The Journal of Physical Chemistry, Vol. 82, No. I , 1978 97
Vibrational Energy Transfer of 1,1,l-Trifluoroethane
(27) J. I. Zink and R. S. Drago, J . Am. Chem. SOC.,92, 5339 (1970). (26) We have also measured the 13C Isotropic shifts and line widths for the cobalt complexes of glycine and alanine with a NMR spectrometer and obtained a pD dependence of the shlft and line width similar to the case for histidine. (29) For CoZt complexes, the experiment values of the electron spin relaxation time T~ are in the range 1 0 - ~ ~ - 1 0s30 - ~at~r m m temperature while the rotational correlation time T , at the same temperature Is approximately 10-io-lO-li s3' for molecules of the size of a histidine-cobait complex in solution. Therefore, under these conditions the relation 1 1 >>~ 1 ~/ ~holds. , Furthermore, we obtain the followin relations in the case of very fast electron spin relaxation time: T> >> TU-', T , = T~ = T ~ Using . these effective correlation times in combination with the relations, W:T: >> 1, w ' T >> 1, the Solomon-Bloembergen equations are reduced
(9)
(38) (39)
TI^-' = T,M-' = {4/3[gZp2r~*S(S + l)/r6] t '/S(S
+ l ) ( A / f i ) '} T ,
(8)
(30) J. Eisinger, R. G. Shulman, and B. M. Szymanski, J . Chem. Phys., 36, 1721 (1962). (31) J. J. Led and D. M. Grant, J. Am. Chem. Soc., 97, 6962 (1975). (32) 2. Luz, J. Chem. Phys., 41, 1756 (1964). (33) 2. Luz and S. Meiboom, J. Chem. Phys., 40, 1058 (1964). (34) R. G. Shulman, H. Sternlicht, and B. J. Wyluda, J . Chem. Phys., 43, 3116 (1965). (35) Large formation constants36for this complex, K1 = lo6.' and K2 = 10' ', at 37 OC, allow us to set the simple ratio, p = [Co2+]/ [ His], and the simple coordination number, q = 2, under the following conditions: (i) in the high pD region where all functional groups of histidine are completely dissociated and (ii) at high concentration of the ligand molecules compared with that of cobalt ion where the possible species, Co(His),, may be almost achieved.
+ His--+ Co(His)+ &(His)+ + H i s Co(His),
Coz+
-+
(40) (41) (42) (43)
Kl
K, (44)
(36) D. D. Perrln and V. S. Sharma, J. Chem. SOC. A , 724 (1967). (37) As is discussed in the previous section, the line widths for Co and imidazole carbon (C,, C4, C,) resonances are controlled by the slow exchange condition which is characterized by the inequality, AwM2 >> TM-', rN-2. he observed paramagnetic shift Awb is thus governed by the equation
The change of 7, with an increase of pD Illuminates a decrease in the observed paramagnetic shift (Awb) at pD >4 for Co and pD >5.5 for imidazole carbons as shown in Figure 3. Followed by eq 9, it is interpreted that Aob decreases as the T u becomes long. Y. Sano and H. Tanabe, J. Inorg. Nucl. Chem., 25, 11 (1963). There have been a number of reference^^'-^* on the analysis of relaxation times induced by a paramagnetic ion which encounters chemical exchange modulation. The paramagnetic nuclear relaxation times have been discussed under the condition that TZMcan be put into the observed transverse relaxation time TZpand the ratio of T i / TPpis set equal to TIM/ T2,. However, the chemical exchange effect does not ailow us to set T,, equal to TPpwhich is so often a function of exchange rate 7M-I. Especially in the presence of a CoZt ion, an exchange mechanism due to 7, is of significant importance as is discussed in the previous section and hence the two relaxation times should be separately analyzed. Therefore the