44
MAKOTO R~ISONOAND YUKIOYONEDA
A Kinetic Study of n-Butene Isomerization over Supported Aluminum
and Magnesium Sulfates by Makoto Misono* and Yukio Yoneda Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Bunkyo-kti, Tokyo, Japan (Received October 4, 1971) Publication costs assisted by the K a w a k a m i Foundation
In order to examine the correlation between the selectivity of acid-catalyzed reactions and the acid strength of catalyst, the isomerization of three isomers of butene was studied kinetically at 40-100° over silica-sup-
ported aluminum and magnesium sulfate catalysts which were previously shown to be typical strong and weak acids, respectively. The activation energy differences between all paths and the relative rate constants have been determined for both catalysts, as well as the overall activation energies. The observed selectivity and the activity were shown to be determined principally by the differences in the energy barriers. For example, the iimease in the 2-: 1-butene ratio from one of 2-butenes with the iiicreasiiig acid strength was quantitatively explained by the decrease in the height of energy barrier of 2-butene formation, relative t o that of I-butene formation. The activation energy difference was small between cis- and trans-2-butene formations, reflecting the c i s : t m n s ratio close to unity. These results were interpreted by the stability of a common intermediate on the basis of carbonium ion mechanism and the principle of linear free energy relationships. The intermediate became more stable and long lived as the acid strength of catalyst increased.
Introduction Stereoselectivity in the catalytic isomerization of 1butene (cis :trans ratio in the products) has been a subject of several investigation^.^-^ The selectivity between cis-trans isomerization and double-bond migration from 2-butene (2-: 1-butene ratio) has also been studied. According to Foster and C v e t a n o ~ i c the ,~ 2-: 1-butene ratio was high over acidic catalysts as the result of the rotation of a carbonium ion intermediate. The ratio was low for basic catalysts presumably because of the restricted rotation of carbanion. We have previously demonstrated that 2- : 1-butene ratio was markedly dependent on the acid strength of catalyst in the case of metal sulfate catalysts. The rate of isomerization and the selectivity of cis-trans isomerization over double-bond migration increased monotonously in the order of H+ > Fe3+ > A13+ > Scy+> Cu2+> Zn2+ > Ni2+2 Co2+> A h 2 + > 1\Ig2+.6 This order agreed witkthat in the acid strength which was estimated from the electroncgativity of the constituent metal ion and tested by indicator^.^^^ These trends found also for other metal salts v-ere explained by the relative rates of proton addition and elimination, or the stability of protonated intermediate.6ps Hightower and Hal19a10 investigated the kinetics of n-butene isomerization over silica-alumina and explained the selectivity by the differences in the energy barriers interconnecting the carbonium ion and the products, assuming a sec-butyl carbonium ion as a common intermediate. The relative reactivities of three isomers were also explained on the same basis. Recently, Lombardo, Sill, and Hall1' suggested that T h e Journal of Physical Chemistry, Vol. 76, ATo. 1, lQY8
the 2-: 1-butene ratio increased with the acid strength of catalyst by the increase in the energy barrier between the carbonium ion and l-butene, relative to that between the ion and 2-butene. I n the present work, we have studied the kinetics and energetics of n-butene isomerization, choosing aluminum sulfate as a typical strong-acid catalyst and magnesium sulfate as a typical weak-acid catalyst out of ten metal sulfates studied before6 It was primarily intended in this work to examine how the kinetic parameters and the energy profile of reaction vary (Table I) with the acid strength of catalyst and to make clear the correlation between the selectivity and the acid strength. Since n-butene isomerization consists only of proton addition and elimination, it is expected that the present workmill provide fundamental information about the rate and selectivity in acid cat a1ysis. (1) W.0. Haag and H. Pines, J . Amer. Chem. Soc., 8 2 , 387 (1960). (2) P. J. Lucchesi, B. L. Baeder, and J. P. Longwell, ibid., 81, 3235 (1959). (3) K.F. Foster and R. J. Cvetanovic, ibid., 82, 4274 (1960). (4) D. M. Brouwer, J . Catal., 1, 22 (1962). (5) H. P. Leftin and E. Hermana, Proc. I n t . Congr. Catal., Srd, 1964, 1064 (1965). (6) M . Misono, Y. Saito, and Y. Yoneda, J . Catal., 9, 135 (1967); 10, 88 (1968). (7) M . Misono, E. Ochiai, Y . Saito, and Y. Yoneda, J . Inorg. Nucl. Chem., 29, 2685 (1967). (8) M. Misono and Y. Yoneda, Bull. Chem. Sac. Jap., in press. (9) J. W. Hightower and W.K. Hall, J . Phys. Chem., 71, 1014 (1967). (10) J. W. Hightower and TV.K. Hall, J. A m e r . Chem. Soc., 89, 778 (1967). (11) E. A. Lombardo, G. A . Sill, and W. K. Hall, J . Catal., in press.
