crystals. 10. Chemical Effects of the 37Cl(n,y)38CINuclear Reaction in K,0sC16-K,0sBr6 Mixed Crystals‘ Horst Miiller,* P. Obergfell, and I. Hagenlocher Institute of Inorganic and Analytical Chemistry, Section Radiochemistry, University of Freiburg, 0-7800 Freiburg i. Br., Federal Republic of Germany (Received: December 9, 1985)
The solid-state reactions occurring during the moderation of recoiling 38Cl,produced by the ( n n ) reaction, have been studied in K20sC16-K20sBrsmixed crystals. The main products are Os38ClC152-,OS’~CIB~:-, and 38Cl-,but even the more intimately mixed species O S ~ ~ C I C I , B( n~=~ ~1,-..., 4) are obtained in significant amounts. The most important reaction found is the replacement process (biliiard-ballprocess). In some cases the recoil atom interacts with more than one ligand and intermediate ligand vacancy (denoted by 0)species O S ~ ~ C ~ C I , O and ~ -OS~~CIB~,~~-:~( n = 0, ..., 4) are formed. The vacancies are assumed to be filled by mobile halides from the lattice. From the products 0~~~ClC1,Br~:-( n = 0, ..., 5 ) and 38Cl-and their yield dependence on the mixed crystal composition the following results were obtained with the help of the program PRIMULA: interstitials 1 1%,primary retention 5%, replacement 6276, replacement plus one additional vacancy 14%, replacement plus two or more additional vacancies 7%. These results together with some experiments with K2ReC16-K2ReBr6targets are in accordance with all other so far known ”C1 and 82Brrecoil reactions in K2M’cl6-K2M”Br6 mixed crystals. They do not confirm the reported ligand vacancy transfer mechanism (kinetic theory) elaborated for 38C1recoil atoms.
= loi2cm-2 s-I; +f(E>100 keV) = 2 X 1Olo s-l; y dose rate = 7 X lo4 Gy h-l; irradiation time: 1 min; temperature: 25 “C). Radioactive recoil atoms, produced by nuclear reactions in Product Separation and Radiometric Analysis. The separation suitable inorganic solids, can be used for examining solid-state of the 38Clrecoil products was accomplished at room temperature damage by chemical methods. Compounds very well suited for by HPLC using a Series 3B pump module (Perkin-Elmer, this purpose are homogeneous mixed crystals of components beUberlingen), a sample loop PL 40 with rheodyne valve 7125 longing to the K2PtC16 type as K2ReCI6-K2ReBrs2 or K2Re(Perkin-Elmer), a 22 X 250 mm column SI 100 Polyol-DEAE Br6-K2SnC16.3 Such substances have been used to investigate 5 pm (Serva, Heidelberg), a spectrophotometer LC 75 (Perkinthe mechanism of reactions which recoil atoms undergo in solids. Elmer), and a fraction collector FOXY (Colora, Lorch) with It is true that such investigations cannot be performed without recorder. Immediately after the irradiation the samples were dissolution, but in this procedure most of the damage centers of dissolved in 1-2 mL 0.25-0.7 M HC104containing already 5 mg the solid are transformed in a simple manner; e.g., halide inof inactive carrier of a mixture of all K2MCl,Br6-, ( M = os or terstitials appear as free halide ions, and all ReCI,Br,?species Re) species (prepared by heating mixtures of K2MCl6and K2MBr6 persist upon dissolution. The numerical results for the K2ReC16-K2ReBr6(n,y)82Br,”7 in quartz ampoules for 24 h at 600 OC’O) and 1 mg of KC1. After K,o~Br~-K~SnCl~(n,y)~~Br,’ and K2ReC16-K2ReBr6(n,y)36c18 the injection the separation was performed within 25-40 min at 12-18 MPa with 0.25-0.7 M HCIOl as eluent, with a flow rate experiments are comparable