observed relaxation time would rather provide us with the information on the kinetics of the system considered here. W. G. Espersen and R. B. Martin, J. Phys. Chem., 80, 161 (1976). W. G. Espersen and R. 8. Martin, J. Am. Chem. Soc., 96, 6111 (1974). R. E. Wasylishen and M. R. Graham, Can. J. Chem., 54, 617 (1976). It should be mentioned that in Figure 5b the slow exchange imR holds for T2;' of C, when the temperature is raised above 45 OC. Thereupon, in order to galn further insight into the complex form at high temperature, we have measured the visible spectra of aqueous solution of 0.04 M CoCI2.6H20 and 0.59 M histidine at elevated temperatures. The resuits show that the spectra vary with raislng the temperature from 25 to 91 OC. The absorption maxima were shifted to larger wavelengths with an increase in the molar absorptivity. The absorption maxima at 25 and 91 OC in the visible spectra were at 506 nm (e 12.8) and 513 nm (e 14.4), respectively. Though the complex formation at high temperature gives rise to such a slight change in the visible spectra, we can speculate that these two spectral changes at high temperature may be due to the amino group associated with metal binding. The longer electron spin relaxation time for Cu(I1) usually results in serious broadening of ligand resonances so that experiments should be performed at histidine to Cu(I1) molar ratios of 10' or greater. Therefore, we have carried out the NMR measurements of the Cu(I1) system in dilute solutions compared with the case of Co(I1) and Ni(I1) systems.
Vibrational Energy Transfer Probabilities of Highly Vibrationally Excited 1,I,I-Trifluoroethane P. J. Marcouxt and D. W. Setser" Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received June 6, 1977) Publication costs assisted by the National Science Foundation
The loss of vibrational energy from chemically activated CH3CF3, initially formed with 102 kcal mol-l of energy via combination of CH3 and CF3 radicals, has been studied at 300 K with 17 bath gases and at 195 K with 5 bath gases. The experimental technique is the measurement of the ratio of the unimolecular decomposition product (CHzCFz)to the collisionally stabilized product (CH3CF3) over a wide range of pressure. By fitting the pressure variation of this ratio with model calculations, the average vibrational energy, ( s ) d o m , removed from CH3CF3per collision was assigned for each of the bath gases. For monatomic, diatomic, and triatomic gases, the ( h E ) d o w , , values ranged from 1 to 2 kcal mol-' and the distribution of transition probabilities has an exponential dependence on AE. For larger polyatomic bath gases, a Gaussian distribution, represented here by a stepladder model, provides a satisfactory description of the transition probability distribution and do, increases to a maximum value of 10 kcal mol-l for the most efficient bath gases. The present data, together with results already in the literature, provide a rather complete description for the vibrational deactivation of chemically activated CH&F3*. Vibrational energy transfer from CH3CF3*occurs somewhat less readily than from many other chemically activated molecules, such as 1,2-dichloroethane,cyclopropane, or alkyl radicals.
-
Introduction The competition between unimolecular reaction and collisional deactivation of highly vibrationally excited 'Present address: Department of Material Sciences, Fuel Science University Park, Pa.