KINETICSTUDYOF %-BUTENE ISOMERIZATION
Table I : Activation Energies and Differences in Activation Energies in the Isomerization of %-Butenes over AI-S and Mg-S, kcal mol-'
_____
Starting butene --l-Dutene----cis-Z-Butene--EP Elt - E I C ~ E c a Eci - Botb
--trans-2-ButeneEt" Et, - Etob
Al-S
7.2 7.OC
7.5
0.0 (1.O)d
1.5 (6.9)d
8.0 8.50
1.4 (6.4)d
Mg-S 9.5 8.5c
0.3
10.0
(1.27)d
0.2 (1.22)d
11.8 11.20
0.3 (1.51)d
Overall activation energy obtained from the Arrhenius plot of the sum of two parallel reactions. Difference in the activation energies between two parallel paths (eq 5 ) . 0 The overall activation energy calculated by eq 7, where those for cis-2butene were from Arrhenius plots of the rate. d Selectivity ratio a t 60'.
Experimental Section Equipment. The equipment was a closed circulating system (124 or 300 ml in volume, including a reactor) connected to a conventional vacuum line, similar to that described before.6 A U-type reactor (Pyrex, 10 mm in 0.d.) was connected to the circulating system with a 4-way stopcock. The temperature of the catalyst bed which was maintained within h0.5" was measured by a thermocouple set in a well installed inside the reactor. The reaction gas prepared as described below was circulated a t a velocity of 120-150 ml/min through the catalyst bed whose top was filled with clean quartz wool for preheating, The amount of catalyst (30-300 mg) was chosen depending on the catalyst and the reaction temperature, so that sufficient gas circulation, relative to the reaction rate, was attained. Satisfactory gas circulation was confirmed by the invariance of the specific activity and the selectivity upon increasing twice or decreasing one-half the amount of catalyst and the circulation rate. Secondary isomerization during diffusion in micropores may be neglected because (i) good agreement between experiment and calculation was obtained (Figure 3, see later section) and (ii) only a trace of multideuterated species was observed in the products which were mainly monodeuterated species in case of the isomerization over deuterated catalysts.12 The fact that the selectivity ratio became higher or lower than the equilibrium ratio depending on catalyst618denies the possibility that the ratio was determined by secondary isomerization. Therefore, under the present experimental conditions, the reactions wcre considered not to be diffusion controlled. Materials. Silica-supported metal sulfates were prepared as described elsewhere.e A1-S and Mg-S denote aluminum and magnesium sulfates supported
45 on silica gel, respectively. Butenes (Matheson, a t least 99.5% pure) were used after dehydration by the passage through a calcium chloride column and distillation at liquid nitrogen temperature. Procedure. Catalysts were evacuated for 1 hr a t 100" in the reactor prior to run. This pretreatment enabled reproducible rate measurement. Reaction gas was prepared by mixing measured amounts of butene and deoxygenated nitrogen in the circulating system. The reaction was started by the introduction of gas to the reactor by 4-way stopcock operation. The reaction temperature and butene pressure studied ranged from 40 to 100" and from 4 to 22 cm. The total pressure was always kept higher than the atmospheric pressure, so that gas sampling with a syringe through a serum cap did not contaminate the system. About 0.5 ml of gas was sampled at appropriate intervals and submitted to glc analysis. Reaction was usually followed until the initial selectivity ratio and rate constant could reasonably be determined (see Figures 1and 3).
