J. Phys. Chem. 1980, 84, 2699-2705
2899
ARTICLES Vibrational Energy Transfer Probabilities of Highly Vibrationally Excited Fluoroethane and 1,P-Difluoroethane Molecules G. Rlchmond and D. W. Setser" Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: January 31, 1980; In Final Form: June 4, 1980)
The collisional loss of vibrational energy from chemically activated CH3CH2Fand CHzFCHzFformed with average energies of 91 and 92.5 kcal mol-', respectively, has been studied at 300 K with four bath gases, SF6, COz,IV2, and He. These chemically activated molecules were formed by combination of CH3with CH2Fand of CH2Fwith CH2F. The data cover an extensive range of pressure and permit the assignment of the mean energy transfer per collision and the form of the transition probability distribution. For He the (A&) values were 1.0 kcal mol-' with an exponential distribution for both CzH$ and C2H4Fp The values for CzH$ or CZII4Fz were virtually the same and ranged from 2.0 to 5.0 kcal mol-' for Nz,COz,and SF6;these transition probability distributions were of the Gaussian type (representedhere by a stepladder model). The results for CH3CHzFand CHzFCHzFare compared to previous findings for CH3CF3and CHzCICHzC1from this laboratory. The deactivation efficiency for SF6is similar for all four molecules. However, the deactivation of CH3CF3by N2 and COz is less efficient than for for the other three molecules. The He deactivation efficiencies for the fluoroethanes are all similar, but substantially smaller than for CzH4Clp Introduction In previous work from this laboratory the vibrational deactivation of highly vibrationally excited CH2C1CH2C1' and CH3CF2 molecules has been characterized by using the chemical activation technique at 300 K. The present study is an extension of that work to include CH3CHzF and CHJ?CHzF. In each instance the activated molecules were formed by radical combination, and the experimental measuremenh consist of the ratio of the stabilization and decomposition product over a wide pressure range in various bath gases. If one uses fully characterized metho d ~ , ' -the ~ mean and the general form of the collisional energy loss probability distribution can be assigned. These probability distributions are most sensitive to the energy range from the mean energy, ( E ) ,of the formed molecules to 20 kcal mol-l below ( E ) . The energy range sampled by our chemical activation studies is roughly 70-100 kcal mol-l. In this energy range the vibrational energy level densities for CH3CH2F and CH2FCHzF are very high 107-10'0 states cm-l, and the energy levels are viewed as a continuum without formal quantum restrictions to the collisional transfer from one level to another. The work with CH3CF3showed that vibrational energy was removed less easily from this molecule, relative to most other chemically activated imolecules, in collisions with monoatomic, diatomic, and small polyatomic partners. However, collisions of CH3CF3with large polyatomic molecules still removed 8-10 kcal mol-l per collision, which is close to the values reported for other highly vibrationally excited polyatomic maleculeuO3In an attempt to identify the molecular property of CH3CF3that might be responsible for the unusual energy transfer characteristics, survey experiments have been done with chemically activated CH3CHzFand CHZFCHZF.We selected He, N2, C02,and SF6 as bath gases in the expectation that these would display the full range of deactivation efficiencies. Experimenh were done over a wide range of pressure so that
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0022-3654/80/2084-2699$0 1.OO/O
the high pressure rate constants, as well as the variation of the apparent unimolecular rate constant with pressure, could be observed. Some less complete chemical activation data for CzHsF and CH2FCH2Ffrom the literature also are discussed in order to include more polyatomic bath gas molecules. The photolysis of CH3COCHzFat 300 K was utilized as the source of CH3 and CHzF radicals. Equations 1-3 deCH3COCHzF-% CO + CH3 + CHzF (1) CH3 + CH2F 4 CH3CHZF* (2) ( E ) = 91.0 kcal mol-' CHZF CHzF CHZFCHZF* (3) ( E ) = 92.5 kcal mol-l scribe the chemical reactions of interest to this work. Assignments of the mean energies of the initially activated molecules were made in accordance with our previous work.Si6 The threshold energies for C Z H Pand 1,2-CzH4Fz are 57 and 62 kcal mol-', respectively. The fact that photodissociation proceeds in a stepwise fashion with formation of CHzF favored in the first step is not of concern to the present work. The competition between unimolecular reaction and collisional cascade is represented in eq 4 for CzHJ?*. The models and thermochemistry that
+
4
0 1980 American Chemical Soclety
The Journal of Physical Chemistty, Vol. 84, No. 21, 7980
2700
Richmond and Setser
we developed5p6for unimolecular HF elimination from C2H4F2and CzH5Fhave been verified by other work+;'' therefore, our previously published values for the RRKM rate constants, kE, and the distribution functions, f(E), of formed molecules were used for analysis of our experimental data. The interested reader should consult ref 5-8 for discussions of the models for the unimolecular rate constants for C2H$ and C2H4F2 Only 1,2-HF elimination occurs; multiple halogen substitution on a carbon is requiredg before 1,l-elimination competes with 1,Zelimination. The collision number, kM, is computed as before (eq 5).2b In this notation M and A are the bath gas and PRESSURE, Torr
CAM
=
(QA
+ UM)/2
tAM/k
[(~~/k)(~~/k)]'/~
activated molecules, respectively. The presentation of the results will follow the organization of our previous paper2on CH3CF3. The chemically activated rate constants are defined in the usual way, k, = k,[M](D/S). The decomposition products are ethene and fluorethene, and the stabilization products are fluoroethane and 1,2-difluoroethane, Reference to the rate constants will be made by using both pressure and s-' units; the former follows from setting kM to unity and using pressure units for [MI. The collisional deactivation models will be either the exponential model for inefficient bath gases or the stepladder model (which is a simplified form that represents a Gaussian distribution) for efficient bath gases. These models are idenitified by the mean down transition probability specified as For exponential models with small ( a d ) . the overdl mean energy transferred per collision, (a), at 300 K is somewhat less than (&?&)e