Section, The Pennsylvania State University, 16802
0022-3654/76/2062-0097$01 .OO/O
molecules can be utilized to obtain vibrational energy transfer probabilities for the molecule providing that the unimolecular rate constants are thoroughly characterized over the appropriate range of energy.l Earlier work from this laboratory has been devoted to study Of the HX (x = F, C1, Br) elimination reactions of chemically activated haloethanes, as well as larger haloalkanes.2 Activation has 0 1978 American Chemical Society
98
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
P. J. Marcoux and D. W. Setser
been provided by exoergic radical combination or methylene insertion reactions. More recently our attention has been focused upon characterization of the vibrational energy transfer processes for the haloethane molecules. The first study was with 1,2-di~hloroethane.~ The present report completes our work with CF3CH3.* A subsequent paper will deal with collisional stabilization of fluoroethane and 1,2-difl~oroethane.~ The study of vibrational deactivation of chemically activated molecules is well d~cumented.'-~>~ The essence of the method is the observation of the enhanced yield of the decomposition product, relative to that expected for unit deactivation, resulting from collisional cascade through the energy region above the unimolecular threshold energy, Eo. The pressure dependence of the decomposition to stabilization ratio (or the apparent unimolecular rate constant) is then fitted by calculations using some assumed collisional transition probability model. The most critical experimental problem is a clean reaction system since data must be collected for a pressure range such that D/S varies a t least from 0.1 to 10. The photolysis of CF3N2CH3(reaction la-d) provides an ex-
hv
CH,N,CF,
CH, t CF, t N,
(la)
2CH, .+ C,H,*
(Ib)
2CF, --t C,F6* CH, t CF, t CH,CF,*
(IC)
( E )= 102 kcal mol-'
(Id)
cellent system4s7for generating CH3 and CF3 radicals, which combine to give CH3CF3*with an average energy of 102 kcal mol-l. The only known side reaction of significance at 300 K is addition of CF3 radicals to trifluoroazomethane followed by combination of the resulting radical with CH3 or CF3 to yield various hydrazines. This is not a serious problem for the measurement of the D / S ratios. The following scheme portrays the nature of the competition between unimolecular reaction and collisional cascade deactivation with loss of AE kcal mol-' of energy per collision: CH,CF,
-
* k(E)
CF,=CH,
+ HF
~M[MI
CH3CF3*
( E )-AE
CH,=CF,
MIM MI
___t
deactivation mechanism, while the former depends both on the deactivation mechanism and the collision cross section. The data are fitted by stochastic master-equation calculations which require as input the initial distribution of formed CH3CF3* molecules, the RRKM specific rate constants (hE),and specification of the collisional transfer probabilities. The data for each bath gas are sufficient to define the collision cross section, the average energy lost per collision, and the general form of the transfer probability distribution. The results from CH3CF3 are in agreement with the following general conclusions established by p r e v i o ~ sstudies. ~ ~ ~ ~(i) ~ The ~ ~ ,energy ~ transfer cross section is T U L J ~ ~ ~ ( ~uLJ ~ ' )is( the T * )Lennard-Jones ; diameter and f12(2,2)( T*) is the temperature-dependent reduced collision integral. (ii) The form of the transfer probability distribution for inefficient gases may be represented by an exponential function in which small energy losses are more probable than large energy losses. (iii) The form of the transfer probability distribution for efficient gases is such that large energy losses are more probable than small energy losses. Such distributions may be represented by several functions; the stepladder form, which is simplified representation of a Gaussian distribution, is used here. Results from monatomic, diatomic, and small polyatomic bath gases are reported. Previously, deactivation by large perfluoroalkane molecules was ~ t u d i e d . Root ~ and coworkerslO have studied a series of fluorinated ethanes. When these efforts are combined, the deactivation of CH3CF3* has been studied with 29 different molecules. The density of vibrational states in CH3CF3* at 90 kcal mol-' is very high, -2 X 1O1O states/cm-l, and the vibrational energy can be treated as being quasicontinuous. The average amount of energy removed from CH3CF3*per collision, ( AE)down, defined in terms of the down-transition probability, ranged from 1kcal mol-' for the rare gas atoms up to 10 kcal mol-' for the large polyatomic molecules. From comparison of the CH3CF3data with other results in the literature, CH3CF3*appears to be somewhat more difficult to deactivate than other chemically activated molecules. Based upon the limited work done at 195 K, there was little change in the energy transfer efficiencies relative to the 300 K data.