Results Figure 1 shows the progress of the reactions of cis2-butene over A1-S and Mg-S plotted according to the first-order rate equation In (x, - X) = --kt
+ In
Xe
(1)
where x and X, represent the conversion at time t and a t equilibrium, respectively. Although this equation is an approximate one for this reaction,1° a practically straight line was obtained up to 98% attainment of equilibrium conversion, when In (z, - X) computed using rate constants given in Table I1 in the exact rate e ~ u a t i o n 'was ~ plotted against time. Therefore, this equation is useful for practical purpose. Over Mg-S, the reaction followed this rate equation, except initial slight deactivation. Over A1-S, after initial rapid deactivation, the rate gradually decreased showing a slow, time-dependent poisoning, until an apparently stationary stage was reached. The rates measured with varied butene pressure indicated that the reaction order was 0.6-0.8. However, if deactivation which was larger in the experiments with higher butene pressure is taken into account, the reaction order seems close to first order. First-order plots (Figure 1) and the (12) M. Misono, N. Tani, and Y . Yoneda, Ann. S y m p . Cutal., Sapporo ( J a p a n ) , 197 1. (13) The exact rate equation for parallel, reversible first-order reactions among three butene isomers have been derived by Haag and Pines.' The rate constants in this paper are defined as
1
The Journal of Physical Chemistry, Vol. 76, No. 1, 1978
46
MAKOTO MISONO AND YUKIOYONEDA
Table I1 : The Relative R a t e Constants of n-Butene Isomerization over Al-S and Mg-S Temp, OC
--
I _ -
---
Relative rate conatants-kot
klt
klC
kc1
-----Selectivity
ratioa----
kta
ktl
kldht
kot/kol
kto/kt1
0.98 (1.0) 0.98 (1.0)
6.75 (6.9) 6.0 (6.2)
6.6 (6.4) 5.9 (5.6)
1.26 (1.27) 1.23 (1.24)
1.21 (1.22) 1.19 (1.20)
1.52 (1.51) 1.46 (1.46)
Al-S 60
1.0
1.020
1.242
0.184
0.469
0.071
80
1.0
1* 020
1.294
0.216
0.537
0.091
Mg-S
a
60
1.0
0.794
0 * 222
0.184
0.083
0.055
80
1.0
0.813
0.257
0.216
0.107
0.071
Selectivity ratios were slightly modified for calculation, so that eq 2 holds exactly.
The original ones are given in parentheses.
2 , 2,5
-
-
At-S
's
'\
2.0 -
\
Y
s
IO,O KCAL/RGLE
I
I
I
I
0
30
EO
90
T I fq E
I
I
120
2.7
'V
(cis/I), (cis/trans)
=
\
i.0
1 N
(2) 1
kct/kcl
The Journal of Physical Chemistry, Vol. 76, N o . I , 197B
2.8
2.9
3,O
3.1
3.2
Figure 2. Log k us. 1 / T plots for the isomerization of cis-2-butene over A1-S and Mg-S.
(3)
where trans/ 1, for example, represents the initial trans-2-butene to 1-butene ratio from cis-Zbutene. Equation 2 supports that the reaction orders of all six paths were approximately the same and the selectivity ratios were the ratio of rate constants, e.g. (trans/l)
4
k,,
10%~
validity of the exact first-order equation13 (Figure 3, see later section) also supported the idea that all reactions were almost first order. In Figure 2, the logarithm of rate constant obtained from the first-order plot is plotted against 1/T. For A1-S, the mean rate constants at the initial ten minutes were used. The activation energies thus obtained (I%, etc., Table I) were about 7 and 10 kcal mol-l for AlS and Mg-S, respectively. Among the selectivity ratios for each catalyst obtained by the extrapolation of the product ratios to zero conversion, the next relationships were confirmed as in other metal sulfate catalysts6
N
Y
'..\
I
Figure 1. First-order-rate plots of the isomerization of cis-2-butene over AI-S and Mg-S a t 80".
(trans/l)
'\
KCAL/MOLE
150
MlN,
(trans/l)(l/cis)(cis/trans)
1.5
'b, 7.5
(4)
Assuming first-order reactions, the six relative rate constants were calculated for each catalyst as summarized in Table 11, following the ordinary method.'^^ I n calculation, the selectivities were slightly modified, so that eq 2 held exactly. I n Figure 3, the composition changes computed using these rate constants and the exact first-order equation13 are compared with the experimental ones. Good agreement shown in this figure between the calculated lines and the experimental points justifies the assumption that the reactions were all parallel, reversible, and nearly first ordcr, and that the selectivity ratios were equal to the ratios of rate constants (eq 4). The temperature dependences of the selectivity ratios are shown in Figure 4. The trans/l ratio, for example, being equal to kct/lccl, is related to the activation energy difference between the two parallel paths from cis-2butene by the following equationg 1 In (kct/kCl)= -(EGl - ICGt) constant
RT
+
(5)
where ECtand Ecl are the activation energy of trans-2-
KINETIC STUDY OF n-BUTENE
47
ISOMERIZATION
given in Table I1 as 3.7:2.8: 1 for Al-S and 13:3: 1 for
Mg-S
C I R C L E S EXPERINENTAL ~
.
