COMMUNICATIONS TO THE EDITOR
4175 original site of activation or by rupture of the original ring (eq 2b).4 Equal amounts of decomposition of the two rings is direct evidence for internal energy randomization prior to decomposition. Variation of pressure D. K. HOFFMAN of the system and the rate of collisional stabilization w of the hot HBC molecule (eq 3)
This is the statistical analog of the maximization of the anti-Helmholtz function for the equilibrium between reacting ideal gases. INSTITUTE FOR ATOMICRESEARCH AND DEPARTMENT OF CHEMISTRY IOWA STATEUNIVERSITY AMES,IOWA50010 RECEIVED AUGUST24, 1970
CF2-CF-CF-CF2*
\/' CHz
Intramolecular Energy Relaxation.
Sir: Working theories of unimolecular decomposition2 assume that internal energy relaxation occurs on a time scale that is short relative to decomposition; effective randomness of the internal energy distribution in excited molecules leads to the prediction of random incidence of decomposition events. Considerable evidence of various kinds and of varying degrees of cogency has now been accumulated that supports the general validity of this p o s t ~ l a t e ,although ~ there can be no doubt that failure must occur for sufficiently short-lived species. We have for some time been attempting to make an even more direct and quantitative experimental test of intramolecular energy relaxation than has hitherto existed. To this end we have applied the technique of chemical activation in order simultaneously to symmetrize and activate a substrate molecule of interest. The substrate finally chosen for study was hexafluorovinylcyclopropane (HVC). Addition of CD2 to HVC produces symmetrical, vibrationally excited hexafluorobicyclopropyl (HBC)
\/
+ lCD2
--j
CHz CF2-CF-CF-CF2"
\/
\/
CH2
CD2
v CH2
\/
CDz
+CF2--CF-CF=CD2
C D2
\/ CHz
\/
CD2
+ ni (3)
provides a time scale against which the rate of internal relaxation can be measured. Obviously one wishes to shorten this time scale by pushing the system to higher and higher pressures; this kind of experiment has the advantage over others that have been made recently at high p r e ~ s u r ein~ ~that ~ three-body effects which can vitiate the lattere as valid tests of the energy-randomization postulate will be self-canceling by the present test. In these experimeonts, HVC was prepared by the photolysis at 2800 A of a ketene-perfluorobutadiene mixture. This fluorocarbon system is preferable to its hydrocarbon analog, which we first investigated, since the reactions of methylene are cleaner with the fluorocarbon; also, decomposition of the hot product proceeds uniquely by split-off of a CF2 m ~ i e t yrather ,~ than by a variety of ring isomerization reactions that ensue for the hydrocarbon analog. The HVC was purified and photolyzed again with ketene-& in the presence of excess CO bath gas which contained 0.5% 02. These diatomics inhibited complicating reaction of any %H2 formed.7i8 After careful and complete gas chromatographic separation of the tetrafluorovinylcyclopropane products from all other components of the reaction mixture, analysis was made with a 3IS9 mass spectrometer. Rupture of products I and I1 by electron impact produces cyclopropyl and vinyl ions and results
(1)
where the asterisk signifies vibrational excitation. Datoms were used as a tracer to distinguish the original and product rings and otherwise differ unimportantly from H atoms. The excited HBC may decompose by disruption of the product ring (eq 2a) which was the CFz-CF-CF-CF2"
\/
CF~--CF-CF-CF~
A Novel
and Direct Test of the RRK-RRKM Postulate la
CF-2CF-CF=CFz
+ M A
+ CFz
(1) (a) Work supported by the National Science Foundation; (b) Standard Oil predoctoral fellow. (2) (a) L. Kassel, "Kinetics of Homogeneous Gas Reactions," Reinhold, New York, N. Y. 1932; (b) R. A. Marcus and 0. K. Rice, J. Phys.Colloid Chem., 55,894 (1951). (3) The first striking evidence was given by J. N. Butler and G. B. Kistiakowsky, J . Amer. Chem. SOC., 8 2 , 759 (1959) ; for a review see L. D. Spicer and B. S. Rabinovitch, Ann. Rev. Phys. Chem., 21, 349 (1970). (4) N . C. Craig, T. Hu, and P. H. Martyn, J . Phys. Chem., 7 2 , 2234 (1968). (5) D. W. Placzek, B. 9. Rabinovitch, and F. H. Dorer, J . Chem. Phys.,44,279 (1966)). (6) J. Aspden, N. A . Kawaja, J. Reardon, and D. J. Wilson, J . Amer. Chem. Soc., 91,7580 (1969). (7) B. A. DeGraff and G. B. Kistiakowsky, ibid., 71, 1553 (1967); 71,3984 (1967). (8) J. W. Simons and B. S. Rabinovitch, J. Phys. Chern., 6 8 , 1322 (1964).
The Journal of Physical Chemistry, Vol. 74,No. $3, 1970
COMMUNICATIONS TO THE EDITOR
4176 in ion peaks at m/e = 47 and 95 from I and m/e = 45 and 97 from 11;unfortunately, the masses corresponding to the vinyl group do not have a unique origin and the intensities of these peaks are confirmatory rather than diagnostic. The energy of the excited bicyclopropyl parent is not known exactly but may be estimated to average 95-100 Table I : Product Ratios on Decomposition of HBC Pressure, mm
0.80 1.0 4.0 9.2 20 60 310
47/45a
1.07 i 0.08' 1.07 f 0.06 1.06 f 0.10 1.12 f 0.03 1.18 f 0.03 1.18 =!= 0.01 1.10 f 0.11
95/97b
1.21 f 0.06' 1.21 f 0.05 1.24 f 0.08 1.23 f 0.04 1.22 =k 0.02 1.26 =!= 0.02
a Uncorrected mass spectral ratios. a 13% of a 97 + 95 overlay correction, only, applied; a more complete correction does not change the ratio significantly. Standard deviation of individual analyses.
The Journal of Physical Chemistry, Vol. 7 4 , N o . $3, 1QYO
kea1 mol-'; the tail of the energy distribution carries energies to above 100 ltcal mol-'. The critical threshold for reaction is knowng to be less than 50 kea1 mol-l, so that the excess energy above threshold is a like amount in this case. Table I shows the product ratios over a 100-fold range of pressures. Their constancy with pressure variation is evident. A minor massspectral H-D isotope effect may be expected to be present but has no influence on the interpretation. The highest pressure corresponds to a relaxation time for collisional stabilization of ~ 1 0 - ' 0 see. The validity of the theoretical post,ulate regarding internal energy randomization is borne out to the extent tested. The work will be extended t o higher pressures and to reverse isotopic substitution. (9) F. P. Herbert, J.A. Kerr, and A. F. Trotman-Dickenson,J . Chem. Soc., 5710 (1965).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF WASHINGTON SEATTLE, WASHINQTON 98105
J. D. R Y N B R A N D T ~ ~ B. S. RABINOVITCH
RECEIVED AUGUST26, 1970