-
+ HF
CH,CF,* ( ( E )- 2AE)(2)
Experimental Section
The l,l,l-trifluoromethylazomethanewas prepared, ~ 1,The RRKM energy dependent specific rate constant is k ~ , purified, and stored as previously d e ~ c r i b e d .The was purchased from PCR Inc. and and kM is the collision number, S C H ~ C F ~ - M ' ( ~ " ~ T / ~1,l-trifluoroacetone )~/'; purified via gas chromatography. Bath gases were purSCHBCFB-Mis the collision diameter. The apparent unichased from commercial suppliers and purified by dismolecular rate constant is defined in the usual way, k , = tillation or if necessary by gas chromatography. The hhl[M](D/S); the yield of CFzCFBis D and the yield of vacuum system used for storage of gases and preparation CH3CF3is S. The experiments are basically very simple, of samples to be photolyzed used glass reservoirs and just the measurement by gas chromatography of the yield stopcocks and routinely pumped to Torr. In order ratio of CH2CFzand CF3CH3over a wide range of pressure. to handle CH3COCH3and CH3COCF3,a special vacuum Photolytic systems are, however, notorious for complisystem (see Results section) was used. Condensible gases cations when yields are examined a t the 1% level. were carefully degased via repeated freeze-thaw cycles Therefore, the photolysis of CH3COCF3 also was invesbefore storing. Noncondensible (-196 "C) bath gases were tigated a t room temperature with nitrogen as the bath gas metered into the photolysis vessel to obtain the desired to corroborate the results from the photolysis of CH3N2CF3 pressure. Condensible gases were measured in a calibrated in nitrogen. gas buret and then transferred into Pyrex photolysis The present work includes 17 bath gases at 300 K and vessels of known volume. For most experiments 0.2 cm3 5 bath gases at 195 K. These data were accumulated over of CH3N2CF3was used in the presence of 210-fold of bath a 5-year period, which presented several opportunities and gas. Samples were allowed to mix thoroughly before necessities for cross-checking of previously obtained data. commencing photolysis. Two aspects of the data are important for each bath gas; At 300 K the samples were irradiated by the output of the limiting high pressure unimolecular rate constant (ham) a water-cooled, Pyrex-jacketed General Electric AH-6 high and the increase of the rate constant hith diminishing pressure mercury lamp. With this lamp and Pyrex vessels pressure. The latter depends only upon the cascade
Vibrational Energy Transfer of l , l ,1-Trifluoroethane
the main absorption by CF3N2CH3is between 3500 and 3800 A. Experiments also were done using a high-pressure Hg lamp (Osram) with filters to isolate the 3600-A region. The results for Nz and COz bath gases were the same for both lamps. For the photolysis of CH3COCF3,the Pyrex jacket of the AH-6 lamp was replaced with a quartz jacket but Pyrex photolysis vessels still were used. In this case the absorption was mainly at 3200 A. The irradiation time varied depending upon the lamp, the vessel, and ratio of CF3N2CH3to bath gas; however, the general range was 10-60 min. The 195 "C experiments were done by placing the photolysis vessels in a cold furnace, which contained ethanol and excess dry ice as coolant, and irradiating for 2 h with the collimated light from the Osram lamp. The cold furnace was an insulated 4-L beaker filled with the coolant. The temperature, as monitored by a thermocouple, was constant for the duration of an experiment. The light path consisted of an evaculated Pyrex cylinder which was expoxied to the beaker and to a quartz window. This prevented the condensation of ice and frost on the window. All analyses were done with a gas chromatograph using a Porapak T column and a hydrogen-flame detector. The gas chromatographic inlet was attached to a standard vacuum line. Photolyzed samples were slowly pumped through a glass-wool packed trap maintained at 78 K to remove the noncondensible gases. The samples then were transferred to a U-tube, mixed with the carrier gas, and injected into the gas chromatograph. Analyses were done only for CH2CF2and CH&F3. After each analysis, the column was baked to 150 " C to elute any higher boiling compounds generated by the photolysis.' Since the quantity of bath gas vastly exceeds the amount of CHzCF2 or CH3CF3,the choice for condensible bath gases is limited to those which do not interfere with the analysis, e.g., bath gases