( m d)
Experimental Section Experiments were done by photolyzing mixtures of CH2FCOCH3and bath gas in Pyrex vessels at 300-320 K followed by gas-chromatographic analysis to obtain the stabilization and decomposition product yields. Gas handling was done by conventional vacuum techniques. To obtain accurately defined pressures we transferred a carefully measured volume of gas sample to photolysis vessels of calibrated volumes. The molar ratios of bath gas to fluoroacetone were 1 0 1 for SF6and C02 and 301 for N2and He. Thorough mixing was achieved by placing glass beads in the vessels and shaking the prepared samples before photolysis. The monofluoroacetone was purchased from K and K Laboratories and purified by gaschromatographic analysis. Photolyses were done by using the unfiltered radiation from a General Electric AH-6 high-pressure mercury lamp. Because of the low quantum yields at room temperature, irradiation times of 1-2 h normally were required, Good product yields were especially difficult to obtain with C02 as the bath gas, presumably because the relatively high pressures needed to deactivate C2H5Fresulted in efficient relaxation of the electronically excited state of CH2FCOCHB. When possible, data for both C2H5Fand C2H4F2were collected from the same experiment. Because of the large difference in the rate constants, separate experiments were required for the extreme ends of the pressure range. In order to measure components from both reactions, gas chromatography with a combination column of 10 f t of Poropak-S and 18 ft of Poropak-T was used with temperature programming. A hydrogen-flame detector was used to detect the product yields. In addition to C2H4, C2H3F,C2H5F,and C2H4F2,ethane and a minor amount
1 3
Flgure 1. Experimentalrate constant (k, = pressure (C&l,/CH,CH$)) vs. pressure for fiuoroethane. The curves are the least-squares fits from eq 6: (0)SF,; (Ul C02; (0) N2;(A)He. Two He points at 50 torr are not shown on the graph because k , > lo4 torr. Wlthln experimental uncertalnty k,' (In pressure units) is the same as for CO, and NP, and only one fitted curve Is shown.
of methyl fluoride were observed. The most troublesome aspect was separating the C2H4and C2Hs. With He and Nz as bath gases, the photolyzed samples were pumped slowly through liquid-nitrogen-cooledtraps, and the condensible materials were then transferred to the injection loop of the gas chromatograph. With C02and SF6 as bath gages, the above procedure was followed to remove the CO generated from photolysis. Then the whole sample, including the co2or SF6,was injected onto the column. The C02and SF6did not interfere with the analysis since the hydrogen-flame detector was not sensitive to either compound. The response of the gas chromatograph was calibrated by using prepared mixtures containing samples of each of the four products. One mixture was diluted with Nzand the second with SFG.These mixtures were injected onto the gas-chromatographic column by using the same procedure utilized for the photolyzed samples. These mixtures were used to periodically check the response of the gas chromatograph. Peak areas, measured by planimetry, were used to obtain the ratio of the decomposition and stabilization product yields. Experimental Results The experimental data are presented as the k, values, in pressure units, vs. the experimental pressure in Figures 1 and 2. Plots of D/S vs. 1/P and S / D vs. pressure extrapolate smoothly to zero, and there are no apparent "extra" sources of either decomposition or stabilization products. The possibility that small amounts of decomposition product, CzH4 or C2H3F, may be removed by radical addition reactiondo could not be explicitly investigated. However, the photolysis temperature was kept at 300 K to suppress such free-radical reactions. The general self-consistency of the data at high and low pressure suggests that any loss of C2H4 or C2HSFmust be minor. However, there obviously is considerable scatter in some of the data. This could be related to side reactions and/or to gas handling problems related to low yields. Radical addition may be less serious for CH3 and CH2F than for CF3 systems.lOJ1 If some removal of CzHl or C2H$ did occur, the present results give lower limit values to k," and (A&). The SF6data for both reactions are of good quality and provide a reference for an efficient deactivating polyatomic bath gas. The high-pressure limiting rate constants are 150 and 18 torr for C2HSFand 1,2-C2H4F2 The order of magnitude difference in the chemical activation rate constants for the two reactions arises from the 5 kcal mol-'
The Journal of Physical Chemistty, Vol. 84, No. 21, 1080 2701
Vlbratlonally Excited CH3CH2Fand CH2FCH2F
TABLE I : Summary of High-Pressure Rate Constants -
He
--
C2H,Fn
k,", torr
bath gas
N, CO, SF, CH,CIF~ (CH,F),CO" (CH,F),CO/SiHCl~ ( CH,F),CO/(CH,),COg
840 1 3501 350i 1501
160 60 50 20
1 4 0 f 20
1,2-C,H4F2
lO'k,", 1331 452 531 25i
s-l
25 6 7 3
lO'k,",
ham,torr 851 9 252 6 351 4 181 2 181 2 g2114 g201 3
14.5 i 3.3 t 4.8t 2.8 1 3.0 f S3.7 f g3.5 f
s-* 1.5 0.8 0.5 0.3 0.3 0.6 0.6
25i 4
The u and e/k values used for C2H4F,were 5.0 A and a The u and e l k values used for C,H,F were 4.7 A and 250 K. ) used for He, N,, CO,, and SF, are 2.67 (10.8), 3.70 (95.1), 4.40 (188), and 5.91 (189), 250 K. The u (and E J ~ values respectively. With the exception of CO,, these values are the same as used previously for CH,CF,; changing uco from 4.47 Data from ref 5; UCH,CIF = 4.3 A and e l k = 400 K were u b d to talto 4.4 A would have no effect on the CH,CF, data. culate kM. e From high- ressure points of ref 12; u(cH,F) c o = 5.0 A and e l k = 520 K; a similar value can be obtained from data reported in ref 11. ?Data from ref 7 were obtained at 340 Kin a 1 : l mixture. For (CH,F),CO the u and e l k values are From ref 6 (similar results were found in ref 13) for a 1:l mixture, given in footnote e ; osiHcl, = 5.8 A and elk = 300 K. U(CH,),CO = 4.6 A and Elk = 520 K; see footnote e for u and e l k for (CH,F),CO.
