72
J . Phys. Chem. 1986, 90, 72-75
TABLE VI: Electrical Conductance A (0-l cm2 equiv-I) and Concentration (mol dm”) for LiBF, in Dimetboxymethane at 25.00 OC run 104~ A 1
78.304 174.61 1 311.22 430.65
6.562 X 5.887 X 6.606 X 8.060 x 10-5
2
1279 2018 5026
2.379 X 4.382 x 10-4 5.662 x 10-3
3
874.6 1080 1574 3550
1.282 X IO4 1.589 X 2.523 x 10-4 1.359 X
electrolyte being dimerized to apolar dimers. An ultrasonic relaxation of Debye type is interpreted as due to the second step of a coupled two-step dimerization process. Kinetic and thermodynamic parameters have been extracted from the data. In particular, the formation constant for the second step of dimerization K2 equals 6.6. The constant for the first step K ,
cannot be determined without a reliable knowledge of a4, the pair-to-pair minimum separation distance. Appendix
Electrical conductance and concentration data for LiBF4 in DMM at 25 OC are given in Table VI. Note Added in Proof: After the completion of the present work we have performed static permittivity measurements a t f = 3.5 MHz with a Bontoon resonator and a cell of capacity Co = 5.0, 0.0, pF of a solution o.0g3 M LiBF4 in DMM at 25 OC. The average of two experiments gave eo = 2.8* f O.O1, a value close to the figure extrapolated from the microwave range to = 2.78 as reported above. This confirms that hypothesis advanced above that the electrolyte is heavily associated to apolar dimers.
*
Acknowledgment. We express our thanks to the Army Research Office, Research Triangle Park, NC, for their support through Grant No. DAAG/29/85/K0048. Registry No. DMM, 109-87-5; LiBF4, 14283-07-9.
Energy Transfer and Pooling between Mg(3P) and Ca Atoms A. T. Pritt, D. Patel, and D. J. Benard* Rockwell International Science Center, Thousand Oaks, California 91 360 (Received: July 2, 1985)
Energy transfer from Mg(3P) to Ca(3P) was found to occur by a fast resonant transfer from Mg(3P) to Ca(’D) and a subsequent slow collisional deactivation of the Ca(’D) to the 3P state. Energy-pooling reactions between Mg(3P), Ca(’D), and Ca(3P) were also observed to populate several highly excited states of Ca up to 45 000-cm-’ excitation.
Introduction Benard and Slafer originally observed excitation of Ca atoms to the 3P state upon effusing Ca vapor into a flow of discharged Mg atoms.’ The primary energy carrier produced by the discharge was the 3Pmetastable state of Mg. Referring to Figure 1, energy transfer from Mg(3P) to Ca(3P) may occur by three routes: (1) direct nonresonant transfer from Mg(3P) to Ca(3P); (2) near-resonant transfer from Mg(3P) to Ca(3D) followed by allowed radiative decay to the 3Pstate; and (3) resonant transfer from Mg(3P) to the metastable Ca(’D) state followed by collisional relaxation of the Ca(’D) either to the 3D state (which radiates to the 3P state) or to the 3P state directly. As will be shown, a fourth route is possible in which energy-pooling reactions produce highly excited states of Ca that radiatively cascade back to the 3P state. To evaluate which of these mechanisms is primarily responsible for the Ca(3P) production, experiments were conducted in which mixtures of Mg and Ca vapors and Ar buffer gas were optically pumped by a pulsed dye laser. By changing the wavelength of the dye laser, initial concentrations of Mg(3P), Ca(’D), and Ca(3D) could be generated. The subsequent rise and fall of the various Mg and Ca excited states produced by energy transfer were then analyzed to determine the kinetic mechanism. In general, only one component of each triplet manifold could be optically pumped or observed in emission due to the strength of the AJ = 1 selection rule.2 The splitting within the triplet states of Mg and Ca, however, are small compared to k T . Therefore, collisions with the buffer gas rapidly equilibrated the nonradiative and the radiative states within each triplet m a n i f ~ l d . ~Conse(1) Benard, D. J.; Slafer, W. D.; Love, P. J. Chem. Phys. L e f t . 1977, 48, 321. (2) Wiese, W. L.; Smith, M. W.; Miles, B. M. Nor/. Stand. Ref: Data Ser., Natl. Bur. Stand. 1969, No. 22.