that either are eluted after CH3CF3or are not detected by the Hz-flame detector. Since the response of the hydrogen-flame detector differs for CHzCFzand CH3CF3and is sensitive to the condition of the column, calibration was necessary for each series of experiments with a given bath gas. The calibrated response was obtained from prepared mixtures of similar composition to the real photolysis samples. In an attempt to reduce the random error associated with the analyses, the ratio of peak heights, the ratio of areas are determined by disk integration, and the ratio of areas from hand, planimetry were recorded for many photolysis and calibration samples. There was no obvious best method and if either of the three methods for measuring the product ratio showed wide deviation, that data point was discarded. The standard errors for the calibration factors were &5%; however, "duplicate" photolyses differed by &lo%, The gas chromatographic analysis gives the ratio of [CF+2H2]/[CF3CH3],which is abbreviated as D / S . This ratio is converted to the apparent unimolecular rate constant in pressure units by multiplying by the pressure, ha = P ( D / S ) . After the collision diameters are selected, the rate constants are converted to s-l units. A typical set of data is displayed in Tables I and I1 for C02 as the bath gas a t 300 and 195 K. Tables of the other data can be obtained by writing to the authors. All of the sets of data show evidence for cascade vibrational deactivation, i,e,, an increase in the values of k , with decreasing pressure. The limiting high pressure rate constant, ha", are of particular importance. In order to directly measure k,", several experiments are required a t D / S I 0.1. As an alternative method, the value for hamwas obtained by regression
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978 99
TABLE I: Data for the Decomposition of CH,CF,* in CO, at 300 K a P, Torr
P',Torr-'
D/S
16.4 18.0 24.9 27.1 27.7 45.4 45.8 50.1 55.0 71.7 106.3 108.7 110.7 181.0 182.9 408.9 453.0 265.0 527.0
0.0609 0.0556 0.0402 0.0369 0.0361 0.0220 0.0218 0.0200 0.0182 0.0139 0.0094 0.0092 0.0090 0.0055 0.0055 0.0025 0.0022 0.0022 0,0019
19.85 16.11 8.07 7.78 7.04 3.68 3.41 2.85 2.70 1.83 1.14 1.11 0.93 0.56 0.57 0.25 0.23 0.20 0.16
k,, Torr 325.5 290.0 201.4 210.8 195.0 167.0 156.6 142.9 148.5 131.2 121.1 120.6 102.0 100.8 103.7 101.0 103.3 92.1 85.9
k,/k," 3.47 3.09 2.14 2.25 2.08 1.78 1.67 1.52 1.58 1.40 1.29 1.29 1.09 1.15 1.11 1.08 1.10 0.98 0.92
a The least-squares fitting t o eq 3 was done in pressure units of cmHg. In these units the least-squares polynominal was k , = 9.39 t 0.146P-' t 358.9P-' 152.5PW3+ 2620.P-4 - 1502P-'; k," = 9.38 k 0.81 cm.
TABLE 11: Data for the Decomposition of CH,CF,* in CO, at 195 K a P, Torr P-',Torr-' 0.256 3.9 0.159 6.3 9.0 0.111 0.0833 12.0 0.0826 12.1 0.0813 12.3 0.0595 16.8 0.0568 17.6 30.2 0.0331 0.0302 33.1 0.0275 36.4 53.9 0.0186 70.5 0.0142 80.9 0.0124 0.0052 193.5 0.0036 278.6 0.0034 293.0 0.0030 332.4 0.0021 470.8
D/S 106.1 40.36 19.88 12.12 12.58 9.56 6.24 5.53 2.62 2.58 2.06 1.33 0.88 0.90 0.30 0.21 0.22 0.18 0.14
ha, Torr 413.9 254.3 178.9 145.4 152.2 117.6 104.8 97.3 79.1 85.4 75.0 71.6 62.0 72.8 58.4 58.5 64.5 61.0 64.2
ka/kam 6.86 4.21 2.96 2.41 2.52 1.96 1.74 1.61 1.31 1.41 1.24 1.19 1.03 1.21 0.97 0.97 1.07 1.01 1.06
a The least-squares fitting to eq 3 was done using pressure units of cmHg. In these units the least-squares polynominal was k , = 6.04 + 3.82P" t 7.77P-' 1 . 5 2 P ; k," = 6.04 * 0.33 cm.
analysis'' in which the experimental ha values were fitted by a least-squares procedure to a polynominal Clearly, the least-squares value for Po provides the least-squares estimate of the limiting high pressure rate constant, k,". All data points for each bath gas were given equal weighting and the data sets were analyzed in terms of eq 3 using second-, third-, fourth-, and fifth-order polynomials. The "best fit" equation was chosen on the basis of the following criteria: minimum in the mean square of residuals, minimum number of parameters, qualitative examination of residuals, and the condition that in the high pressure region the limiting slope be positive, Le., a negative slope has no physical meaning. This last condition was the most important criteria in the determination of the "best fit" equation from which k," was determined. A plot of ha vs. P'is shown for some typical data in Figure 1.