and could be attributed to interstitial of 9-15 mL min-’. The exact conditions were dependent upon halides, primary retention, simple replacements (billiard-ball the age of the column. The solution leaving the column passed The substititon), and processes of larger disorder. continuously through the photometer adjusted to 217 nm. Due K2ReC16-K2ReBr6(n,y)38cl experiment, however, required for to the addition of the carrier substances C1- and all seven its explanation an entirely different model of ligand vacancy MCl,Br6-?species were located and collected with the help of transfer, the “kinetic t h e ~ r y ” . ~This . ~ model has not remained the fraction collector which was operated manually. The elution uncontradicted, and it seemed worthwhile to repeat experiments diagrams are similar to those obtained in low-pressure column with 38CIrecoil atoms. The system K2OSC16-K20SBr6 was used chromatography.8 Without the addition of the inactive carrier as target, mainly because less activity is produced in Os than in substances, only MC162-and MBr62- could have been located. It Re, but some additional experiments were performed with K2was further shown that no gross ligand exchange occurred between Rec16-K$eBr6 mixed crystals likewise. MC162- and MBr6’- during the activations resulting in mixed MC1,Brh2- species. For this purpose activated samples have been Experimental Section separated without addition of inactive carriers. In these cases only Materials and Irradiation. The preparation of the K2ReMCls2- and MBr62- were found spectrophotometrically. C&,-K2ReBr6 mixed crystals has been described elsewhere.2 The activity of the fractions containing the different 38Clrecoil K2OsCl6-K20sBr6 mixed crystals were prepared by following the labeled species was determined (after adjustment of all samples same p r o c e d ~ r e . Neutron ~ activation of 5-mg samples was carried to the same volume by addition of solvent when necessary) by y out in polyethylene capsules in the TRIGA swimming pool reactor spectroscopy using the 1643 and 2167 keV lines. The spectra were at the Institute of Nuclear Chemistry, University of Mainz (& obtained with the help of a Silena multichannel analyzer 79434096 with automatic peak search and peak net area computation and a DSG Ge-Li detector, Model LGC 15, (active volume 48 (1) Part 9: Muller, H.; Bekk, P.; Bicheler, U. Radiochim. Acta 1984, 36, cm3, efficiency 15.2%, peak/Compton ratio 37.6, resolution 2.1 115. keV at 1.33 MeV). The measuring period was 10 min for all (2) Miiller, H.;Martin, S. 2.Anorg. Allg. Chem. 1978, 445, 47. (3) Miiller, H. Z. Anorg. Allg. Chem. 1965, 336, 24. samples, and the resulting standard deviations are specified in the (4) Miiller, H.; Martin, S. Inorg. Nucl. Chem. Letr. 1969, 5, 761. tables of results. The dead time of the counting equipment never (5) (a) Bell, R.; RBssler, K.; Stkklin, G.; Upadhyay, S. R. Report Kernexceeded 2% of the real time; therefore no special dead-time forschungsanlage Jiilich, JuL-625-RC, 1969. (b) Bell, R.; Rhsler, K.; correction as used by Otterbach6 and Rossler et aL7was necessary. Stkklin, G.; Upadhyay, S. R. J. Inorg. Nucl. Chem. 1972, 34, 461. Introduction
(6) Otterbach, J. Report Kernforschungsanlage Jiilich, JuL-832-RC, 1972. (7) Rhsler, K.; Otterbach, J.; Stkklin, G. J. Phys. Chem. 1972, 76, 2499. ( 8 ) Miiller, H.; Diefallah, E. M.; Martin, S. J. Phys. Chem. 1981,85, 3514. (9) Muller, H.; Bekk, P. Reo. Chim. Miner. 1985, 22, 809.
