NOTES
2685
because of a contribution to the broadening from viscosity. The half-width of the 981-cm-1 line of sulfate ion (the same line studied in the proton transfer experiments) has been investigated for 1.2 M ammonium sulfate solutions containing different amounts of glucose, used to vary the viscosity. Ideally a colorless substance was desired which, when added in trace amounts, would increase the viscosity substantially. Neither glycerol nor Kelzan, a water soluble polymer supplied by the Kelco Chemical Co., were satisfactory. Aqueous glucose solutions have two broad bands located at 1070 and 900 cm-l. It was possible to obtain a good 981-cm-' line profile between these two. Five solutions were prepared. Each was 1.2 M in ammonium sulfate and contained sufficient glucose to provide viscosities of 2.0, 11.5, 85.0, 274.0, and 500.0 cP, respectively. The viscosity was measured in a Brookfield small sample chamber viscometer, 0.1". Solujacketed for temperature control to 25 tions were contained in a Raman cell fitted with a water jacket and were maintained at 25 f 1". Spectra were excited by the 435.8-nm mercury line and obtained on the Cary 81 spectrophotometer. A slit width of 10 cm-l was used. The half-widths were measured, both manually and with the aid of a computer routine for the study of line shape.8 For each solution a width of 14.5 =k 0.5 cm-I was found. Within a wave number the halfwidth of the symmetric stretch of sulfate does not change when the viscosity of the medium changes from 2 to 500 cP. This independence is expected. The depolarization ratio of the symmetric stretch of a tetrahedral molecule is zero and the polarizability tensor is isotropic; in this case rotatory motions have no effect on the width of lines.' Any anisotropy, induced by the environment in the liquid phase, will be small.'O I n light of these results it is believed that the Raman line broadening data for sulfuric acidgand bisulfatess require no correction for viscosity.
*
(10) W.F. Murphy, M. V. Evans, and P. Bender, J . Chem. Phys., 47, 1836 (1967).
Experimental Section Hot 38Clmas produced by means of the 37C1(n,y)38C1 reaction using the 1-Mw reactor of Washington State University. Samples of the desired reactant were sealed in Vycor ampules along with small amounts of Ar, 02,and dichloroethylene. All liquids were degassed in the vacuum system prior to use. Gases were obtained from Matheson Co. and used without purification. Dichloroethylene, used as a scavenger, was obtained from Eastman Co. Reagent grade CCI, was used directly. Product analysis was by conventional radiogas chromatography using columns of either silicon GE SF-96 or silicon DC 550 on firebrick. Adsorption of halocarbons by stopcock grease is a frequent problem in this work, particularly if the compound being determined is present only at tracer levels. To eliminate this difficulty, the inlet system of the gas chromatograph was constructed using Teflon stopcocks. (1) (2) 86, (3) (4)
Replacement Reactions of Hot Chlorine Atoms in Chlorofluoromethanes
R. Wolfgang, Progr. React. Kinet., 3 , 97 (1965). C. M. Wai, C. T . Ting, and F. S. Rowland, J . Amer. Chem. Soc., 2525 (1964). C. M. Wai and F. 9 . Rowland, J . Phya. Chem., 71, 2752 (1967). C. M. Wai and F. S. Rowland, ibid., 72, 3049 (1968).
(5) C. M. TVai and F. S. Rowland, (1968).
by S. C. Lee and C. 0. Hower* Department of Chemistrg, University of Idaho, iMoscow, Idaho (Received October 14, 19YO)
used with great success to study the inertial and steric factors that affect hydrogen-atom reactions. There is now considerable interest in extending these studies to hot halogen reactions in order to build a general model of the factors controlling the reactivity of monovalent chemical species. Recoil studies of chlorine atoms were initiated by Wai and R o ~ l a n d ~and - ~ by Spicer and Wolfgang.8 Recoil fluorine has received relatively more a t t e n t i ~ n . ~Spicer, Todd, and Wolfgang have measured yields of hot fluorine replacement products in fluoromethanes'" and hot chlorine replacement products in chloromethanes," and discuss the trends observed in terms of steric factors and a "translational inertial" effect. We report here the results of measurements made on the absolute yields of hot replacement products in chlorine-atom reactions with chlorofluoromethanes. These results represent the first systematic study of C1 for F replacement reactions and the first comparison of C1 for F with C1 for C1 replacements in the same molecule. Yield patterns are similar to those reported by Spicer, et ai., but some significant differences are also observed.