z
ou W1d0
3 1 0 -
3
10
' ' f'"',' 103 PRESSURE ,Torr '
' " " 1 1
10'
Fbwe 2. Experimental rats constants (k,
' I
= pressure (C&13F/C2H4F2))
vs. pressure for 1,2difluoroethane. The curves are the least squares flts from eq 6 (0)SF,; (U)GO,; (0)N,; (A)He. The smaller increase In k, relative to Figure 1 Is a consequence of the higher range of S I 0 for the C2H4F2reaction (see Figures 3 and 4).
lower threshold energy for C2HJ?.6*6The convenient range of pressure is from 30 to 300 torr; good data at high pressure for C2H6F(except for SFs) and low pressure for C2H4F2presented some experimental problems. As shown in Figure 1, data points in the true high-pressure region (pressures sufficiently high that k, = constant, Le., SID 2 5) could not be achieved with C2HPfor the less efficient gases; however, the low-pressure C2H6F data are rather extensive. The onset of the increase in k, with reduced pressure occurs at higher pressure for the less efficient gases, i.e., PHe > PN,> Pco, > PsF6.The high-pressure data for C2H4F2are of good quality for each bath gas. The k, values for N2and C02are nearly the same for both C2H4F2 and C2H6F. An attempt was made to extend the C02/ C2H4F2data to below 110torr of pressure; however, the data were quite scattered (as shown by the three points at 5.2 torr) and were not reliable. The data were analyzed in the same way as beforee2 First the experimental rate constants, in pressure units, were fitted by least squares to a power series in pressure-l (eq 6) in order to obtain a value for the high-pressure
ka = PO + B~(l/pll+ P2(1/Pl2+ + Pn(1/Pln (6) intercept, Po, which in the limiting high-pressure rate constant, k,". The retention of higher order terms in eq 6 was investigated to obtain the best fit (see ref 2a). As an aid in the selection of the best value for Po, the k, data also were fitted to a power series in D I S . In either case Po must be positive, and the calculated curves must be
1o9 0.001
0.01
0.10
S/D
10
Figure 3. Comparison of the best-fit calculated rate constants from assumed deactivation models wlth the experimental data for fluoroethane: (e)SF,; (U) COP;(0)N,; (A)He. The preferred fits are the solid curves. The scallng factors which were applied to the data are 0.82, 0.87, 0.70, and 0.90 for SF,, COP,NP, and He,respectively. The (A€)ev = 2.0 curve was multiplied by 0.51 In order to match the calculated kamto that of the (A€), = 3.0 calculated curve. The = 0.0 kcal mol-' curve was multiplied by 1.I 1 to convert the k, to the 5.0 kcal mol-' value.
(k€),
monotonic decreasing functions. Although the pressure range is extensive as shown in Figures 1 and 2, only a limited number of points could be obtained for SID > 2 for C,HJ?. This is shown more effectively in the k, vs. SID plots, Figures 3 and 4,used in the next section to assign the (A&> values. The shortage of high-pressure results for He, C02, and N2with CH8CH2Fmade assignment of k," especially difficult. For these data the expected shapes of the k, vs. S I D curves in the high-pressure region were also a factor in selecting the best K," values. The k, vs. P' plots are not a convenient way to display the data, and plots of k, vs. pressure, Figures 1 and 2, were selected. However, the curves on these plots are the computed least-squares values obtained from eq 6. The flat part of the curve at high pressure reflects the value. The best k," values are collected in Table I and converted to s-l units by using our best estimates for the collision diameters. The data for C2H6Fand 1,2-C2H4F2 are not as good as the results for CH3CF3;2however, this is the best that could be done with CH8COCH2Fas the radical source. Data in the literature that could be effectively analyzed were included in Table I. The CH2C1F data cover a large enough pressure range that a complete analysis using the procedure outline above could be done. For the other data, the k," values were obtained by methods that are equiv-
2702
The Journal of Physical Chemistry, Vol. 84, No. 21, 1980
TABLE 11:
(A&)
Richmond and Setser
Values for Fluoroethane and l,2-Difluoroethane at Room fluoroethanea
bath gas
kcal/mol 5.0, SL 2.0, SL 3.0 (2.51, . ,. SL 2.0, exp 1.0, exp
SF, COa
10' haw,s-' calcd exptl 20 43 29 (35) . , 57 154
25 53 45
1,2-difluor~ethane~ kcal/mol 5.0, SL 2.5, SL 3.0 (2.0), . .. SL
10*kaW, s-' calcd exptl 2.5 4.2 3.5 (5.1) . .