0022-3654/86/2090-3418$01 .50/0
(10) Muller, H.; Bekk, P.; Hagenlocher, I. Z. Anorg. Allg. Chew. 1983,
503, 15.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3419
Nuclear Transformation in Mixed Crystals TABLE I: Radioactivity of (in mol % K209Cb)0 K20sC16 os3Bc1c1522.4 5.8 9.1 20.6 36.3 41.2 63.1 78.1 90.1 100.0
4.2 (27) 8.3 ( i 3 j 16.3 (14) 24.1 (16) 34.0 (20) 43.9 (IO) 55.2 (8) 67.0 (12) 77.0 (6) 86.3 (6)
Labeled Species (in %) Following ”Cl(n,y)”CI Nuclear Reaction in K#sC&-K20SBr6 Mixed Crystals 0 ~ ~ ~ C l C l ~ BOrS~~ -~ C I C I , B ~ ,O~ -S ~ ~ C I C ~ ~ OS~~CICIB~,ZB~,~0 0 0 0 85.6 (64) 0 2.5 (1 1) 66.9 (34j 0 0 3.1 (12) 71.4 (31) 0.0 (18) 0 0 4.9 (24) 3.8 (21) 5.1 (10) 4.9 (6) 4.5 (8) 3.9 (4) 0.0 (I)
0 1.6 (9) 1.3 (4) 1.6 (2) 1.3 (3) 1.7 (2) 0
4.2 (IO) 5.3 (12) 4.9 ( 5 ) 3.6 (3) 2.8 (3) 1.7 (1) 0
0 0.8 (8) 1.4 (3) 1.0 (2) 1.2 (3) 0.5 (1) 0
‘Mean counting errors (in units of the last digit) are given in parentheses. *No
53.4 43.6 32.9 21.0 12.5 5.4 0
OsBr20
’8Cr 10.2 (20) 22.3 (20j 8.1 (10) 13.3 (11) 6.8 (9) 8.6 (4) 11.2 (4) 10.5 (4) 9.2 (2) 13.7 (2)
0 1.1 (16) 0 4.0 (15) 1.9 (4) 1.4 (2) 0.3 (3) 0.5 (1) 0
(27) (24) (10) (5) (6) (2)
activity should be found in this fraction.
TABLE II: Radioactivity of %CI Labeled Species (in W ) Following 37Cl(n,y)”CI Nuclear Reaction in KzReQ-K2ReBr6 Mixed Crystals (in mol % KZRa6)“ K2ReC1, Re38C1C152- Re38CIC14Br2- Re38C1C1,Br2- Re38C1ClzBr32- Re38C1CIBr2- Re38C1Br52ReBr2-b 5.0 10.4 (13) 3.5 (IO) 0 0 0.0 (4) 65.3 (39) 6.7 (20) 11.6 15.5 (10) 4.3 (6) 1.4 ( 5 ) 0 2.7 (6) 61.4 (25) 3.9 (10) 22.7 27.3 (10) 5.1 (8) 0 0 3.5 (6) 49.4 (16) 3.4 (11) 88.OC 74.8 (8) 4.7 (2) 0.5 (2) 0.5 (2) 1.7 (2) 6.6 (3) 0
38~1-
14.1 (14) 10.8 (8) 11.3 (6) 10.1 (2)
‘Mean counting errors (in units of the last digit) are given in parentheses. b N o 38Clactivity should be found in this fraction. We suppose that this activity arises from the tailing of Re38C1Br52-which occurred in the worn-out column used for these experiments. In accordance with this interpretation we find the Re38C1Br5Zyield smaller than the respective yield (compare Figure 1). e 1.1 ( 5 ) W 38Cl activity was found in the region between and Re38C1C15z-. TABLE III: Numerical Evaluation of the Experimental Results Corresponding to the Quation Y = p n + p l y
Po = yo +12.4 (24) Os38CICI5~+5.4 (11) O S ~ ~ C I C I ~ B ~ - ~-1.0 (8) O S ~ ~ C I C I , B ~ ~ ~ --0.5 (3) O S ~ ~ C I C I ~ B ~ , ~ --0.4 (2) 0~~~ClClBr:+1.1 (5) 0 ~ ’ ~ C l B r ~ ~ - +81.0 (30) 38~1-
PI -2.3 (42) +80 (2) +24 (4) +7.0 (18) +5.5 (10) +17 (3) -123 (16)
P2
-22 -6 -5 -18 +43
(4) (2) (1) (3) (16)
Apo, %
Apl, W
19 20 78 66 49 51 4
181 2 17 25 19 17 13
+ p2y
a
Ap2, %
mean error of single measurement
18 29 21 16 36
4.6 2.0 1.1 0.5 0.3 0.8 4.3
YI w +10.1 +85.5 +0.9 +0.6 +0.2 -0.3 +1.1
Y is the calculated yield in percent; y is the mole fraction of K20sCI6;() indicates mean error of po, p l , and p 2 in units of the last digit; Apo, A p l , and Ap2 are mean errors of the respective coefficients in percent; p o = Yois the calculated yield for pure K20sBr6(y=O); Yloois the calculated yield for pure K2OsCI6(y=l).
