KOTES
June, 1963 pared to the molecular size. For the rubber systems, the D' values a t 7 = 0 niay represent the mobility of the benzene molecules among the polymer chains. The present experiments suggest that the extrapolated D' values are close to the true D' values at T = 0. This idea as well as the others presented here should be investigated by further experimental and theoretical studies. Recently, other workerss have studied the diffusion of water in a sodium carboxymethyl cellulose gel by a different spin-echo method and a conventional diffusion method involviiig about 24 hours diffusion time. The two methods yielded the same diffusion coefficient values. For this system, the diffusion distance niay be large compared to the average molecule-to-barrier distance. (6) W. T. Higdon and J. D. Robinson, J . Chem. Phgs., 37, 1161 (1962).
1367
W
s 0
20
40
60
80
IO0
120
140
160
180 ZOO
PRESSURE OF I,5-HEXADIENE tmmi.
Fig. 1.-Rates of formation of komeric products as a function of the pressure of lj5-hexadiene: A, allylcyclopropane; 0, bicyclol2.l.l] heqane. Room temperature, mercury resonance radiation (2537 A. ).
MERCURY PHOTOSEXSITIZED ISOMERIZATION OF 1,6-HEXADIENE1 BYR. SRINIVASAX International Business Machines Corporation, l'homas J . W-atson Research Center, Yorktown Heights, N e w York
I1
Received December I & 1968
It has recently been shown2 that the mercury photosensitized isomerization of 1-butene leads to methylcyclopropane as one of the products. The quantum yield of tthe isomer was found to change with the pressure of 1-butene, the value going through a maximum a t about 65 mm. Since the double bonds in 1,5-hexadieiie are both terniinal and isolated from each other, the mercurysensitized isomerization of this compound may be expected to resemble that of 1-butene. Experimental 1,5-Hexamdienefrom K and K Laboratories (Jamaica 33, as obtained. Vapor phase chromatography showed that it contained a small amount of another hydrocarbon as an impurity but the latter did not seem to undergo photosensitized reactions in the system, since i t was recovered unchanged. The light source was a bank of 12 germicidal lamps (General Electric G8T5) arranged in a circle with their long axes parallel to the narrow (10 mm. diameter) cylindrical quartz cell. The intensity of the 2537 A. line at the center of the cell was 7.3 x 1016 quanta/cc ./sec. A conventional vacuum line in which stopcocks were not excluded was used to degas the sample, evacuate the cell, and fill it. Irradiation was carried to conversions which ranged from about 10% a t the lowest pressure to less than ly0a t the highest pressure. Product analysis was ca,rried out by gas chromatography on a 2-meter Ticon oil column (Perkin-Elmer Column R,) a t room temperature.
N,Y.) was used
Results Mercury-sensitized decomposition of 1,5-hexadiene gave two isomeric products as well as numerous other products which are belj.eved to be of free radical origin. The isorners were identified to be allylcyclopropane (I) and bicyclo [2.1.1]hexane (11). (1) This is part I1 of "Kinetics of the Pliotocheinical Dimerization of Olefins to Cyclobutane Derivatives." For part I , see J . Am. Chem. Soc., 84, 1141 (1962). 12) R. J. Cvetanoric and L. C. Doyle, J . Chem. Phgs., 37, 513 (19G2).
The infrared spectrum in each case agreed with that of an authentic sample prepared by the mercury-sensitized decomposition of norcamphor.3 Quantitative analysis of these products was conducted by measuring the areas under their peaks in the chromatogram. In Fig. 1, the rates of formation of allylcyclopropane and bicyclohexane are plotted as a function of the initial pressure of 1,j-hexadieiie in the system. The rates may be taken as being proportional to the quantum yields. It also was observed that the products of free radical origin decreased steadily with an increase in pressure. Discussion The formation of allylcyclopropane from 1,thexadiene in this system is analogous to the formation of methylcyclopropane from 1-butene on mercury photosensitization. CHB CH2--CHz--CH=CHz \ / 'CH
(1)
The fact that the yield of allylcyclopropane goes through a maximum at a pressure of 28 mm. of l,5hexadiene while the yield of free radical products decreases contiiiuously with increasing pressure suggests that the details of the photochemical processes in the two systems may also be similare4 Cvetanovic and Doyle2 have proposed the following mechanism for the 1-butene-mercury system
B
+ Hg* --+
B*(+ Hg)
B* --+ decomposition I3* ----f
c*
(2) (3)
(4
(3) R. Srinivasan, J . A m . Chem. Soc , 83,4923 (1961) (4) It can be estimated t h a t the quantum Sield for allylcyolopiopane ai the maximum IS about 0.10 which 18 of t h e same order of magnitude as the quantum yield reported for metliylc>clopropane in the 1-butene-mercury system. 2
1368
NOTES B* C*
+M+B +M +
C* +decomposition M+ C M
+
(5) (6)
(7)
where B* and C* refer to excited molecules of 1-butene and methylcyclopropane, respectively. This mechanism will predict a maximum for the quantum yield of
C. On the assumption that a similar mechanism may be operative in the present study, equations 2 t’o 7 can be rewritten as (with B and C representing 1,5-hexadiene and allylcyclopropane, respectively)
B
+ Hg*
B*
B* +H* H*+ M+H
+M
( 14)
(15)
H* -+ decomposition (16) These equations will lead to an expression of the following nature for either Cor H .