2.8 4.8 3.3
133 1.0, exp 16.2. 14.5 He 5.0 (6.0). CH.CIF~ 2.5 12.21 3.0 ,. SL ( Cd,F),CO 26,S 2 2.2 * ' 53.7 (CH,F),CO/(CH,),CO -6, SLf 18 25 a The uncertainty in ( A E d ) for SF, and CO, is f 1 and f 0.5 kcal mol-', respectively. The N, results are less reliable than for CO,, but within the experimental error the assignments are the same as for CO,. See text for further discussion. For (AEd)exp = 1.0 kcal mol-' the overall mean energy, ( A E ) ,lost per collision is -0.3 kcal mol-' less than the mean down-transition value, (A&), given in the table. For (&Ed),, = 5.0 kcal mol-' the ( A E d ) is the fame as ( A E ) ; for t A E d ) s L = 2.0 kcal mol-' the ( A B is 0.5 kcal mol-' less than (A&). See text for discussion of the N, results. Within the experimental error the assignments for N, are the same as for CO,. Data for ref 5. e Assignment based upon absence of any dependence of k , on pressure; see ref 7. f Assignment based upon dependence of k , on pressure noted in ref 13.
I 10
S/D
100
Figure 4. Comparison of the best-fit calculated rate constants from assumed deactivation models with the experimental data for 1,2difluoroethane: (0)SFB;( 0 )COP;(0)N2; (A)He. The scallng factors are 0.90,0.73, and 1.0 for SF,, COP,and He, respectively. If = 2.5 kcal mol-' is selected for the COPdata, the scaling factor would be nearer unity, and ( AEd)8L= 2.5 is preferred over 2.0 kcal mol-' on this basis (see Table 11). The N2 points have not been scaled; see text for discussion of the model for NP.
alent to taking the mean of all of the rate constants with S / D 2 1. This method may give a slight overestimate of the true k,", but the systematic error is within the f15% uncertainty of the mean values. The k," values for the polyatomic bath gases agree closely with those obtained for SF6,which give confidence that the SF6data represent an efficient bath gas. The kamvalue from the SiHC13/ (CH2F)&0 mixture is for 340 K, which partially explains why the rate constant is higher than for CHzCIF or SFe Assignment of Collisional Deactivation Models The transition-probability models were assigned by matching calculated k , values from assumed models to the experimental values. Fitting the absolute values of k," and the variation of k, with pressure are both important. The methods used for the model calculations have been explained before.% Briefly, the ratio of the decomposition and stabilization product yields are calculated from the steady-state distribution of molecules. The steady-state distribution, ni, is obtained from the master equation formulation. For each of the ith energy regions, eq 7 applies. The product Rfi is the rate of formation in the dni/dt = 0 = Rfi - kini - k~[M]n@ij I
+ k~[M]CPijnj I (7)
energy region i, ki i s the RRKM rate constant of this region, Pij is the probability of transfer from energy region i to energy region j in a collision, and kM was defined in eq 5. In the present calculations the energy regions were 0.5 kcal mol-l in width. Exponential and stepladder distributions, with variable ( u d ) , were used to represent the down-transition probabilities. The up transitions are obtained from detailed balance. The RRKM rate constants and distribution functions of formed molecules were taken from our former work.6y6 Using the known values of ki, fi, and kM and the assumed values of Pi,, we calculated the steady-state relative populations and, hence, the k, values for a range of pressures for each transition-probability model by using the method of Duewer, Coxon, and Setser.14 The contribution to deactivation by fluoroacetone was explicitly included by using a stepladder model with (Ai&) = 8 kcal mol-' weighted into the collision transition probability matrix according to the mole fraction of fluoroacetone (0.10 for SF6 and C02and 0.03 for Nz and He). The computed k, values were matched to the experimental data by using plots of k, vs. S/D.2bTo match the calculated and experimental results, we scaled the experimental rate constants (see figure captions) so as to obtain the best fit, over the whole pressure range, to a given calculated curve. Such scaling is equivalent to altering the collision diameter, sm, or shifting the RRKM ki values by a constant factor. Since neither the collision diameters nor the rate constants are known absolutely, the need for some scaling is not surprising. However, for a well-characterized unimolecular reaction the scaling factors should show a systematic and small deviation from unity. The best fits of the calculated and experimental k, values are shown in Figures 3 and 4, and a summary is given in Table 11. The scaling factors, which are based on fitting over the entire presure range, are very close to the k,"(calcd)/k,"(exptl) ratio except for the He data. The k,"(calcd) and k,"(exptl) values are included in Table 11. The SF6 data with CzH$ are well described by ( h E d ) S L = 5 kcal mol-'. The uncertainty should be of the order of fl kcal mol-', as displayed by the calculated curve for (Ah!d)sL = 6.0 kcal mol-', adjusted to the k," value of the ( hE)sL= 5.0 curve. The assignment for COz is somewhat uncertain because of the scatter in the data point for S/D 2 0.5. Nevertheless, the ( = 2.0 kcal mol-' curve provides an excellent overall fit. To match the curvature with an exponential assignment, ( must be 4 . 5 kcal mol-l, and that would lead to a large overestimate of k,". On the basis of the magnitude of 12," we favor ( m d ) S L
The Journal of Physlcal Chemistry, Voi. 84, No. 21, 1980 2703
Vibratlonally Excited CH3CH2Fand CH2FCH2F
TABLE I11 : Comparison of Some Relativea Collisional Efficiencies
--
bath gas --He
NZ
--
1,2-CaH4Fa
CZHP (A&)
1.0,exp 2.!4 SL 2.0, SL 5.0, SL
CH CF36
pkJ
(AEd)
BEJ
(AEd)
PtJ
0.19 0.55 0.47 1.0 91
l.0,exp 2.5, SL 2.5, SL 5.0, SL
0.19 0.85 0.58 1.0 92.6
1.0, exp 1.5, exp 2.0, exp 6.0, exp
0.12 0.13 0.23 1.0 102
1,%C2H,ClC (AEd)
1.4,exp 4.0, SL