The counting rates were adjusted for equal time. In addition to the statistical errors of the activity measurements, errors caused by the incomplete chromatographic separation must be taken into account. These errors are estimated to be smaller than or equal to 10% of the final value, but not larger than &3% absolute (see, however, footnote a of Table 11). No activity was found in the region between C1- and MC162which shows that during the runs no hydrolysis producing free 38Cl- occurs and that no 38Cl labeled aquation products of MCl,Br6-;- are present.
Results Table I and Table I1 present the contributions of 38Cllabeled reaction products obtained in K2OSCl6-K20SBr6 and K2ReC16K2ReBr6 mixed crystals, respectively. The yields of the main products M38C1C152-,M38C1C14Br2-,M38C1C1Br42-,M38C1Br52( M = Os, Re), and 38Cl- are shown in Figure 1; the other two species are present only in minor quantities. In order to find an unambiguous way for constructing curves for the different yields Y, functions of second order with least-squares fit have been calculated. (In no way it is assumed that the reactions leading to the different species follow first- or second-order curves; in fact, the model described in the discussion needs even higher order curves in some cases.) Table I11 shows the numerical results for the equation y = Po + PIT + PzY2 where Y is the percentage yield of the respective labeled species and y is the mole fraction of Kz0sC16 in the mixed crystals. Included are the absolute and the percentage errors of the
coefficients po, pl, and p z , the mean errors of the single measurements, and the extrapolated yields for zero content (y = 0) of K2@C16 (Yo= po) and for pure (y = 1) K20sC16( Yloo). Some of the extrapolated Yoand Y l , results are of negative sign, which cannot happen physically, but the absolute value is within the region of the respective errors. The data for KZReCl6-KzReBr6 mixed crystals were not used for these calculations. The results of Rijssler et al.’ for 38Clrecoil reaction products in unannealed KZReCl6-KzReBr6mixed crystals are included in Figure 1. (On annealing only in K2ReC16 rich mixed crystals (y 3 0.5) part of 38Cl-reacted back to Re38C1C1:-.) As can be seen there is a large disagreement especially for mixed crystals with lower KzOsCls content (y < 0.5). The present results, therefore, do not support the conclusions of Rossler et al.7 concerning the “kinetic theory”. Possible reasons for these discrepancies have already been discussed.8 Further it must be mentioned that very similar yield curves as obtained here for 37Cl(n,y)38C1recoil reaction products were found in the past in all other 36Cland 8zBr experiments in mixed crystal^.'^^-^ Discussion
In a mixed crystal with vanishing concentration of KzOsC16only three main reaction products were obtained. The first is 12.4% free chloride, 38Cl-, resulting from recoil atoms which at the end of their trajectory are trapped at vacancies in the lattice. The second, 5.4% O S ~ ~ C ~represents C ~ ~ ~ -the , experimental primary retention which more sophisticatedly can be divided into (1) species which have never been destroyed, (2) species whose the O S - ~ ~ C I bond was broken for a “short” time but re-formed from the debris before dissolution, and (3) species in which 38Clchanged its site
3420 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 100.0
100.0
0s36c1c L,z80.0
lM-o/
60.0
1
A
,'
:'p/
40.0
tt
80.0
___jj_l
60.0
40.0
lw'o
'M.0
20.0
20.0
0.0
Miiller et al.
ty 3
I
I
20.0
I
I
40.0 60.0 mol.-% K,OsCL,
0.0
80.0
0s"C 1C 1,Br* 100.0
100.0
80.0
80.0
60.0
60.0
" c 140.0
40.0
20.0
0 t 4 - A
0.0
20.0
'....
-
A
0 I
0 I
I
20.0
40.0
60.0
O S ~ 1CC 1BrF
z43I
0.0
80.0
mol-% K,OsCl,
mol-%
K,OsCL,
Figure 1. Yield of 38Cl-labeledspecies resulting from the "Cl(n,y)'*C1 nuclear process in K@SC16-K20SBr6 mixed crystals (0),error bars represent f a ;fitted yield curves (-); calculated yield curves (-- -). Included are own experimental results (A)(with error bars) and Rossler et al.'s7 fitted yield curves (-. .) for KzReCl6-KzReBr6(n,y)'*C1 experiments.