0022-3654/86/2090-0072$01.50/0
quently, in analyzing the data, the triplet states were each treated as single kinetic entities. Experimental Section The Mg and Ca vapors were supported in a cross-shaped, electrically heated stainless steel heat pipe oven at temperatures near 900 K, where the vapor pressure of Mg is approximately 1 torr and the Ca vapor pressure is approximately two orders of magnitude ~ m a l l e r . ~ Details of the oven’s construction are contained in previous publication^.^ From 20 to 500 torr of Ar buffer gas was added to the oven to prevent condensation on the windows. The vapors within the oven were optically pumped by the output of an excimer-driven dye laser (2-mm-diameter beam, 10 mJ/lO ns, 0.01 nm fwhm), which was capable of saturating6 the Mg(’S 3P) transition at 457 nm. The resulting laser-induced fluorescence was viewed at right angles to the laser beam by a cooled GaAs photomultiplier tube with spectral discrimination provided by a 0.25-m monochromator or selected interference filters. The photocurrents were passed to a wide bandwidth preamplifier and a transient digitizer/signal averager where the signals were time resolved and summed over repetitive shots of the dye laser to reduce noise.
-
Results and Discussion In most of the experiments, optical pumping of the Mg/Ca vapor at 457 nm was employed to generate an initial concentration of Mg(3P). Following the laser pulse, the Mg(3P+’S) emission (3) Wright, J. J.; Dawson, J. F.; Balling, L. C. Phys. Rev. A 1974, 83. (4) Nesmyanov, A. N. ‘Vapor Pressure of the Elements”; Academic Press: New York, 1963. ( 5 ) Malins, R. J.; Benard, D. J. Chem. Phys. Lett. 1980, 74, 321. (6) Pritt. A. T.; Patel, D.; Benard, D. J. Chem. Phys. Lett. 1984, 105, 667.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 73
Energy Transfer between Mg and Ca
3
P
J
-
1!
I
I
I
I
I
0
100
200
I
I
I
I
I
I
'Pl
458 nm492 nmI
500
600
Figure 3. Effect of Ar buffer gas on initial rate of Ca(-'P)production.
MAGNESIUM
300 400 PRESSURE (torr)
CALCIUM
Figure 1. Energy level diagram for Mg and Ca showing the transitions that were optically pumped (1) by dye laser and those that were subsequently observed in emission (1).
I
I
I
I
I
I
0
50
100
150
200
250
TIME I PSI
Figure 2. Time profiles of Ca(3P) following pulsed dye laser excitation of Mg(lP) in Ar buffer gas.
intensity decayed to half its initial peak value in approximately 100 ps. This behavior was observed in a similar experiment6 in pure Mg vapor with H e buffer gas and is attributed to rapid Mg(3P) self-annihilation reactions. In our previous work the Mg(3P) decayed to half its initial value in approximately 10 ps; longer Mg(3P) decays were obtained in these experiments by operating the heat pipe a t slightly lower temperatures which reduced the Mg vapor pressure. The presence of Ca in the heat pipe also tended to reduce the Mg vapor pressure through Raoults law due to the sohbility of M g in liquid Ca. These changes had the effect of reducing the initial Mg(3P) concentration that was produced by the dye laser, which in turn increased the duration of the second-order decay of the Mg(3P). Since the Mg(3P) concentration was approximately two orders of magnitude larger than the Ca atom concentration, the decay of the M B ( ~ P was ) not significantly affected by the presence of the Ca vapor. Energy Transfer. Figure 2 shows the rise and fall of the Ca(3P) concentration at three different buffer gas pressures, following pulsed dye laser excitation of Mg/Ca vapor at 457 nm. The initial
rise of the Ca(3P) occurs on the time scale that is commensurate with the decay of the Mg(3P). The decay of the Ca(3P) which occurs on a longer time scale is influenced by the quenching of Ca(3P) due to collisions with the buffer gas and ground-state Mg atoms5 as well as by the presence of residual Mg(3P) atoms at long times. Therefore, it is difficult to gain an understanding of the energy-transfer process from the long-time behavior of the Ca(3P). Figure 3 demonstrates a linear relationship between the initial rate of Ca(3P) generation and the buffer gas pressure. This strong pressure dependence is incompatible with production of Ca(3P) by transfer from Mg(3P) to Ca(3P) directly or transfer from Mg(3P) to Ca(3D) which then radiates to the 3P state since neither of these processes depends on collisions with the buffer gas. The nonzero intercept that is shown in Figure 3 implies that some Ca(3P) production does occur in the absence of buffer gas, equal to the increase in the rate of Ca(3P) production that is obtained by addition of approximately 50 torr of Ar. Therefore at 500-torr buffer gas pressure 90% of the Ca(3P) that is