C,Had
FtJ
PkJ
PkJ
0.24 0.77 >0.75e 1.0 88
0.12 0.26 0.61
0.21 0.45 1.05 1.0 112
COZ 7.0, SL SF, energy, (E), kcal/mol threshold 57 62 68 60 51 energy, E,, kcal/mol (I The collisional efficiencies are defined relative to SF, as kam(SF,)/kaw(M). From ref 2. From ref l b . From ref 15 and 16; the collisional efficient-y for SF, with C,H, seems surprisingly low, and we have used two reference values, SF, (second column) and the mean value for C,F,, CH,Cl, and CF, (first column). e Estimated.
= 2.0-2.5 kcal mol-' for COP Within the experimental error, k," for N2 is the same as for C02. However, the curvature for N2 may be slightly less than for COz, and ( u d ) s L = 3.0 or ( A&d)exp = 2.0 kcal mol-' provides an adequate fit. Since the scaling factor for ( h E d ) S L = 3.0 kcal mol-' is 0.70, which is of the same general magnitude as for the other gases, (A#Y),, = 3.0-2.5 kcal mol-' may be prefereable to (AE), = 2.0 kcal mol-l, which would require a scaling factor of1.27 to lift the experimental k," value to 5.7 X lo9 s-lQ There is an abrupt decline in the He experimental k, values for S / D > 1.2, and seven pohta in the higher S / D range do not lie on any calculated curve that fits the rest of the data. If these points are totally ignored, ( A&)exp = 1.0 kcal mol-l and a scaling factor of 0.80 gives an e x c e l h t fit. If the high S / D points are weighted, a somewhat smaller k," is suggested (as in Table I), and we have set the scaling factor at 0.90 but retained ( m d ) e x p = 1.0 kcal Im01-l. The C2H4F2data with SF6 are matched with ( m d ) S L = 5.0 kcal mol-'. The He results are fitted with ( h E d ) e x p = 1.0 kcal mol-l. As with C2HJ?,the He data show a rather abrupt lowering of the experimental rate constants for the highest SID points. In fitting the C02data, the four lowest pressure points ( S / D < 0.10) have been ignored because of the scatter. The ( h E d ) s L = 3.0 curve (with a scaling factor of 0.73) provides a good fit; however, the curvatures for ( h E d ) s L = 2 and 3 kcal mol-' are virtually identical on the scale of Figure 4. Considering the uncertainty of the experimental k,"(C02), the best assignment is (A#Yd)sL = 2.5 f 1 kcal mol-l. The N2 data with C2H4F2present a special problem, A niodel with < 2 kcal mol-' is required to fit the steep increase in k, with reduced pressure. But, k,"(N:J is apparently lower than Iz,"(C02), suggesting that (A&)sL is larger than for COP The only reasonable decision in that within experimental error the N2 and C02 deactivation models are the same. Exponential models provide an even less satisfactory fit for the N2 or C02 data. The entries in Table I1 for CH2C1F,S(CH2F)2C0,12and (CH2F)2CO/(CH3)2CO'3 are based upon data already in the literature. For CH2ClF (A&) was assigned in the same way as for the He, N2, C02, and SF6 data. The present ( a d ) assignment is considerably lower than the old value of 11 f 3 kcal mol-I. This reduction arises because the lz,' selected earlier was too large and because of a greater weighting given to data for S I D 1 0.4 in the earlier analysis. The data are less extensive for the other gases; and k," values and the qualitative variation of It, with pressure at low SID were used in making the assignments in Table 11. The scaling factor8 required to shift the data to the calculated curves are all quite modest and are only slightly
less than unity for both reactions. The systematidy small deviation from unity lends support to the sm values (Le., the gas kinetic cross sections) and the RRKM rate constants that were used in the analysis. Discussion The objective of this study was to survey the collisional deactivation efficiencies of chemically activated C2H$ and C2H4F2.For polyatomic molecules of the SFs,CH2ClF, or acetone variety, the deactivation efficiencies for monoatomic, diatomic, and triatomic molecules with C2H6F and 1,2-CzH4F2tend to be the same. Both of these chemical activation systems should be amenable to more detailed study. Because of the large difference in the k," values, there was no great advantage in simultaneously acquiring data for C2H,F and 1,2-C2H4Fz. The deactivation of C2H6Fformed by radical combination was studied earlier13with the bath gases N2, CHI, and a 1:lmixture of (CH3)2C0and (CH2F),C0, which was the radical source. The data are not complete and a k," value was reported only for the acetone mixture (see Tables I and 11). On the basis of the nonlinear variation of D / S with (pressure)-l,the authors13assigned ( values of approximately 6,2.5, and 1kcal mol-' for the acetones, CH4 and N2 bath gases. These results are in qualitative agreement with our assignments; the acetone data in particular are consistent with our interpretation for SF6 and with the results2 from CH3CF3with acetone bath gases. An unusual finding of the early study13 was an apparent strong enhancement of (AE)for N2 with increasing temperature. More complete studies2Jswith other molecules have found only mild dependence of upon temperature, and the variation of (a) with temperature reported for N2 and C2H6Frequires further verification before being accepted. Our main interest was to acquire data that could be compared with the CH3CF3results.2 Such a comparison is qualitatively in Table I11 along with some other molecules activated to a similar energy. For photoactivated C7H8only the relative efficiencies, oCu, are listed because the (AE)values are either not available or not sufficiently reliable to be useful for the present comparison. The SF molecule was adopted for reference in making the PCL! comparison since its collisional efficiency tends to be constant for the haloethanes. Unfortunately with cyclois lower than that for C02,CH3C1, heptatriene16oCu(sF6) and CF4and only slightly greater than that for CHI. Either there is experimental error in the data or SF6 has an analomously low efficiency for C7Hs. Using the k," for C2F6,CF4,and CH3C1as the reference (first column under C7HSof Table 111) probably gives a more representative comparison for our purposes. Inspection of the table shows
