NOTES
4135
Pressure Effects in the Photolysis
of CH3CD3at 1470 A
by Hajime Akimoto and Ikuzo Tanaka h b o r a t o r y of Physical Chemistry, Tokyo Institute of Technology, Ohokayamu, Meguroku, Tokyo, J a p a n (Received June 9, 1967)
on the photolysis of ethane in the Several vacuum ultraviolet region have been reported in respect to the primary-decomposition modes and the nature of intermediate excited ethylene or ethylidene. As to the photolysis of CH3CD3,the following reaction scheme has been proposed both at 1470 and at 1236 A in the earlier paper.lr2 CHaCD3
+ h~ +HZ + C2HD3* + Dz + CzHD
YD+
+M
CzHDi
(1) CHaCD3
+ h~
Dz
Results and Discussion The data are presented in Figures 1 and 2. Figure 1 shows the pressure dependence of the ratio of ethylene to acetylene produced in the phot'olysis of C2H6 and CH3CD3 at 1470 A, room temperature, and at very low conversion less than 0.01%. The figure also con-
+ CzH3D* + H2 + CzHD
YD+
C2Hz C2HD (1')
+M
CH3CDi
Experimental Section The experimental procedures used in these experiments are the same as described previously.'s2 Since some of the experiments were performed under very low conversion (less than 0.01 %), CzH6 obtained from Takachiho Chem. Ind. Co. and CH3CD3from Rlerck Sharp and Dohm Ltd. were purified by gas chromatography until the impurity ethylene was reduced to less than 0.0003%. Activated charcoal column was used to detect the ethylene and acetylene in the huge amount of unreacted ethane. The reaction cell composed of a magnetic circulator and glass tube of 15 mm 0.d. was designed so that the reactant might be circulated most effectively especially near the window.
+ h~ +H + D + C2HzD2
(2)
On the other hand, Hampson and McNesby4 have studied the pressure dependence of the formation ratio of ethylene to acetylene for the photolysis of C2H6 and clarified that the intermediate excited ethylene decomposes into acetylene and hydrogen unimolecularly competing with collisional stabilization to ethylene. They have also suggested the presence of another intermediate ethylene species which cannot decompose even without collisional stabilization, and proposed the following scheme.
+ h~ +Hz + C2H4** CzHe + h~ +2H + CzH4
CZH6
C2H4**
C2H4*
CzH4**
+ hl
+ Hz CzH4 +
CzH4*
+ 31
CzH4
C2H4** + CzHz
+ 34
(3) (4)
(5) (6)
(7) (8)
This work has been conducted in order to study the photolysis of CH3CD3at 1470 A in more detail and to check the validity of reaction schemes 1 and 2 and 3-8.
't 0
100
200
300
P (mm) Figure 1. Ethylene to acetylene ratio us. pressure of ethane in the photolysis of C2He (0) and CHICTI, (0)at low conversion, and Hampson's data' ( A ) for C2Hs at 310'. (1) R.F. Hampson, Jr., J. R. McNesby, H. hkimoto, and I. Tanaka, J . Chem. Phys., 40, 1099 (1964). (2) H . Akimoto, K. Obi, and I. Tanaka, ibid., 42, 3864 (1965). (3) R. F. Hampson, Jr., and J. R . McNesby, ibid., 42, 2200 (1965). (4) R. F. Hampson, Jr., and J. R. McNesby, ibid., 43, 3592 (1965).
Volume 71. Number 12
.Vovember 1967
4136
NOTES
is extrapolated to zero. However, this is not the case in Figure 2, but the yields of CzH3Dand CzHD3start from a nonzero value a t zero pressure and increase with pressure. This fact suggests directly that one part of C2H3D and CzHD3 results from some process which does not need collisional stabilization, and the other part of them descends from some excited ethylene species which can decompose competing with collisional stabilization. The presence of ethylene which can be formed without collisional stabilization strongly supports the assumption proposed by Hampson and McN e ~ b y . ~On the other hand, the absence of pressure dependence of CzHzDzsupports the assumption that the formation of CzHzDzcorresponds to reaction 2. Thus we have the modified reaction scheme
0.61
+ CzHD3** 5DZ+ CzHD HD + CzDz \Hz + CzHDs* -+ CzHD, (10) (11) CH3CD3+ h v -%-Dz + CzH3D**+Hz + CzHD
CH3CD3
1 0
I
100
I
I
200
300
I
Loo
500
k.’[M1
tains several plots of Hampson’s data4 a t 310”. It has been shown’ that internal scavenging of hydrogen atom produced in reaction 4 occurs a t room temperature (9)
so that the ethylene to acetylene ratio should decrease as a function of conversion. On the other hand, no scavenging should occur at 310” as postulated by Hampson and M ~ K e s b y . ~The ? ~ agreement of our present data with those at 310” implies that essentially no scavenging of hydrogen atom occurs at low conversion less than O.Ol%, and therefore, the ratio in Figure 1 reflects the primary yields of ethylene and acetylene. Now that the ratio of ethylene to acetylene has been found to be dependent on the ethane pressure, it must be instructive to investigate how the isotopic distribution in ethylene in the photolysis of CH8CD, changes with pressure. Figure 2 shows each yield of CzH3D, CZH2Dz1and CzHD3 as a function of pressure. The yield is calculated based upon the isotopic distribution of ethylene and the primary formation ratio of ethylene to acetylene which has been obtained in Figure 1 and is normalized so that the total primary yield of ethylene and acetylene should be unity. Assuming that the isotopic distribution in ethylene is independent of conversion, the yield in Figure 2 can be recognized as the primary one of each ethylene. According to reactions 1 and 2, the yields of CzH3D and C2HD3must be zero when the pressure of CH3CD3 The Journal of Physical Chemistry
\ kd
Figure 2. Yields of CZHDZ(o),CZH~D ( O ) , and C2H2DZ(0)v.9. prwsure in the photolysis of CH3CD3.