SOLID L I N E S : COMPUTED
Discussion
10
Correlation between Selectivity and Energy Barrier.
It was previously reported for n-butene isomerization catalyzed by supported metal sulfates that the rate of isomerization and the selectivity of cis-trans isomerization over double-bond migration increased monotonously, as the acid strength increased from MgS04 to H2S04.6 Present results demonstrate that this change in the selectivity has its origin in the variation of the energy barriers of the parallel paths. Increase in trans/l ratio from RIg-S to Al-S was
2-
t E k-
Y 5 2 w2 Y
0
10 P E R
20
30
40
C E N T
EO
50
70
C O N V E R S I O N
Figure 3 . Experimental and computed product ratios plotted against conversion. Reaction temperature: 80".
(trans/l) for Al-S 6.9 - 5.7 (trans/l) for Ng-S 1.2
(at 60")
This ratio agrees within experimental error with that calculated from the activation energy difference as exp [{ (Bo1 -
Bct)Al-S
(Ed
-
- Ect)M@,-S}/RTl = exp[(1.5 - 0.2)/RT] = 7
2,7
2,8
2,9
3,O
3,l
3.2
10%~
Figure 4. Temperature dependence of selectivity ratios; Al-S; --, Mg-S.
----,
butene formation and that of 1-butene formation, respectively. Analogous equations are derived for the other selectivity ratios, as well. The activation energy differences thus obtained from the slopes in Figure 4 are given in Table I. The difference between trans2-butene and 1-butene formations and between cis-2butene and 1-butene formations were larger for Al-S (1.4-1.5 kcal mol-') than for AIg-S (0.2-0.3). They were reported to be 0.8 kcal mol-l for silica-al~mina.~ The difference, El, - Elt, was small for both catalysts, as it was for silica-al~mina.~These trends may be compared with eq 3. The following relation which is expected from eq 2 is to be noted among the activation energy differences. (Ec1
-
Ect)
+ (Eit - EiJ
4- (Et, - E$i)
N
Similarly, for cis/l ratio it was 6.4/1.5 = 4.2, as compared with exp[(l.4 - 0.3)/RT] = 5 . 2 , Consequently, the 2-/l-butene ratio increased with the acid strength, as the height of energy barrier to 2-butene became lower, relative to that to 1-butene. The &/trans ratio, on the other hand, was nearly unity for both catalysts, where Elt - El, 5 0.3 kcal mol-'. Table I11 shows the activation energy differences expected from the selectivity ratios and the observed ones. Considering the experimental error and the simple assumption of the neglect of the entropy terms, the agreement is good in general.
Table 111: Comparison of the Observed Activation Energies with those Expected from the Selectivity Ratios, kcal mol-'
----
Found
_-_._Mg-S---Calod'
1.3(2.0) 1.2(1.9) 0 (0)
1.5 1.4 0.0
0.15 (0.9) 0.25(1.0) 0.15(0.15)
A1-S
Eel
-
Eot
Et1 - Et, Elt - El,
___-
Caloda
Found
0.2
0.3 0.3
a Calcd, for example, as trans/l = exp{(E,t - E,,)/RT]. Figures in parentheses were calcd as trans/l = ('/a)exp((E,l E , t ) / R T ] , considering equivalent three hydrogen atoms in 1butene formation.
0 (6)
The relative rates of the isomerization of three isomers were roughly 1:cis:trans = 3 : 2 : 1 over A1-S and 10:2.5:1 over RIg-S at GO". These ratios agreed with those calculated from the relative rate constants
The relative reactivities are also explained by the activation energies. Smaller activation energy for Al-S was reflected in its higher activity. The difference in the overall activation energies calculated independently T h e Journal of Physical Chemistry, Vol. 76, No. I , 1972
48
MAKOTO MISONOAND YUKIOYONEDA
using the temperature dependence of the relative rate constants in the following approximate equation In (hot
+ kOd/(ktc+
&I)
=
(Et - Q/RT
+ constant, etc.