(a)
2704
Th8 JOW&
of PbyShl Chemisty, Vd. 64, NO. 21, 1980
that completely general statements about a given bath gas for all five activated molecules cannot be made. However, three points emerge with regard to CzH5Fand CzH4Fzvs. CH3CF3: (i) the efficiency for He is similar for all three fluoroethanes;(ii) nitrogen (and presumably other diatomic molecules) and COz are much more efficient with C2H5F and C2H4F2than with CH3CF3;and (iii) although the Nz and C02efficiencies are similar with CzH5Fand C2H4F2, COPis more efficient than N2with CH3CFp Comparison of the fluoroethanes with C2H4C12shows He, N,, and COz to be less efficient for all three fluoroethanes. The deactivation efficiencies of C7H8resemble those for CzH5Fand C2H4Fz,except that C02seems more effective than Nz. If CzH4 is used as a reference rather than SFe, then some relative Pcu values can be given for He (0.22) and Nz(0.72) with cyc10propane.l~On the basis of these comparisons, cyclopropane and 1,2-CzH4ClZare the most readily deactivated, CH3CF3 is the least readily deactivated, and CzHJ?, 1,2-C2H4F2,and C7Hg are between these extremes. Direct comparison with efficiencies of thermally activated systems3will not be made because, in general, the energy range is much lower and because of the somewhat different physical nature of the P, values. After reaching the qualitative conclusions above regarding the variation of the efficiencies of a common bath gas with different activated molecules, it would be desirable to explain the trends in terms of molecular properties, The general model that qualitatively fits the trends observed for vibrational relaxation of highly activated polyatomic molecules is the transitional-mode view of Lin and Rabinovitch.ls During the collision encounter, energy is presumed to flow to the new “vibrational modes” developing in the weakly bound A-M* complex. Dynamical restriction^,^^^^ which cannot be fully defined at present, limit the accumulation of energy in the transitional modes to less than the statistical amount given by energy and momentum conservation laws. These dynamical restrictions are crucial because the observed ( a values ) are much smaller than the statistical values. Another experimental observation which downplays the importance of the densities of states or restrictions associated with the nature of the internal modes of the polyatomic excited molecules in limiting (AE)is that the (a) values tend to remain constant when the densities of states of A* are altered, but other factors in the collision are held constant. Two examples are the constancy of ( a for ) C2H4ClZ relative to CZD4Cl2 with several different bath gaseslb and the constancy of (AE)for a series of sec-alkyl radicals,21 which increase in size from C4 to Cg, with common bath gases. Information theoretic analysiszzof the atom plus polyatomic case based upon a prior distribution of AE that included only the internal vibrational states of the polyatomic molecule and the relative product translational energy had only a very modest surprisal (A N 0.05-0.10) relative to the experimental results. Since rotational states of the polyatomic molecule were excluded from the prior distribution, some of the dynamical restrictions discussed by others are implicitly taken into account. Trajectory calculationsz3of atom plus vibrationally excited triatomic molecules (HzO, 0,) with Lennard-Jones intermolecular potentials provide qualitative support for the importance of long-range interactions in determining the energytransfer probabilities. These calculations found exponential-like transition probability distributions in agreement with experimental findings. Although Bunker and Jayich’s trajectory c a l ~ u l a t i o for n ~ ~vibrationally excited CH3NC colliding with He, Xe, Nz, and Hzused only repulsive pairwise interactions for the intermolecular po-
Richmond and Setser
tential and gave energy-transfer results in approximate agreement with experimental findings, the bulk of the data are best explained by models that emphasize attractive interactions. Within the context of the transitional model, the well depth and the anisotropic properties of V(A*-M), and the related dynamical restrictions,should be important in determining the energy-transfer probabilities. We believe that the differences in the interaction potentials between the various haloethane molecules and the common bath gas molecule, rather than the internal properties of the excited molecule, explain the different energy-transfer behavior. Little is known about the V(A*-M) potentials for the haloethanes. Without doubt, V(C2H4C1*,,M) will be deeper than for the fluoroethanes, and this may explain the higher deactivationefficiencies for C2H4CI2.The dipole moments of the fluoroethanes are about the same,z6but there is some difference in the boiling points for CzH6F (-38 “C), CH2FCHzF(31 “C), and CH3CF3(-47 “C), and the boiling point of CH3CF3is the lowest even though ita molecular weight is the highest. The methyl group of l,l,l-trifluoroethane is isoelectronic with a fluorine atom, and the V(CH3CF3-M) potential may have some characteristics of that for CF4-M, in spite of the permanent dipole for CH3CFp We postulate that the smaller well depthz6 and the more isotropic potential for CH3CF3, relative to CH3CHy or CH3FCH2F,are responsible for the greater restriction on energy flow to the transitional modes for the CH3CF3-M complex, whenever M corresponds to a molecule with intermediate efficiency, i.e., diatomic and triatomic cases. Acknowledgment. We thank Dr. Paul Marcoux for his assistance in doing the least-squarescurve fitting displayed in Figures 1and 2. This work was supported by the National Science Foundation (MPS75-02793 and 77-21380).