with another ligand of the same complex anion. These retention mechanisms, however, cannot be distinguished experimentally. The third, 8 1.O% Os3ECIBrz-,was produced by a replacement or billiard-ball reaction 38c1 OsBrs2- Br O ~ ~ ~ C l B r 5 2 -
+
-
+
At first sight the production of 1.1% 0 ~ ~ ~ C l C l in B the r ~ most ~diluted mixed crystals is surprising; we will return to this point later. If only the mentioned three mechanisms were responsible for all reaction products, then in mixed crystals with increasing K20sC16content exclusively the o ~ ~ ~ C l yield C 1 ~should increase a t the expense of O ~ ~ ~ C l B r because 52now the replacement reaction 38C1 osc162c1 Os38ClC152-
+
-
+
may occur, and nothing else should happen. We observe also, however, all other mixed species in mixed crystals containing both components (compare Table I, and for 0 ~ ~ ~ C l C l Bonce r 2 - more Figure 1 ) . Therefore a model is needed which allows for the production of these species. It should be mentioned that Rossler et al.' did not separate these species because they believed that 38CI"almost exclusively appears in the monosubstituted complexes and the free halide".
The formation of the more intimately mixed 38Cllabeled species has been discussed with some success in the case of 82Brrecoil atoms in K20sBr6-KzSnC16 mixed crystals with the help of the "impact-induced multiple ligand abstraction" (IMULA) model. A detailed description has been given in part 9.' In this model it is assumed that besides the normal recoil atom/ligand substitution in some impacts further ligands are lost by the complex ion hit resulting in ligand vacancy complex anions O ~ ~ ~ C l X , o ~ - n 2 (n = 0, ..., 4; X = C1, Br). Reactions of the recoil halide within its original complex anion are designated as L,(n) and reactions with foreign complex anions as Lf(n). The parameter n is defined as the sum of substitutions which is zero or one (38Clsubstitutes C1 or Br) plus the number of transient ligand vacancies which can be occupied by halide atoms from the lattice which is between zero and five. As an example the &(3) process produces in K20SC16-KzOSBr6 mixed crystals transient O S ~ ~ C ~ C I (from ~O,~OsClS2-) and Os3*C1Br3022-(from OsBrs2-) the ratio of which is mainly dependent on the mixed crystal composition. The ligand vacancies are assumed to be filled by halides from the lattice; the exact contributions of C1 and Br are dependent on the mixed crystal composition and are expected to follow more or less statistical laws. In the example mentioned O~~~ClC152(by addition of 2 Cl-), Os38C1C1,BrZ- (by addition of 1 CI- and 1 Br-), and Os38C1C13Br2Z-(by addition of 2 Br-) are obtained from
The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3421
Nuclear Transformation in Mixed Crystals TABLE I V Comparison of Results Obtained for K209Cb-K20sBr6(n,~)38C1and K20sBr~-K~~nC~6(n,y)8*Br' Nuclear Reactions parameter 82Br 38C1 374" 528b E,(max), eV
Edav), eV Dc
free halide, % LSl), % L,W, 9 i
Lf(l), Lf(21, ?h LX3) a
+ Lf(4) + LX5) + Lf(6), 5%
100" 1.04 7.1 7.9 1.7
52.5 22.0 8.0
294b
1.30 11.4 5.4 0
62.0
14.0 7.0
the present calculations N was fixed at 40 owing to the greater halogen recoil energy. It has been shown, however, that other values (e.g., N = 12 or N = 100) only change the results within the error range. In other words, the experiments do not lead to a value for N . The free chloride yield, 38Cl-, which was found almost constant for all mixed crystals investigated was fixed at 1 1.4%. The application of the IMULA model to the experiments gives the final results (together with the estimated errors): D, = 1.30 f 0.05 D , = 1.00 f 0.05 L,(1)
+ L,(O) = 5.4 f 1.0%
Reference 12. Reference 13.