(7) generally agreed with thosc from Arrhenius plots as shown in Table I. The differences among isomers were smaller for Al-S (0.8-1.5 lical mol-l) than for Mg-S (2.3-2.7), as the natural consequence of the smaller activation energy differences between parallel paths for Mg-S (0.2 - 0.3). Those observed activation energies were in accord with the observed reactivity order of 1-butene > cis-2-butene > t~ans-2-butene and the larger difference in rcactivity over Rlg-S than over AI-S. Energy PTofile of Butene Isomerization Cowelated with the Acid Strength of Catalyst. Although discussion given above is valid regardless of the kind of mechanism, the present results are better understood if one considers a common sec-butyl carbonium ion intermediate.l42l5 If a see-butyl carbonium ion (C+) is a common intermediate, the reaction scheme may be written as 1 k,t
kli
cis
trans
According t o this scheme, the selectivity ratios bccome the ratios of rate constants of the parallel paths from the ion, vix., (trans/l) = k , t / k , ~ , etc. The activation energy differences, E,1 - Ect, etc., are then attributed to those between two parallel paths from the ion. The initial rate of the isomerization of each butene may be expressed by the next equations on the assumption of first-order reaction and a stationary concentration of carbonium ion.
v=-
+ + +
d(cis-2-butene) - k,I(klt k , ~ ) .p'sa, etc. dt kil kic kit
(8)
where p and sa. are the butene pressure and the acid content, respectively. Upon the basis of this mechanism, the energy profiles of butene isomerization via a carbonium ion may be drawn as in Figure 5 , following Hightower and Hall.9 In this figure, possible weakly adsorbed states of butenes were added, although their stability is unknown. The overall activation energies, .Ec, etc., give the height of the transition state measured from the starting butene, because eq 8 becomes v = (1/2) k,, p s, for A1-S since k,, N k,t >> k,l (experimentally, 6.9 'v 6.4 >> 1.0 a t 60") and that for Mg-S is v = (2/3) k,, p s, (experimentally, 1.5 _N 1.2 _N 1.0). The heights of transition states were estimated from E,, - Ect,etc. The Journal of Phusical Chemistry, Vol. 76, No. 1, 1972
Figure 5 , Schematic energy profiles of n-butene isomerization via carbonium ion intermediate over A1-S and Mg-S, kcal mol-'.
According to Hammond16 and Leffler and Grunwald, l7 the transition state bears greater resemblance to the less stable state between the initial and final ones. In other words, the transition state more nearly resembles the initial state as the reaction becomes more exothermic. The extent of the resemblance can be represented by a! in LFER, 8G* = a8G, where 8G* and 8G denote small variations in the free energy (or enthalpy) of the transition state in either the initial or final state, re~pectively.~7*~~ If one applies this postulate in the present case, it is expected that the transition state more nearly resembles the carbonium ion as the acid strength of catalyst decreases, since the ion must be less stable over a weak acid than over a strong acid, Little differences among E,I - ECt,etc., observed for JIg-S (0.2-0.3 kcal mol-', therefore, a s 0.1) agreed with this expectation. Large differences for Al-S (1.4-1.5 kcal mol-l, a 0.7) may be explained as the transition state reflecting the energy difference in butenes. Since the doublcbond migration and the cis-trans isomerization would sometimes be dissimilar reactions, LFER is applied here, not in rigorous meaning, but for convenience of better understanding. Similarly, the fact that difference among isomers in the overall activation energies was greater for RIg-s (2.3-2.7 kcal mol-') than for Al-S (0.8-1.5) can be explained as the ion formation was more exothermic
-
(14) Several studies have suggested a carbonium ion mechanism for n-butene isomerization over solid acids such as silica-alumina
and nickel sulfate.8-"$16 The result that deuteration of products, similar t o the former investigations,'ol'6 was observed for the isomerization over deuterated AI-S and Mg-S provides an evidence of protonic acid sites.12 The observed deuterium distribution indicated a carbonium ion mechanism. (15) 8. Ozalti and K. Kimura, 1.Cafal., 3, 395 (1964). (16) G. S. Hammond, J . Amer. Chem. Soc., 77, 334 (1955). (17) 0. A. Lefflerand E. Gqunwald, "Rates and Equilibria of Organic Reactions," Wiley, New York, N. Y., 1963. (18) Y. Yoneda, Int. Congr. Catal., 4th, (1968).