References and Notes (1) (a) Setser, D. W.; Hassler, J. C. J . Phys. Chem. 1987, 71, 1364. (b) Setser, D. W.; Siefert, E. E. J. Chem. Phys. 1972, 57, 3613, 3623. (2) (a) Marcoux, P. J.; Setser, D. W. J. Phys. Chem. 1078, 82, 97. (b) Marcoux, P. J.; Siefert, E. E.; Setser, D. W. Int. J . Chem. Kinet. 1975, 7 , 473. (3) Tardy, D. C.; Rabinovitch, B. S. Chem. Rev. 1077, 77, 369. (4) Fwst, W. “Theory of Unbnolecular Reactions”, Academic Press: New York, 1973. (5) Chang, H. W.; Setser, D. W. J. Am. Chem. Soc. 1009, 91, 7648. (6) Chang, H. W.; Craig, N. L.; Setser, D. W. J. Phys. Chem. 1972, 76, 954. (7) Kerr, J. A.; Timiin, D. M. Int. J. Chem. Klnet. 1971, 3 , 427; Trans. Faraday SOC.1971, 67, 1376. (8) (a) Day, M; Trotman-Dickenson,A. F. J. Chem. Soc. A . 1989, 233. (b) Kirk, A. W.; Trotman-Dickenson, A. F.; Trus, B. L. IbM. 1988, 3058. (c) Cadman, P.;Kirk, A. W.; TrotmaflCkellson,A. F. J. chem. SOC., Faraday Trans. 11978, 72, 996, 1428. (9) (a) Klm, K. C.; Setser, D. W.; Holmes, B. E. J . Phys. Chem. 1973, 77, 725. (b) Kim, K. C.; Setser, D. W. IbM. 1974, 78, 2166. (c) Holmes, 8. E.; Setser, D. W.; Prltchard, 0. 0. Int. J. Chem. Klnet. 1976, 8 , 215. (10) (a) Neely, 8. D.; Carmichaei, H. J . Phys. Chem. 1973, 77, 307. (b) Pettijohn, R. R.; Mutch, G. W.; Root, J. W. Ibkl. 1975 79, 1747, 2077. (c) Hoit, P. M.; Kerr, J. A. Int. J . Chem. Kinet. 1977, 9 , 185. (11) Kerr, J. A.; Timlin, D. W. Int. J . Chem. Kinet. 1971, 3 , 1, 69. Venugopalan, M.; Graham, T. F. J . Phys. Chem. (12) Pritchard, G. 0.; 1904, 68, 1788. (13) Kerr, J. A.; Oolady, B. V.; TrotmanDickenson,A. F. J. Chem. Soc. A . 1909, 275. (14) Duewer, W. H.; Coxon, J. A.; Setser, D. W. J . Chem. Phys. 1972, 56, 4355. (15) Luu, S. H.; Troe, J.; Ber. Bunsenges. fhys. Chem. 1973, 77,326; 1074, 78, 766. (16) Luu, S. H.; Glanzer, K.; Troe, J. @e. Bunsenges. Hys. Chsm. 1975, 79, 855. (17) Simons, J. W.; Rabinovltch, B. S.; Setser, D. W. J . Chem. Phys. 1084, 4 7 , 800. (18) Lin, Y. N.; Rabinovitch, B. S. J . fhys. Chem. 1970, 74, 3151. (19) Oref, I.; Rabinovitch, B. S. Chem. Phys. 1077, 26, 385.
J. Phys. Chem. 1980, 84, 2705-2707 (20) BhattacharJee,R. C.; Forst, W. Chem. Phys. 1978, 30, 217. (21) (a) Tardy, 0. C.; Rabinovltch,B. S. J. Chem. phys. 1968, 48, 5194. (b) Georgakakos, J. H.; Rabinovitch, B. S. IbU. 1972, 56, 5921. (22) Jensen, C. C.; Steinfeld, J. J.; Levlne, R. D. J . Chem. Phys. 1978, 68, 1432. (23) Stace, A. *I.; Murrell, J. N. J. Chem. Phys. 1978, 68, 3028.