generated involves collisions with the buffer gas. This behavior can be explained by resonant energy transfer from the Mg(3P) state to the metastable Ca('D) state which then requires collisions with the buffer gas to decay to the 3P or 3D states provided that deactivation off Ca(lD) is the rate-limiting step in the formation of Ca(3P). Since Ca('D) is the only metastable state of Mg or Ca, apart from the 3P states, no other intermediate state is capable of producing the observed pressure dependence. Any intermediate state which decayed by allowed transitions would be incapable of delaying the peak of the Ca(3P) time profile significantly on the 100-ps time scale, as shown in Figure 2. To further verify the production of Ca(3P) via transfer through the ID state, a series of experiments were performed in which ovens containing pure Ca vapor or Mg and Ca vapor mixtures and buffer gas were optically pumped at 492 and 458 nm to produce Ca()D) and Ca('D), respectively. Opical pumping of the 3D state produced Ca(3P) emission which rose to its peak in less than 100 ns with or without Mg present. Since the Ca(3D) state decays to the 3P state by allowed transitions,' this result was expected. Optical pumping of Ca(lD) also produced Ca(3P) both with and without Mg present but with Mg present, we were able to observe the production of Mg(3P), which rose to its peak concentration in less than 200 ns. The peak intensity of Ca(3P) produced by pumping the Ca('D) state in the absence of Mg was comparable to that which was obtained by pumping Mg(3P) in the presence of Ca. These results establish a pathway from Mg(3P) to Ca(3P) via the Ca('D) state and imply that the transfer of energy from Mg(3P) to Ca('D) occurs at a gas kinetic rate on the order of cm3/s. Therefore, the Ca(lD) concentration equilibrates with the (7) Diffenderfer, R. N.; Dagdigian, P. J.; Yarkony, D. R. J . Phys. B 1981, 14, 21.
74 The Journal of Physical Chemistry, Vol. 90, No. 1, 1986
Pritt et al.
--
-
-
4. The following transitions were particularly intense; 423 nm ('P IS); 430 nm ('P 3P); 445 nm ('D 'P); 488 nm (IF ID); 551 nm (IS 'P); 559 nm ('2D 'D);586 nm ('D 'P); 614 nm (3S 3P); 646 nm ('F 'D), and 672 nm ('P ID). These lines, the resonance lines of Mg and Ca at 285 and 423 nm, respectively, and the 518-nm Mg(3S-3P) transitions were of comparable intensity and between one and two orders of magnitude brighter than the 657-nm Ca(3P IS) transition. The Mg lines at 518 and 285 nm are due to Mg('P) energy pooling with itself and have been observed by Breckenridge'O and Bourigignon" in previous experiments in pure Mg vapor, as well as in our own laboratory.6 The low intensity of Ca(3P-1S) emission that is shown in Figure 4 actually represents a substantial concentration of Ca('P). During the first 100 p s after the laser pulse saturates the Mg(IS-+'P) transition, the average Mg(3P) concentration is about 67% of the total Mg-atom concentration. A_450 500 550 600 650 During this time the recorded intensity of Ca(3P-+1S) emission WAVELENGTH is about one tenth of the Mg('P-+'S) emission. The effective Figure 4. Emission spectrum produced by 457-nm pulsed dye laser IS transitions in Mg and Ca are 71 radiative rates of the 'P excitation of Mg/Ca vapor at 900 K in 500 torr of Ar buffer gas. and 870 s-', respectively,*and the instrument response was a factor of 1.6 larger at the wavelength of the 3P 'S transition of Ca Mg('P) on a time scale that is essentially "instantaneous" with than for Mg. These results imply that the average Ca('P) conrespect to any significant changes of the Mg('P) or Ca('P) concentration was approximately 30% of the total Ca-atom concencentrations. tration if a Mg to Ca vapor pressure ratio of 100 is assumed. If The approximate rate of Ca( 'D) collisional deactivation can the ratio of the Ca('P) to Ca('S) concentrations exceeded the 9 be obtained from the 500-torr curve that is shown in Figure 2. to 1 degeneracy ratio, then a population inversion would exist During the first 100 p s after the laser pulse, the Ca(3P) conwhich may allow lasing on the 657.1-nm transition. This does centration approaches its peak value in an exponential manner not seem a likely possibility, however, in view of the relative rates 50 ws, which would imply a colwith a time constant of about of Ca('D) and Ca('P) collisional deactivation by Ar mentioned cm3/s. Since the lisional deactivation rate of about 4 X earlier. The more intense transitions which were pumped by Mg('P) concentration is also decaying during this time, the true energy pooling radiatively decay on the 10- to 100-ns time scale; time constant is somewhat longer. Quenching of Ca('P) also tends