49
ESRSPECTROSCOPY OF BISDITHIOOXALATONITROSYL IRON AKION over Mg-S. The stability of carbonium ion is also spcculatcd upon the same basis. According to the above discussion, the ion is a t least 10.0 - 7.5 = 2.5 kcal morc stable over Al-S. Further, if one should assume a to be 0.5 in S& = d c H , where suffix C means stabilization concerning the catalyst, the ion would be (10.0 - 7 . 5 ) / 0 . 5= 5 kcal more stable over Al-S. In conclusion, over a strong acid the carbonium ion intermediate was rather stable and the energy barrier of 2-butene formation was lower than that of 1-butene formation, probably because the transition states reflected the energy difference in butene isomers. Therefore, the high 2-: 1-butene ratio, as well as the high rate of isomerization and the small difference in the reactivities of three isomers, was observed over A1-S. On the contrary, low activity, low 2-: 1-butene ratio and larger difference in the reactivities of butenes were observed over Mg-S. Over a weak acid, the
intermediate must be less stable and vould bear less resemblance to butene, effecting little difference in the energy barriers between parallel paths. It seems also probable in this case, by analogy with the variation of the elimination mechanism from El to E2, e.g.,19 that the reaction appears more concerted, proton addition becoming slower and the elimination more rapids6,* A concerted mechanism would also favor the doublebond migration.20 These two explanations for weakacid catalysts may not be mutually exclusive as suggested by Lombardo, et al."
Aclcnowledgement. The authors arc indebted to Dr. W. E(. Hall of Gulf Research and Development Company and Professor Y. Saito of thc University of Tokyo for their helpful suggestions. (19) J. F. Bunnett, Sum. f r o g r . Chem., 5 , 353 (1969). (20) J. Turkevich and R. K. Smith, J . Chem. Phys., 16, 466 (1948).
Electron Spin Resonance Spectroscopy of the Bisdithiooxalatonitrosyl Iron Anion by W. V. Sweeney and R. E. Coffman* Chemistry Department, Unitersity of Iowa, Iowa City, Iowa
62240
(Received J u l y 19, 1971)
Publication costs assisted by the National Science Foundation
Liquid and rigid solution electron spin resonance studies were conducted on the bisdithiooxalatonitrosyl iron dianion. The rigid solution spectrum was matched usiiig an efficient FORTRAN IV simulation program. The magnetic symmetry is axial, with spin-Hamiltonian parameters: gl = 2.0162, gl = 2.0345, A I , = 0.00156 cm-l, A1 = 0.0127 cm-l. These parameters were found to be both temperature and solvent dependent. On addition of anhydrous SnC14. a ligand perturbation effect leads to the conclusion that the unpaired electron is in an orbital of symmetry a1 or a2 in the point group Cz,. Consideration of evidence from previous studies on related complexes indicates that the half-filled orbital has d,z character.
The study of the esr spectroscopy of pentacoordinate sulfur-bonded iron nitrosyls is interesting because of the known delocalization of the unpaired electron over the Fe-NO group and because the spin-Hamiltonian g values give information concerning the relative ordering of the one-electron NO'S in relation to the niI0 containing the unpaired electron. Previous studies of molecules of this typel-6 have not yet resulted in agreement concerning the ordering of the predominantly Fe-3d MO's. 3 , 6 The dithiocarbamates and dithiolenes, for example, are so similar that one would expect that one set of Fe-3d molecular orbitals should suffice for both. Accordingly, the dithiooxalate complex should be interesting for purposes of comparison.
We report here a study of the esr spectroscopy of the benzyltriphenylphosphonium salt of the bisdithiooxalatonitrosyl iron dianion, Fe(DTO)2NOZ-,in liquid and rigid solution. The synthesis and structure of this molecule have been reported by Coucouvanis, et al.' (1) J. Gibson, Nature, 64, 196 (1962). (2) A. McDonald, W. D. Phillips, and H. F. Mower, J . A m e r . Chem. Soc., 87, 3319 (1965). (3) J. A. McCleverty, N. M. Atherton, J. Locke, E. J. Wharton, and C. J. Winscom, ibid., 89, 6082 (1967). (4) N. S. Garif'yanov and S. A. Luchkina, Dokl. A k a d . N a u k SSSR, 189, 779 (1969). (5) J. A. McCleverty and B. Ratcliff, J . Chem. SOC.A, 1627 (1970). (6) B. A. Goodman, J. B. Raynor, and M . C. R. Symons, ibid., 2572 (1969). The Journal of Phusical Chemistry, Vol. 76, N o . 1, 1972