100
The Journal of Physical Chemistry, Vol. 82, No. 1, 1978
P. J. Marcoux and D. W. Setser
I
0
01
00
1
1
0.2
1
I
1
0.4
I
0.6
[Pressure,torr]
I
I
-1 0.8
J
Figure 1. Plots of k, vs. pressure-' for C02 and SF8 at 300 and 195 K. The lines are the least-squares fit of the data to eq 3.
700 N2
SATH GAS
k0 500
-w
nr
Y
3 00
100
0
200
400
GOO
800
Pressure,t o r r Figure 2. Plot of k, vs. pressure for N2 bath gas for photolysis of CH2N2CF3at 300 (e)and 195 K ( 0 )and CH,COCF, at 300 K (0). The lines are calculated results from the least-squares fit to eq 3.
The k, vs. P' plots are a rather poor way to display the data because of the compressed nature of the abscissa. A k, vs. pressure plot is preferred and the N2 data, plus the computed fit from eq 3, are shown in Figure 2. The regression analysis provides an excellent fit for the Nz data over the whole pressure region. In the following section the experimental data and the computed best fit to the data, obtained from eq 3, will be presented as k, vs. pressure plots. The D I S value for a given data point can be obtained from these plots by dividing the k, value in Torr units by the pressure.
Experimental Results Tests for Experimental Complications. Inspection of the data show that the S I D ratio extrapolates smoothly to zero when plotted vs. pressure. Conversely, the D I S ratio goes to zero on a P1plot. Thus, there are no "extra" sources of CH2CFz or CH3CF3. The initial work on chemically activated CH3CF3* by Giles and Whittle13 suggested that secondary reactions could remove substantial amounts of the decomposition product, CH2CF2, via reaction with CF3 radicals. This suggestion was pursued by Neely and Carmichael13and later by Root and co-workers1° in systems using cophotolysis of acetone and hexafluoroacetone. Neely and Carmichael13 sought to correct for CH2CFz loss in the experiments at 468 K by extrapolation to zero conversion. Root and co-workers
I
l
20
l
l
40
1
1
GO
I
I
80
Pressure, t o r r
l
i
100
I
I
:2d
Figure 3. Summary of the rate constant data for CpFBfrom this work and ref lob. The lines are calculated results from the least-squares fit of of our data to eq 3.