therefore, the peak concentrations of the corresponding excited to shorten the observed time constant. Therefore, the value of states are actually three to four orders of magnitude smaller than 4X cm3/s should be considered an upper limit on the rate the peak concentration of Ca(3P). of collisional deactivation of Ca('D) by Ar. This result shows As with the transfer of energy from Mg(3P) to Ca('P), several that collisional deactivation of Ca('D) is the rate-limiting step possible routes can lead to production of the highly excited Ca in the production of Ca('P) and also explains why no deviation states by energy pooling. The most frequent energy-pooling from linearity is observed in Figure 3 up to buffer gas pressures collisions occur between two Mg(3P) atoms. Since Mg('P) of 500 torr. The rate of Ca('P) quenching by Ar measured by self-annihilates at a high rate, Mg('P) energy pooling may rapidly Malins and Benard5 is nearly equal to the upper limit that we have populate the excited 'S, 'P, and 'S states of Mg from 35 000- to obtained on the rate of Ca('D) collisional relaxation. Therefore, 44 OOO-cm-' excitation. The excited 'S state would decay radithe peak concentration of Ca('P) tends to remain relatively atively to the 'P while the 'P and 'S excited states decay radiatively constant as the buffer gas pressure is increased even though the to the 'S ground state and the 'P state, respectively. Because of initial rate of Ca('P) generation increases significantly with Ar the large concentrations of ground state Mg('S) and metastable pressure as shown in Figures 2 and 3. Mg('P), the IP and 'S states could have prolonged lifetimes due Our result for the collisional deactivation of Ca('D) by Ar is to radiation trapping. Transfer from these states to ground-state also much slower than Wright's* measurement of the rate of Ca atoms could then produce the highly excited Ca states that collisional deactivation of Ca(IP) to the 'P state. Wright suggested were observed in emission. Typical radiation trapped lifetimes,I2 that Ca(lP) decayed to the 3P state by first radiating to the 'D on the order of 10 p s or less, however, are too short to allow state which subsequently decayed by collisions and radiation to efficient transfer to Ca atoms in this experiment. With typical the 'P state. The comparison of our results to Wright's suggests Ca concentration of 10'4/cm3 and gas kinetic transfer rates of that perhaps another, more efficient mechanism is operative in lo-'' cm3/s, only 10% of the highly excited Mg atoms would Wright's experiments, such as collision-induced intersystem transfer to Ca prior to decay in our experiment. Excited states crossingg from the lP state to the 'D energy levels which then up to 37 000 cm-' can be generated by energy pooling between radiate to the 'P state. Since we did not try to detect the 1.96-pm Mg('P) and Ca('P) since substantial concentrations of Ca(3P) Ca(3D-3P) emission, we are unable to determine whether the are formed before the Mg(3P) decays significantly. Since the rate Ca('P) in our experiment is produced from the Ca('D) by colof Ca(ID) deactivation is small compared to the rate of transfer lisional deactivation to the 'P state directly or by quenching to from Mg('P), a significant Ca(ID) population coexists with the the 'D state which radiatively decays into the 'P state. UnderMg(3P). Therefore, energy-pooling reactions between Mg(3P) standing this difference may be critical to resolving the discrepancy and Ca(lD) may also account for the production of the highly between the rates of Ca('P) and Ca( lD) collisional deactivation. excited Ca states. In this case the Ca('D) lifetime' is sufficiently If the Ca('D) quenches directly to the 'P state, while the Ca('P) long to allow efficient energy pooling to occur with Mg(3P). deactivates to the 'D state, then the rate of Ca(IP) deactivation Energy-pooling reactions between two excited Ca atoms unshould be larger since the 'P-'D splitting is smaller than the ' E 3 P doubtedly occur but at much lower rates than the corresponding splitting. Mg/Ca reactions. Energy Pooling. When the Mg/Ca vapor was optically pumped By studying the pressure dependence and time history of the at 457 nm to produce Mg('P), the vast majority of the visible states p r o d u d by energy pooling, one can in many cases ascertain emission occurred out of high-lying states of Ca between 30 000and 45 OOO-cm-' excitation. The spectrum of the visible emission during the first 100 p s after the laser pulse is shown in Figure
--
-
6
I
nw
(8) Wright, J. J.; Balling, L. C. J. G e m . Phys. 1980, 73, 1617. (9) Hale. M. 0.; Leone, S. R. J . Chem. Phys. 1983, 79, 3352.