os38c1c1302-,and 0 ~ ~ ~ C l C l ~ B0~~~ClClBr:r?-, and Os38C1Br:from Os38C1Br3022-,respectively. In the present case the contributions of the different L,(n) reactions are derived easily from the results for the most diluted mixed crystals. L,(O) + L,( l ) , which cannot be distinguished experimentally, amount to 5.4%; L,(2), L,(3), and Ls(4) amount B r ~ be ~ -the result to zero. 1.1% observed yield of 0 ~ ~ ~ C l C lmight of 1.6% L,(5) or 2.9% &(6) or a combination of both but it is hard to understand why L,(5) and/or L,(6) are not zero if L,(2), L,(3), and L,(4) are. Another interpretation will be presented, and L,(5) and L,(6) will be considered as zero. The contributions of the different Lf(n) processes cannot be derived directly from the experiments because most species are resulting from more than one reaction. A program PRIMULA was developed which allows the calculation of all possible reactions leading to 0~~~ClCl,Br~-:-species. The program starts with assumed initial L,(n) and Lkn) contributions. Then the vacancies of the transient species are occupied by halide from the surrounding lattice following a hypergeometric distribution. The processes are summarized calculated yields from all LJn) and Un) for each reaction product and are compared with the experimental results. Subsequently in the next loop L,(n) and Ldn) values are varied in such a way that experimental and calculated results converge. PRIMULA contains 15 free parameters. Twelve of them are the different L,(n) and L f ( n ) contributions (n = 1, ..., 6). L,(O) is included in &(l), and LXO) is physically indefinite. The parameter N stands for the number of "quasi-free" ligands in the reaction zone which are thought available for filling the vacancies. Ligands which have become free during formation of O S ~ ~ C ~ X , O ~ - > transient species are added to this number. The complex discrimination factor D, simulates for Lf(n) processes a preferred (0, > 1) or less favored ( D , < 1) impact probability of the recoil 38Clwith O S C ~ ~The ~ - . ligand discrimination factor D , simulates for &(n) and Ldn) processes a preferred ( D l > 1) or diminished (0, C 1) reaction probability of O S ~ ~ C ~ X , ~ ~transient -:species with C1 from the reaction zone. It may be suspected that every experimental result can be simulated with 15 free parameters. This problem has been discussed in our previous work with the consequence that there are strong restrictions for most of the parameters, which are almost the same in the present case.' Calculations-that means comparison between experimental and calculated results-were performed for mixed crystals with mole fractions of 0.0, 0.1, 0.2, ..., 0.9, 1.0 K20sC16. The respective yields of 38Cl-and the six 0~~~ClC1,Br~:species were obtained from the yield equations presented in Table 111. This kind of calculation was easier than the one using the experimental values directly with subsequent averaging. In previous mixed crystal experiments with Re and Os recoil atoms with recoil energies smaller than 100 eV it was found that the reaction zone contained between 12 and 18 halide ions." For (1 1 ) Muller, H. Proceedings of the IAEA Symposium on Chemical Effects of Nuclear Transformations, Vienna, 1964; IAEA: Vienna, 1965; Vol. 11, p 359. (12) Robinson, M. T.; Rossler, K.; Torrens, I. M. J . Chem. Phys. 1974, 60, 680.
Lf(1) = 62.0 f 2.0%
L,(2) = 0
L f ( 2 ) = 14.0 f 2.0%
L,(3) = 0
Lf(3) = 3.5 -+ 1.0%
L,(4) = 0
Lf(4) = 2.5 f 1.0%
L,(5) = 0
Lf(5) = 1.0 f 1.0%
Ls(6) = 0
L f ( 6 ) = 0.0 f 1.0%
38Cl- = 11.4 f 1.0% As can be seen from Figure 1 there exists a very good agreement between the experimental and the calculated yields of 38Cllabeled species. The IMULA model therefore is able to interpret the experiments not only qualitatively but even quantitatively. While for mixed crystals with vanishing K20sC16content (y i= 0) the calculated yield of 0 ~ ~ ~ C l C l is Bzero, r ~ ~the - observed yield amounts to 1.1%. The reason for the production of this species is that the recoil range of 38Clatoms is so small that ligand vacancy complex anions may be produced adjacent to those complex anions from which the recoil atom came; therefore