86
Communications to the Editor
(4) D. S.Matteson and R. A. Bailey, Chem I d . . , 191 (1967); J Am Chem Soc., 90, 3761 (1968). (5) J. 9.Moffat and R. J. Collens, J. Mol. Spectrosc., 27, 252 (1968). 16) C. C. Costain and B. P. Stoicheff. J. Chem. Phvs.. 30. 777 (19591. (7) J. D. Morrison and A. J. C. Nicholson, J Chem k y s ; 20,' 1021i1952j. (8) C. Leibovici, J. Mol. Sfruct., 9, 177 (1971). (9) P. A. Mullen and M. K. Orloff, Theor. Chim Acta(&rL), 23,278 (1971). (10) R. Radeglis and E. Gey, J. frakt. Chem., 314, 43 (1972). (1 1) W. J. Hehre, R. F. Stewart, and J. A. Pople, J. Chem. Phys., 51, 2657 (1969), and subsequent papers; QCPE program no. 236.
(12) W. S.Wilcox, J. M. Goldstein, and J. W. Simmons, J. Chem. fhys., 22, 516 (1954). (13) K. Bolton, N. L. Owen, and J. Sheridan, Spectrochim. Acta, fat? A, 26, 909 (1970). (14) D. H. Liskow, C. F. Bender, and H. F. Schaefer, 111, J. Am. Chem. Soc., 94, 5178 (1972). (15) D. Booth and J. N. Murrell, Mol. fhys., 24, 1117 (1972). (16) P. K. Pearson, H. F. Schaefer, and V. Wahlgren, J. Chem. Phys., 62, 350 (1975).
COMMUNICATIONS TO THE EDITOR
Competitive Radiotracer Evaluation of Reiatlve Rate Constants at Stratospheric Temperatures for Reactions of 3eCiwith CH, and C,H, Vs. CH,=CHBr' Publication costs assisted by the U S . Energy Research and Development Administration
Sir: The rate constants and activation energy for the abstraction of H from CHI by atomic C1, as in (l),have C1 + CH,
-+
HCl t CH,
(11
been evaluated by several techniques over the past decade and especially during the last 2 yearsa2t3The rate constant is of special interest in the 210-270 K range because of its importance with respect to the stratospheric C10, chain reaction which results in ozone removal.4* Since estimates of the ozone depletion corresponding to a given stratospheric chlorine concentration vary almost linearly with this rate constant (slower rate corresponding to more ozone depletion), an error of a factor of 1.5 at 225 K assumes considerable practical significance. We have now applied a radiotracer method to the study of the competitive reactions of 38Clwith a hydrocarbon (methane, ethane) vs. reaction with vinyl bromide, using 38Clatoms formed in situ by irradiation of gaseous CClF3with thermal neutrons of the ambient temfrom a nuclear r e a ~ t o r .Variation ~ perature of the samples during irradiation between 243 and 361 K has permitted evaluation of the relative activation energies and relative rate constants for the reactions with the two gaseous hydrocarbons,and could be readily applied to other hydrogen-containing compounds.8 Our measurements indicate a rate constant for kl at 243 K of 3.2 f 0.6) X cm3 molecule-l s-l, in agreement with the lower limits of values considered in the past several years. The fundamental measurement in each system is the percentage yield of CH2=CH3sC1from 38Clreactions with CH2=CHBr, following the sequence of reactions outlined in equations 1-4. The 38Clatoms, although formed with
+ CH,=CHBr C2H,Br38C1*+ M C,H,Br38C1 + HI T l
-f
-+
-f
C2H,Br38Cl*
-+
C,H3Br3'C1*
(2)
C,H,Br38C1 + M
(3)
C,H,Br38C1 + I
CH,=CH38C1 + Br
The Journal of Physical Chemistw, Vol. 81. No. 1, 1977
(4)
(5)
excess kinetic energy from the 37Cl(n,y)38C1 nuclear reaction, are thermalized by multiple collisions with the excess of inert CC1F3.7 The competitive deexcitation of CzH3Br38C1* by collision in (3), and subsequent reaction of the thermalized radical can be confirmed through the observation of CH2BrCH238C1 in the presence of a hydrogen-donating scavenger molecule such as HI. In the present experiments molecular O2was used as a scavenger to remove the stabilized radicals of (3) without the release of CHz=CH3sC1. At any particular temperature and pressure of CC1F3a fixed fraction of the radicals formed in (2) will subsequently decompose to CHz=CH38C1 by reaction 5. However, when hydrogen abstraction from a hydrocarbon is also available for removal of 38Clatoms, as in (1)or (6), 38c1+
C,H,
~ 3 8 c 1 +C,H,
(6)
the yield of CHz=CH38C1will be correspondingly reduced to the extent that the thermal 38Cl atoms have been prevented from reacting with CHz=CHBr through prior reaction with the hydrocarbon. This competition is expressed in eq 7 for CH4 and CHz=CHBr, in which
1 Y(CH,,=CHJ8 C1)
=
A
+p-)(
)
(CH4) Bh2 (CH2=CHBr)
(7)
YcHFcH38Cl is the fractional yield of 38Clas CH2=CH38C1 and all of the 38Clis assumed to be available for reactions 1, 2, and 6. (If small corrections are made for "hot" reactions with CCIFBthe numerator on the left-hand side is about 0.95 instead of 1.00.) The intercept parameter A and the slope parameter B both involve reactions 2-4, but are independent of the concentration of RH as long as (RH)