-
-
(10) Breckenridge, W. H.; Nikolia, W. L.; Stewart, J. J. Chem. Phys. 1981, 7 4 , 2073. (1 1) Bourgignon, B.; Rostas, J.; Taieb, G. J . Chem. Phys. 1982, 77, 2979. (12) Hqennekens, J.; Gallagher, A. Phys. Reu. A 1983, 28, 238.
J. Phys. Chem. 1986, 90,75-78 the parentage of the different excited states. Positive or negative pressure dependence indicates production involving the Ca(3P) state or the Ca(lD) state, respectively. A sudden rise in less than 1 ps shows the involvement of Mg(3P) or Ca('D), whereas production involving Ca(,P) shows a nearly linear rise over the first 100 ps. The Ca(,S) state was observed to have an initial quadratic increase with time, indicating that it was produced primarily by energy pooling between two Ca(3P) atoms. This nearly resonant reaction has previously been observed by Benard and M a l i n ~ .The ~ excited states of Ca that were produced between 37 000 and 44000 cm-' can only be accessed by Mg(3P) + Ca(ID) energy pooling or transfer from the radiation trapped Mg('P?S) states. The latter possibility can be ruled out since all of the excited Ca states above 38 000 cm-I rose to their peak concentrations in a small fraction of a microsecond. With knowledge of the initial Mg(3P) concentration and the radiative lifetime of Ca(,P), one may infer approximate rate constants for the energy pooling reactions on the order of lo-" to cm3/s from the relative intensities of the Ca(3P-1S) emissions that resulted from the energy-pooling reactions. Since
75
the transitions excited by energy pooling decay either to the 3P state or to states which in turn decay to the state by radiation or collisions, some Ca(3P) can be produced by energy pooling and radiative decay. This route to Ca(,P) generation does not depend on collisions with the buffer gas and therefore is of secondary importance at pressures greater than 50 torr. Summary The transfer of energy from M&P) to Ca atoms can generate the 3P state by a variety of mechanisms. Above 50-torr buffer gas pressure, resonant energy transfer from Mg(,P) to Ca('D) followed by collisional deactivation of the Ca( 'D) state becomes the dominant process. The rate of transfer from Mg(,P) to Ca(ID) is gas kinetic, while the rate of collisional deactivation is much slower and is the rate-controlling factor in the production of Ca(,P) in the range of 50 to 500 torr. During the energy-transfer process, energy-pooling reactions between Mg(3P), Ca(ID), and Ca(3P) produce a variety of more highly excited states of Ca that decay radiatively by allowed transitions. Registry No. Mg, 7439-95-4; Ca, 7440-70-2.
Energy Barrier for 1,P-Hydrogen Migration in Benzylbromocarbene Michael T. H. Liu* and R. Subramanian Department of Chemistry, University of Prince Edward Island, Charlottetown, Prince Edward Island, Canada C I A 4P3 (Received: July 5, 1985)
Thermolysisand photolysis of 3-bromo-3-benzyldiazirine with tetramethylethylene in isooctane yielded E- and Z-0-bromostyrene and 1-benzyl-1-bromo-2,2,3,3-tetramethylcyclopropane. The intramolecular-intermolecular rate constants derived from the thermolysis and photolysis, when applied to an Arrhenius plot, fall on the same straight line. An A factor of s-l and an activation energy of 4.7 kcal mol-' were obtained for the 1,2-H migration in benzylbromocarbene.