in the next step chloride ligands released from these anions may fill vacancies in the ligand vacancy complex. Effects connected with the small recoil range of 38Clatoms are, however, outside the scope of the IMULA model; therefore it cannot predict the production of 0 ~ ~ ~ C l C l in B rsuch ~ ~ very diluted mixed crystals by Lf(n) processes with n 3 2. The yield of 0 ~ ~ ~ C l C l Binr ,the ~ - most diluted mixed crystals can be used to obtain some information about the chloride content of the reaction zone. 0 ~ ~ ~ C l C l isB produced r ~ ~ - if exactly one of the ligand vacancies of O S ~ ~ C ~ B ~ , ~ ~ - , Os38C1Br2032-,and respectively, is occupied by chloride. The 0 ~ ~ ~ C l C l yield B r ~depends ~on the chloride concentration in the reaction zone as follows YOs38CICIBr4'-
= Lf(2) IC11
+ 2Lf(3)IC11 [Br] +
+ 4Lf(5)[C1][BrI3 with [Cl] and [Br] the respective concentrations and [Cl] + [Br] 3Lf(4)[C1][BrI2
= 1. If the observed y ( O ~ ~ ~ C l C l B r=2 -0.01 ) 1 and the calculated Ldn) contributions are used, the equation is solved by [Cl] = 0.035. This leads to a relation between the chloride content of the reaction zone and the number m of lost chloride ligands: N = m/0.035 = 28.5m. Because in the most diluted mixed crystals (y = 0) all chloride ions in the reaction zone must originate from that OsC1,Zspecies which lost one ligand as recoil 38Clatom we have 1 6 m < 5 and subsequently for the number of halide ions in the reaction zone 27.5 < N < 137.5. Any arguments to restrict the region of m and subsequently of N do not seem convincing, but to our belief m 2, N 60 is a reasonable choice. It may be suspected that the mechanism described may result even in 0 ~ ~ ~ C l C l ~ B if rtwo ? - chloride ions enter the ligand vacancy complex ion. Due to the low chloride content of only 3.5% the calculated contribution is almost zero. At the beginning of the discussion 8 1.O% of O S ~ ~ C I Bwas ~,~regarded as being produced by simple replacements (Lf(1) processes). It is now obvious that Lf(l) is only responsible for 62 f 2%, 14 f 2% stem from the Lf(2) process and the rest from the other Lf(n) processes. In the most dilute (y 0) K20sCl,-K2OSBr6 mixed crystals all Lf(n) processes produce
- -
-
(13) Ferro, L. .I.Spicer, ; L. D. J . G e m . Phys. 1978, 69, 1320.
7.Phys. Chem. 1986, 90, 3422-3429
3422
but the respective contributions are not known. Table IV shows the most important IMULA parameters for both investigated systems K20SC1,-K,0SBr6(n,y)3*cl and K20sBr6-K2SnC16(n,~)82Br’ including the respective maximum and average recoil energies. As can be seen the simple reactions interstitial halide, L,( 1) and L,-(l) account for most of the reaction products indicating that in solids only little disorder is produced by (n,r) recoil atoms. The differences, however, are very hard to understand. It is, indeed, expected that with increasing recoil energy L,( 1) decreases which is found, but unexpectedly the Ls(2) process disappears totally. Moreover the LA 1) process increases and the L,,-(2)process decreases with increasing recoil energy which is quite opposite the expectation which predicts that processes responsible for more disorder increase when recoil atoms with higher energies react. Only ad hoc hypotheses might explain the large difference in D,. Unfortunately it was not yet possible to apply IMULA to our previous K2ReC16-K2ReBr6(n,y)36Cl experiments’ (maximum 36CIrecoil energy 1099 eV, average recoil energy 634 eVI4); it cannot be said if this failure is caused by deficient experimental data or because the model cannot be applied in its present state to such high recoil energies. (14) Chang, J.; Ferro, L. J.; Spicer, L. D. J . Chem. Phys. 1983, 79, 6419.
Conclusions
It has been shown that the chemical effects of the nuclear process K20sClsK20sBr6(n,y)38cl can be quantitatively discussed by using the impact-induced multiple ligand abstraction (IMULA) model. For a more profound understanding of the relation between recoil energy and recoil atom mass, respectively, and the contributions of the different IMULA processes, further investigations seem necessary, using other (but similar) systems and other recoil energies.