2705
(24) Bunker, D. L.; Jayich, S. A. Chem. PAYS. 1978, 73, 129. (25) Nelson, R. D.; LMe, R. D., Jr.; Maryott, A. A. Natl. Stand. Ref. Data Ser. ( U . S . Natl. Bur. Stand) 1987, No. 10. (26) The suggestion that elk of CH,CF, Is smaller than for C2H,F $r CH2FCH,F will have no practical effect on the computation of SM , for whlch the same d k values were assumed.
Hydrated Electron Cleavage of 1,3-Dlmethyluracil Dimers in Aqueous Solution I. Rolsenthalt and M. Faraggl" Chemistry Department, Nuclear Research Center-Negev, Beer-Seva, Israel (Received Aprll3, 1980)
The efficiency of the radiation-induced cleavage of the four stereoisomers of 1,3-dimethyluracilcyclobutane-type dimers is dependent on their stereochemistry. In slightly acid solutions for each electron donor, e,- or CO, two rnonomers are formed. In'alkaline solutions, the cleavage of cis,syn and &,anti dimers occurs via a chain mechanism. In pulse radiolysis the dimers behave similarly to the monomer.
Introduction A major part of mutagenic and lethal effects of UV irradiation on biological systems is attributed to photochemical alterations of pyrimidine residues in nucleic acids. The formation of cyclobutane-type dimers is one of the major photolesions in nucleic acids, as defined in chemical terms at the molecular level. This damage can be repaired through irradiation of the infected nucleic acid with visible light in the presence of a photoreactivating enzyme. The photochemical reaction responsible for this process has been demonstrated to be the monomerization of cyclobutane dimer moieties. For this reason, the mechanism of dimer cleavage, that is the repair of damage, has attracted a great deal of research eff0rt.l Hence, several works indicated that ionizing radiation cleaves cis,syn ("ice") thymine dimer." The effects of ionizing radiation on all four isomeric photodimers of 1,3-dimethylthymine in the crystalline s t a h have also been reported? In crystals this process is complex and depends on size and perfection of the crystals. The aim of the present investigation was the study of the effect of ionizing radiation on aqueous solutions of the four stereoisomers of' 1,3-dimethyluracil (DMU) dimers. Materials and Met hods The four stereoisomers of DMU (Figure 1) were prepared by acetone sensitization and separated as described? The solutions were prepared in triply distilled water. The y irradiations were performed in a 17-kCi 'Wo source, which provided a dose rate of lo4rd/min. Gas saturation of the solutions was achieved by continuous bubling of purified gases (Matheson Co., Inc.) for 30 min. Since the only result of the hydrated electron reaction with all four dimers was the cleavage to monomer, the rate of conversion was followed by recording the increase of the absorption at 265 nm (Cary 17 spectrophotometer). At this wavelength €dimer = 650 M-' cm-l and EDMU = 8200 M-l
-
cm-'.
Pulse Radiolysis. The experiments were carried out by using the Hebrew University linear accelerator. It was operated at 5 MeV, 200 mA, and a pulse length of 0.1-1.0 PS, giving a dose range of 150-2000 rd per pulse. Other experimental procedures, the apparatus optical detection Also at the Volcani Institute of Agricultural Research, Department of Technology, P.0 Box 6, Bet-Dagan, Israel. 0022-3654/80/2084-2705$01 .OO/O
system, the cell filling technique, and the evaluation of the kinetic curves have been previously described.'
Results and Discussion In y-irradiated aqueous solutions, most of the energy is absorbed by the solvent. This energy absorption produces hydrated electron (ea;), H atoms, and OH radicals as reactive species, which in turn could react with the solute. The radiochemical yields, G values, expressed as the number of species produced per 100 eV of energy, are G, = 2.8, GH = 0.55, and GOH = 2.9. In the presence ogf tert-butyl alcohol, hydroxyl radicals are efficiently removed to yield a relatively nonreactive radical (eq 1). ArgonOH + (CH3)3COH (CH3)zC(CHz)OH+ HzO (1) saturated solutions of DMU dimer (5 X M) in the presence of 0.1 M tert-butyl alcohol in lo-" M phosphate buffer were irradiated at different pH values (4.7 IpH 9). Linear plots of DMU concentration, as measured a t X = 265 nm, vs. dose (from lo00 to 9OOO rd) were obtained. From the slope of the curves, G(DMU), representing the number of DMU monomer molecules produced by 100 eV, was obtained. Table I summarizes these results. In order to define the reactive species responsible for the cleavage of the dimers, we also performed the radiolysis in acid solution (pH 2). Under these reaction conditions, e, is entirely converted to H atoms (eq 2) which become eaq- + H30+ k + HzO (2) the exclusive reactant in the medium. At this pH, the extent of cleavage was very small, G(DMU) = 0.03, which indicates that the H atoms are not reactive toward the monomer formation. Another chemical species present in the reaction mixture is the reaction product of OH with tert-butyl alcohol, that is, (CH3)zC(CHz)0Hradical. Although very sluggish, this radical might react with the DMU dimer. However, in experiments with NzO-saturated solutions where e, is removed by conversion to OH (eq 3), the amount of monomer formed was very small [G-
-
-
H+
eaq- + NzO OH + N2 + OH(3) (DMU) = 0.061. When radiolysis was performed in the absence of tert-butyl alcohol, in NzO-saturated solutions, the apparent yield of DMU formation was again very small (G = 0.03-0.05), which indicates that the OH radical does not cleave the dimer to DMU. Thus we conclude that the 0 1980 American Chemical Society