The intramolecular 1,Zhydrogen migration to a divalent carbon has been a subject of considerable interesP3 and certainly is an important process in the chemistry of carbenes. Extensive calculations have been performed on the barrier of this rearrangement for a variety of ~ a r b e n e s . ~In general, there are discrepancies among the calculated results reported by different laboratories.2 In 1983, Burnett, Stevens, Feigerle, and Lineberger5 examined the vibrational structure in the vinylidene photoelectron spectrum and deduced that there should be at least some barrier to hydrogen migration. In particular, their observation tends to rule out the very low barriers calculated by some research g r o ~ p s . More ~~.~ recently, the energy barrier for benzylchlorocarbene arrangement (6.4 kcal mol-') has also been reported.6 We now report on the energy barrier for 1,2-H migration in benzylbromocarbene. Experimental Section The 3-bromo-3-benzyldiazirine was synthesized according to Graham's method.' Isooctane and tetramethylethylene were ~
(1) 'Carbenes", R. A. Moss and M. Jones, Jr., Ed., Wiley, New York Vol. I, 1973; Vol. 11, 1975; W. Kirmse, 'Carbene Chemistry"; 2nd ed., Academic Press, New York, 1971. (2) H. F. Schaefer, 111, Acc. Chem. Res., 12, 288 (1979). (3) W. M. Jones, 'Rearrangements in Ground and Excited State", Vol. 1, P. DeMayo, Ed., Academic Press, New York, 1980, pp 95-160. (4) (a) N. Bodor and M. J. S. Dewar, J. Am. Chem. Soc., 94,9103 (1972); (b) E. P. Kybar, Ibid.,99, 8330 (1977); (c) J. A. Pople, K. Raghavachari, M. J. Frish, J. S. Binkley, and P. v. R.Schleyer, Ibid., 105, 6389 1983; (d) Y. Osamura, H. F. Schaefer 111, S. K. Gray, and W. H. Miller, ibid.,103, 1904 ... (19x1) --,( 5 ) S. M. Burnett, A. E. Stevens, C. S. Feigerle, and W. C. Lineberger, Chem. Phys. Lert., 100, 124 (1983). (6) M.T. H. Liu, J . Chem. Soc., Chem. Commun., 982 (1985).
fractionally distilled before use. Irradiation was carried out with a 275-W G E sunlamp until all of the diazirine was destroyed. Temperature control during photolysis was within fO.l OC. A CS-052 filter was used to allow transmission of wavelength > 350 nm to avoid product isomerization. N M R spectra were recorded on Varian T60 and NICOLET 360 spectrometers. Relative yields of products were determined by gas chromatographic analysis on a Varian VISTA 6000 instrument using a 6 ft X 2 mm (i.d.) glass column packed with CSP-2OM. Peak areas were integrated by a H P 3390A recorder. Peak area ratios were converted to mole ratios by application of correction factors obtained by the analysis of mixtures of products of known composition. The isolation of the products was carried out by column chromatography using silica gel and eluting with 9 5 5 pentane-diethyl ether. Thermolyses were carried out in a sealed Pyrex tube immersed in an oil thermostat which could be maintained to fO. 1 OC in the temperature range 70-100 OC. Photolysis of 3-Bromo-3-benzyldiazirine, 1 , in TME. The diazirine (0.3 g, 1.4 X mol) in 5 g of T M E was irradiated for 16 h at 20.0 OC. Removal of T M E and chromatography on silica gel gave 175 mg of @-bromostyrene[nmr (CDCl,) 6 7.06 (phenyl), AB quartet with the other pairs centered at 6 6.53 and 6.98 (E-olefinic H), and AB quartet with the other pairs centered at 6 6.30 and 6.92 (Z-olefinic H)*] and 112 mg of l-benzyl-lbromo-2,2,3,3-tetramethylcyclopropane[nmr (CDCl,) 6 7.21 (s, 5 H), 3.32 (s, 2 H), 1.33 (s, 6 H), and 1.18 (s, 6 H)].
\ - -
0022-3654/86/2090-0075.$01.50/0
(7) W. H. Graham, J . Am. Chem. SOC.,87, 4396 (1965). (8) L. J. Dolby, C. Wilkins, andT. G. Frey,J. Org. Chem., 31, 110 (1966).
0 1986 American Chemical Society