developed a protective scavenging method to inhibit secondary reactions with CH2=CF2. They found that the loss of CH2CF2 was the most severe for highly fluorinated ethanes as bath gases. The system was apparently selfscavenged with CzH6as the bath gas. Root's work shows that for some bath gases the loss of CH2CFzcan be significant at 307 and 373 K for photolysis of acetone and hexafluoroacetone. In effort to check the experimental data from the CH3N2CF3system, additional experiments were performed using CF3COCH3as the photolytic source with Nz as the bath gas. As shown in Figure 2, the agreement between the two data sets becomes progressively poorer with increasing pressure. The lower k, values in the high pressure region from photolysis of CF3COCH3 probably can be attributed to loss of CH2=CFz via secondary reaction. However, we take the general agreement between the two photolytic systems to be encouraging. As a further check of our reliability, the new results with CzF6as a bath gas can be compared to the data obtained by Root and coworkers using their scavenging technique. As illustrated in Figure 3, the agreement is satisfactory over the entire pressure range. Our current data give k," = 23 f 1 Torr; Root and co-workers report 21.6 f 1.4. The C2F6 data of our previous study48 gave k," = 16 f 3 Torr, when analyzed by the regression method, Since CzF6was the bath gas that was the most sensitive to added scavengers in Root's study, the agreement between the two data sets strongly suggests that removal of CHzCFz by secondary reactions is not serious for the CH3N2CF3photolytic system. As final evidence for this claim, we found that the D I S ratios were invariant with length of CHzNzCF3photolysis time for C2F6 and CH3Cl as the bath gas. A t 195 K the radical addition process would be even slower and no secondary effects would be expected. The explanation of why CHzCFzis not attacked by CF, radicals in the CH3N2CF3photolysis system is the ease of addition of CF3 to the azo compound. This is demonstrated by the product yields in the pure system:' CzFG (0.6%), CzHG (19%), CF3CH3 + CFzCHz (7%), CFsH (0.6% ), (CF,)ZNz(CH3)2 (29% ), CF3CH3NzCF3CH3 (22% ), (CF3)zN2CH3CF3(16%), and CF3CH3Nz(CH3)2(12%). The addition products account for -70% of the observed yield. The small quantities of CzFG and CFSH show that the steady-state concentration of CF3 is quite low. These qualitative findings are consistent with the rate contants of Table 111,which indicate that addition to RNNR (R = CH3 or CF3) by either methyl or trifluoromethyl radicals is faster than addition to CFz=CHz. This trend should hold for CH3N2CF3.Thus, the parent molecule acts as the
The Journal of Physical Chemistry, Vol. 82,No. 1, 1978 101
Vibrational Energy Transfer of 1,1, I-Trifluoroethane
TABLE 111: Arrhenius Parameter for Some Radical Addition or Abstraction Reactionsb Molecule' log A Ea h(300 K ) Ref log A Ea CZH, &-Butene CH,NNCH, CF,NNCF, *CH,=CF, CH,= CF,* CH,COCH, CH,COCF, CH,N,CH, a
10.52 11.15 10.70
CH, (addition) 7.7 8.13 X l o 4 8.1 1.77 x 105 6.0 2.13 X l o 6
14 14 15
11.10
9.9
7.73 x 103
16
11.40
CH, (abstraction) 9.5 3.02 x 104
10.96
7.8
1.9 x 105
14,18
k(300 K )
Ref
11.30 10.50
CF, (addition) 2.0 6.97 x 109 0 3.16 X 10"
14 14
10.16 11.30 10.90
3.5 8.9 10.9
4.08 x 107 6.54 x 104 9.11 x l o 2
15 16 16
11.50 10.33
CF, (abstraction) 8.2 3.36 x 105 6.6 3.33 x 105
14 14
14
The units are cm3 mol-' s-' for A and kcal mol-' for E,.
The asterisk identifies the addition site.
TABLE IV: Experimental High Pressure Rate Constants
70tt
@
CH3COCHj
m
^F,COCj
Temp,
K
300
301-
-- -a - -- -- --
'
20
I0L I
I
I
I
4
10
20
30
43
50
Pressure, t o r r Figure 4. Plot of k , vs. pressure for (CF3)zC0 and (CH&CO at 300 K. The lines show the calculated values using the least-squares fit to eq 3; the dotted curve is the fit obtained if the two highest pressure points for (CH&CO are not included in the analysis.
scavenger in the photolysis of CH3N2CF3and the decomposition yield (CH2CF2)is effectively protected from radical addition. Rate Constants for Efficient Polyatornic Bath Cases. Mole ratios of CH3N2CF3to bath gas of 1:15 were used for CzF6,CH3COCH3,and CF3COCFB.This was increased to 1:20 for SF6. Data also were collected at 195 and 300 K for CzF6and SF6. The results are shown in Figures 1,3, and 4 and the k," values are tabulated in Table IV. Since acetones are difficult to handle in vacuum lines containing stopcocks, these mixtures were prepared with a greaseless vacuum rack. To ensure that the acetone and hexafluoroacetone were not photolyzed, the reaction vessels were shielded by a 1/8-in. plexiglass filter. The filter had a short wavelength cutoff at 340 nm (