89849
Publication costs borne completely by The Journal of Physical Chemistry
In the last decade, the reactions of translationally hot tritium atoms, produced by nuclear recoil, have been
J. Amer. Chem. Soc.,
90, 3638
C. M. Wai and F. S. Rowland, i b i d . , 91, 1053 (1969). C. M . Wai and F. S. Rowland, J . Phys. Chem., 74, 434 (1970). L. Spicer and R. Wolfgang, J . Chem. Phys., 50, 3466 (1969). See for example (a) C. F. McKnight, N. J. Parks, and J. W. Root, J . Phys. Chem., 7 4 , 217 (1970); (b) T. Smail and F. S. Rowland, {bid., 7 4 , 1859 (1970). (10) L. Spicer, J. F. J. Todd, and R. Wolfgang, J . Amer. Chem. Soc., 90, 2425 (1968). (11) L. Spicer and R. Wolfgang, ibid., 90, 2426 (1968). (6) (7) (8) (9)
The Journal of Physical Chemistry, Vol. 76, N o . 17, 1971
2686
NOTES I
T
Table I : Replacement Product Yields for CFJCl as a Function of Xeon Concentration Neon,
IO
a
CFaCl yield, %
CFzClz yield,
%
0 65 88
0.74 i 0.09 0 . 4 4 dz 0.05 0.03 i= 0.01
0 . 6 5 i= 0.06 0.32 + 0.03 0.05 It 0.01
%
5.0
-I
->
w
W
I-
3
-1
0 v)
m U
1.0 I
Ts-
I
I CCIZ8CIF
0
1
2
3 4 5 6 7 8 MOLE % C z H z C I z
9
1
0
Figure I. Dependence of product yields on dichloroethylene concentration for chlorine atom reactions with CF2Cli. O2 (1%) and Ar (1%) are also present.
Labeled compounds were detected with an external proportional counter. Absolute yields were determined by using the 40Ar(n,y)41Ar reaction as an internal monitor. Both 41Ar and 38Clwere counted with the same detector and no correction was made for a difference in detection efficiency. These nuclides decay by high energy p transitions, and since the counter window was thin (5 mg/cm2), error due to this assumption is small. We estimate a possible 5% error in the reported absolute yields from this source. An additional systematic error of 10% is estimated for the measurement of the ratio of argon to freon in the samples. Only errors due to counting statistics and data analysis are reported with the results.
Results and Discussion The goal of this work was to measure the absolute yields of the hot C1 for C1 and C1 for F replacement reactions in the compounds studied. The assumption that these reactions proceed primarily by means of hot atoms was first tested by malung scavenger and moderator studies. Dichloroethylene and oxygen were used as a scavenger combination following the experience of Wai and Rowland3in scavenging thermal C1 atoms from the CH3C1 system. Figure 1 shows the dependence of product yields on C2H2C1,concentration for a mixture of CF2Cle,O2 (l%),and Ar (lye). The results indicate that 3% C2H2C12and 1% O2 is an efficient scavenger combination in CF2Cl2. This same combination was The Journal of Phgsicol Chemistru, Vol. 75, N o . 17, 2971
used with all of the reactant gases, the assumption being made that the scavenging efficiency would be about the same in the other reaction systems. Table I shows the effect on yields of adding neon to scavenged CFaC1. The yields of labeled CF3C1 and CF2CIz decrease as neon concentration increases, and approach zero at high neon concentration. We are confident, therefore, that the replacement reactions measured in well-scavenged freons are due only to hot atoms. Hot replacement reactions mag frequently deposit sufficient excitation energy in the primary product to cause its subsequent decomposition. It has been reactions d e r n ~ n s t r a t e dthat ~ ~ F~ for ~ ~ F~ replacement ~~ are followed by extensive decomposition, amounting in some cases to 80% of the total replacement yield. The extent of decomposition following hot chlorine replacement reactions is not known, but at least some decomposition may be assumed to occur. Decomposition will compete with stabilization by collision, and product yields may consequently be sensitive to pressure. It mas therefore of interest to study product yields as a function of pressure. The yields of labeled CF2C1, and CFC& from chlorine atom reactions with CF2C12were measured at six different pressures in the range 3 to 46 cm. Yields mere found to be independent of pressure within this range. This result cannot be interpreted t o mean that decomposition is not occurring. It may be that these pressures were not high enough to bring about stabilization of a significant fraction of excited primary products. However, this does demonstrate that small variations in pressure from sample to sample will not perturb the yield patterns which we observe. The absolute per cent yields of products resulting from C1 for C1 and C1 for F replacement reactions in CF4, CF3Cl, CF2Cl2, CFCls, and CCl, are given in Table 11. Pertinent measurements by Spicer and Wolfgang on replacement reactions in chloromethanes are included in the table to facilitate comparisons. I n all cases the reaction mixture consisted of the reactant molecule, 3% C2H,C12, 1% 0 2 , and 1% Ar. Total pressures were about 60 cm. Reactions a i t h CF, were studied in approximately equal molar mixtures of CCb (12) C. F. McKnight and J. SV. Root, J . Phus. C h e m , 73, 4430 (1969). (13) Y . K.Tang, T. Smail, and F. S. Rowland, J . Amer. Chem. SOC., 91, 2130 (1969).
2687
NOTES Table I1 : Yieltls of Hot Chlorine Replacement Reactions Reactant molecule (from ref 11)
70
Reactant molecule (this n.ork)
Yield,
%
cc1,
C1 for C1 Replacement 2.4a CFsCl 1.7" CF2Clz 0.6" CFCls 0.6" CCl,
0.73 f0.05 1 . 0 =!co0.15 1 . 4 7 =t 0 . 1 5 0.28 i 0.10
CHBF
C1 for F Replacement CF, CF3C1 CFzCl, 3 ,