which is exactly similar to equation 1 that was derived by Cvetanovic and Doyle2 in the case of 1-butene. It follows that the yields of both allylcyclopropaiie and bicyclo [ 2 . l .llhexane should show a maximum with pressure whereas in fact (Fig. 1) only the former shows such a maximum. The explanation for the disagreement may be one of two possibilities. (i) I n equation 17, the pressure a t which @ is a niaximum will be a function of a, b, and c which in turn are ratios of the rate constants k 8 . . . . k16. Since the rate constants for the formation and decomposition of allylcyclopropane and bicyclohexane may differ considerably, the maximum for the latter will be different from that for the former.6 Cvetanovic has pointed out6 that a combination of a relatively high value for Mmax and a relatively low value for emax may make it experzmentally difficult to locate the maximum in the case of bicyclo [2 . 1. 1]hexane. (ii) A second explanation is based on an alternative mechanism. The electronically excited triplet 1,shexadiene molecule that is formed in reaction 8 may contain up to 20 kcal./mole of vibrational energy which it may lose stepwise in several collisions. If the vibrational levels are designated by subscripts, the molecule of B that is formed in (8) will he Bn*. The deactivation reactions may be represented by the general equation
31
+ Bn* +M + B , *
Bn*
(18)
From three of these levels (or sets of levels), n, m, and the three products, free radicals, C, and H may be formed 0,
( 5 ) It can be predicted t h a t the maximum for bicyclo[Z.l.l]hexane mill occur at a higher pressure of l,5-hexadicne than the maximum for allylcyclopropane. (6) R. J. Cvetanovic, private communication.
+free radicals
(19)
B,* +C Bo* +H With the addition of one more step to provide formation of excited 1,S-hexadiene molecules zeroth vibrational level
+
M R,* it can be derived that @free radical = h 9 / k 1 9
@C
+ EIg
(8) and so on with equation 9 through 13 replacing equations 3 through 7. The following additional steps would account for the formation of bicyclo [2.1.1]hexane ( H ) . --+
Vol. 67
add
aH
+
--+
Bo*
+M
kl8lCf
+ k&T)(k:o + kz2M)
=
kl&zoM/(klg
=
k18k22M2/(klg klsW!)(k20 k&)
+
+
(20)
(21)
for the in the (22)
(23) (24) (25)
This mechanism is seen to predict the pressure dependence of all three of the products ~ o r r e c t l y . ~I n particular, the expression for @c,is identical with eq. 17 and is capable of the same simplifications to give a linear dependence of (M/@C)”~ us. M. The present study admittedly does not resolve this conflict. However, it is clear that the experimental pressure dependence curves can be generated on the basis of more than one mechanism. Acknowledgment.-The author wishes to thank Dr. R. J. Cvetanovic for his friendly criticism during the course of a lengthy correspondence. He is grateful to the members of the organic chemistry group a t the Research Center for their advice and encouragement. (7) T h e prediction of the trends in the formation of bicyclohexane is only qualitatively correct. A comparison of the curve in Fig. 1 with eq. 25 clearly shows t h a t other steps which involve H should be included in the mechanism.
A LANGMUIR DETERMINATIOS O F THE SUBLIMATION PRESSURE OF BOROK1 B Y ROBERT c. P.4ULE
AND
JOHN L.
MARGRAVE
Department of Chemistry, Unisersity of Wisconsin, Madison, Wiseonsin Received January 9, 1968
Alt,hough several different groups of workers have reported Knudsen vapor pressure measurements on boron, no two sets of measurements agree and there is a difference of about 5 x lo4 in vapor pressure values between the two extreme sets of measurements. Searcy and Myers2 have made B-vapor pressure measurements using an initial and final weighing technique with C, Ta, and ZrBz Knudsen crucibles. The lids and orifices of all three types of crucibles were made of Ta. Considerable reaction occurred between the B and the C or Ta crucibles and lids. ilfter the experiments, some free B was found in the Knudsen cells. Since the borides of T a and C are considerably less volatile tha,n B, i t was assumed that the vapor pressure of B was measured. Searcy and Myers gave a higher reliability to the vapor pressure measurements made in the ZrBz crucibles (with T a lids and orifices), since thesc cells minimized boride formation. Akishin, et aL,3 made B-vapor pressure nieasurements (1) Abstracted, in part from the thesis of Robert C. Paule presented in partial fulfillment of the requirements for the Ph.D. degree of the University of Wisconsin, January, 1962. ( 2 ) A. W. Searcy and C . E. Myers, J . Phgs. Chem., 61, 967 (1967). (3) P. A. Akishin, 0. T. Mikitin, and L. N. Gorokhov, Dokl. A k a d . Nauk S S S R , 129, 1077 (1959).