+ H + M +CZHS + M
HZ
pa
Wmm)
CzH4
+ hv
k8IMI
I
\
HD + CzHz (10’)
+ CzH3D* +CzH3D CHaCD3 + hv ’2 H + D + CHzCDz Dz
(11’) (12)
Assuming the above scheme, the following equations are obtained @(CzHDz) = = ~ @O(CZHDO)
3
;
(13)
p2
@o(CzHaD)=
1 - = 1(1 @(CzHDd - @o(CzHD3) pi
493’
1 + k,--)[MI kd
(14) (15)
where @(CzHD3) is the normalized yield of CzHD3and (PO(CzHD3) is that a t zero pressure, etc. Figure 3 shows the plots of experimental values of the lefthand side of ey 15 and 15’ against the reciprocal of CH3CD3 pressure. Using the data in Figures 2 and 3, the values for PI,493, PI’, 93’, 492, k s l k d , and ks’/kd’ are obtained as follows: PI = 0.23, pa = 0.32, pl’ = 0.20, 493’ = 0.10, Pz = 0.15, k,/kd = 0.010 mnl-’, ks’/kd’ = 0.004 mm-’. It should be noticed that) the present values of PI ‘p3 = 0.55, PI’ p3’ = 0.30, and PZ agree well with earlier ones,’ 0.50, 0.35, and 0.15 respectively, which were calculated from isotopic distribution of ethylene and acetylene without knowledge of the pressure effect. This agreement assures the validity of the relative ratio of each primary process
+
+
XOTES
4137
CH3CDa
+ h~ +Hz + C2HD3**
kd
+Dz
k . 1 ,.[MI\
HD
+ CzHD + C2Dz
C2HD3* 4CzHD3
30 '
(18) etc., but we are not at the stage of choosing one of the alternatives. Further work on the photolysis of ethane at the absorption edge is under investigation.
I 0
I
I
I
0.01
4 mm")
1
0.02
I
1 0.03
(
Figure 3. Reciprocals of @(CtHDa) - %(CzHDa) (0) and @(C,HsDI - Qo(CzHaD)( 0 )us. reciprocal of pressure of CHaCDa.
(5) hf. C. Sauer, Jr., and L. M. Dorfman, J. Chem. Phys., 3 5 , 497 (1961). (6) H. Akimoto and I. Tanaka, unpublished data. (7) H. M. Frey, J . Chem. SOC.,2293 (1962). (8) C. L. Kibby and G. B. Kistiakowsky, J . Phys. Chem., 70, 126 (1966). (9) H. hf. Frey and I. D. R. Stevens, J . Chem. SOC.,1700 (1965). (10) D. P. Chong and G. B. Kistiakowsky, J . Phys. Chem., 68, 1793 (1964).
The Magnetic Properties of Some Europium Chelates
reported earlier.' The present values of (p3/(p1 = 1.4 or (o3'/$01' = 0.5, and k , / k d = 0.010 mm-l or k a ' / k d ' = 0.004 mm-l are also compared to 1.22 and 0.0047 mm-l for the photolysis of CzH6 reported by Hampson and McNesby. As to the assignments of CzHD3**and CzHD3*,both of them can be distinguished from the electronic excited singlet state attained by direct light absorption in the point of reactivity. Direct photolysis of ethylene has been studied to give both hydrogen molecule and atoms following to the reaction5
The relative importance of each process has been deterP ~ l.05 ~ and 0.66 at 1470 and 1634 mined to be ( P ~ ~ / ( = A, respectively. Since the C2HD3**produced in reaction 10 should have maximum energy 7.0 ev, which is equivalent to 1770 A, it should also yield hydrogen atoms if it is in the electronic excited singlet state. But this is not the case in our experiments, and the inclusion of such an atomic process could not explain the product analysis reported ear1ier.I~~The circumstances are the same for the photolysis of d i a z ~ e t h a n e , rnethyldia~irine,~ ~?~ and methylketene,lO and the formation of hydrogen atom was not reported in either case. Our present data can also be interpreted by the alternative scheme which is equivalent to reactions 3-8
by T. M. Shepherd United Kingdom Atomic Energy Authority, Atomic Weapons Research Establishment, Aldermaston, Berks, England Accepted and Transmitted bg The Faraday Society
(April 4, 1967)
Considerable interest has been shown recently in the coordination chemistry of europium diketone systems mainly on account of their liquid laser applications. The magnetic properties of europium chelates have only received limited attention.' This paper reports the results of magnetic susceptibility measurements of several salts of europium tetrakisbenzoyltrifluoroacetonate, EU(BTFA)~-, over the temperature range - 150" to 100". Trivalent europium and samarium differ from the other trivalent lanthanides in exhibiting a relatively small spin-orbit coupling constant. With Eu3+, the ?F1and 'Fz states are sufficiently close to the lowest J state, the 7F0, to be populated at ordinary temperatures. The Curie-Weiss relationship is therefore not obeyed and the magnetic susceptibility becomes a complicated function of temperaturea2 Experimental data obtained for europium salts have been in good agreement with Van Vleck's theoretical values except at very low temperatures.2J (1) T. Moeller, D. F. Martin, L. C. Thompson, R. Ferrds, G. R. Feistel, and W. J. Randall, Chem. Rev., 65, 1 (1965).
Volume 71. Number 19 Xovember 1967