Acknowledgment. This investigation was supported by the Bundesministerium fiir Forschung und Technologie as part of the program “Erforschung kondensierter Materie-Nuklearchemie”. We thank Dr. N . Trautmann, Institute of Nuclear Chemistry, University of Mainz, for performing the irradiations and U. Knitz for her help with the activity measurements. The program PRIMULA was written by U. Bicheler. The numerical calculations were performed with the UNIVAC 1100/82 computer of the University of Freiburg. Registry No. K20sCl6, 16871-60-6; K20sBr6, 16903-69-8; O S ~ ~ C I C I102494-71-3; ~~-, 102494-72-4; 0~”ClClBr~~-, 102494-73-5;OS~~CICI,B~~-, 102494-74-6;Os38C1C13Br~-, 102494-75-7; 14158-34-0. O S ~ ~ C ~ C I ~102494-76-8; B ~ , ~ - , OsBr$-, 16920-04-0;
Thermal Reactlons of Cyclic Ethers at High Temperatures. 2. Pyrolysis of Tetrahydrofuran behind Reflected Shocks Assa Lifshitz,* Menashe Bidani,’ and Shimon Bidani Department of Physical Chemistry, The Hebrew University, Jerusalem 91 904, Israel (Received: January 8, 1986)
The thermal decomposition of tetrahydrofuran was studied behind reflected shocks in a single-pulse shock tube over the temperature range 1070-1530 K and overall densities ranging from 2 to 8 X mol/cm3. Over this temperature range the following products were obtained: H2, CH,, C2H4, C2H2,C3H4(allene), C,H4 (methylacetylene), 1-C4H8,C4H6,C4H4, C4H2, and small quantities of c-c3&, C6H5,c&6, and c4H40. From a series of experiments using mixtures of tetrahydrofuran and tetrahydrofuran-d8 and also partially deuterated (3,3,4,4-d4) reactant, the following initiation steps were suggested: tetrahydrofuran C2H4 (CH2),-0, k,,, = 3.30 X 10l6exp(-83 X 103/RT)s-’; tetrahydrofuran C3H6+ CH,O, kuni = 8.25 X lOI5 exp(-83 X 103/RT) s-I. The (CH2),-0 residue neither isomerizes to acetaldehyde nor dissociates from a potential surface of the latter. It dissociates to methyl and formyl radicals, producing carbon monoxide as the only oxygenated product of this channel. It was shown that ethylene is formed by elimination from the tetrahydrofuran ring at the 2-3 (4-5) and 3-4 positions at a ratio of -2.2:l. A secondary isotope effect of 1.65 in the production of both ethylene and propylene in favor of tetrahydrofuran over tetrahydrofurand, was noticed. It was also shown that allene and methylacetylene preserve the original skeleton of the tetrahydrofuran and are probably formed directly from propylene. 1-Butene on the other hand is produced by an attack of methyl radicals on propylene and does not preserve the skeleton of the tetrahydrofuran.
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Introduction
Whereas a lot of effort has been devoted to the study of the pyrolysis of aliphatic and aromatic hydrocarbons, very little has been done in trying to elucidate the pyrolysis pattern of heterocyclic compounds. Tetrahydrofuran is a very stable five-membered ring ether. The only attempt to study its thermal decomposition dates back some 35 years to Walters et al.,,s3 who studied the decomposition in a heated bulb over the temperature range 530-670 O C . As far as we are aware, nothing has been done since then to repeat the study or extend its temperature range in order to obtain more (1) In partial fulfillment of the requirements for a Ph.D. Thesis submitted to the Senate of the Hebrew University by M.B. (2) Klute, C. H.; Walters, W. D. J. A m . Chem. Sor. 1946, 68, 506. (3) McDonald, G.; Lodege, N. M.; Walters, W. D. J. Am. Chem. Sor. 1951, 73, 1757.
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accurate and elaborate information that could lead to the understanding of the pyrolysis mechanism. In the early study,2 the overall decomposition rate was determined from time-dependent pressure observations over the temperature range 529-569 OC at pressures ranging from 50 to 300 Torr. The authors concluded that the overall decomposition rate of tetrahydrofuran obeyed a IS-order rate law with a rate constant’ k = 1.15 X loL2exp(43.0 X 103/RT) L’I2 mol-’/2 s-I. Among the reaction products obtained in the pyrolysis, ethylene and carbon monoxide were present at the highest concentrations, followed by methane together with smaller percentages of higher (unspecified) unsaturated compounds, and also hydrogen and ethane. Special care was taken in order to analyze for aldehydes during the pyrolysis, as acetaldehyde and formaldehyde might play the role of active intermediates in the reaction. A timedependent analysis did reveal the presence of aldehydes, which increased in the beginning, reached a peak, and then disappeared 0 1986 American Chemical Society
