HgpP1) Quenching Cross Sections
4 09
expected assuming an allyl resonance of 4 8 kcal/mol for a dimethyl allyl radical.31 By analogy i t is assumed that the transition state leading to the 1,3-pentadiene also loses about one-third of the expected 12.6 kcal/mol resonance energy. This decreased resonance means that the Et n P r rotation can be taken as a hindered rotation with an increased barrier rather than a three electron torsion, which raises the preexponential factor to a value of 1014.1. -+
References a n d Notes, (1) Robert A. Welch Foundation UndergraduateScholarship Recipient. (2) (a) W. Kirmse, "Carbene Chemistry," 2nd ed, Academic Press, New York, N.Y., 1971. (b) V. Franzen, Chem. Rev., 95, 571 (1962). (3) B. Grzybowska, J. H. Knox, and A. F. Trotman-Dickenson, J. Chem. Soc., 4402 (1961); 3826 (1962). (4) H. M. Frey, Trans. Faraday Soc., 58, 516 (1962). (5) (a) T. L. Rose, J. Amer. Chem. SOC., 95, 3500 (1973); (b) J. fhys. Chem., 76, 1389 (1972); (c) T. L. Rose, R. J. Seyse, and P. M. Crane, Int. J. Chem. Kinet., in press. (6) P. J. Robinson and K. A. Holbrook, "Unimolecular Reactions," Wlley, New York, N.Y., 1972. (7) H. Fujimoto and R. Hoffman, J. fhys. Chem., 78, 1167 (1974). (8) C. A. Weillngton, J. fhys. Chem., 66, 1671 (1962).
(9) M. C. Flowers and H. M. Frey, J. Chem. Soc., 3547 (1961).
(IO) K. W. Egger and S. W. Benson, J. Amer. Chem. SOC., 87, 3311, 3314 (1965'); 88, 241 (1966). (1 1) H. M. k e y , A. M. Lamont, and R. Walsh, J. Chem. SOC.D, 1583 (1970). (12) H. E. O'Nealand S. W. Benson, J. fhys..Chem.,72, 1855(1968). (13) S. W. Benson and H, E. O'Neai, Net. Stand. Ref. Data Ser., Net. Bur. Stand., 2'f, 63 (1970). (14) M. R. Hoare and T. W. Ruijgrok, J. Chem. fhys., 52, 113 (1970). (15) (a) J. W. Slmons and G. W. Taylor, J. fhys. Chem., 73, 1274 (1969); (b) G. W. Taylor and J. W. Slmons, IbM., 74, 464 (1970). (16) S. W. Benson, "Thermochemical Ktnetics," Wiley, New York, N.Y., 1968. (17) C. S. Elllot and H. M. Frey, J. Chem. SOC.,345 (1965). (18) H. E. O'Neal and S. W. Benson, J. Chem. Eng. Data, 15, 266 (1970). (19) H. E. O'Neal and S. W. Benson, Int. J. Chem. Kinet., 2, 423 (1970). (20) J. D. Rynbrandt and B. S. Rabinovitch, J. Phys. Chem., 74, 1683 (1970). (21) D. W. Setser and E. E. Siefert, J. Chem. fhys., 57, 3623 (1972). (22) W. P. L. Carter and P. C. Tardy, J. Phys. Chem., 78, 1579 (1974). (23) G. B. Kistiakowsky and B. B. Saunders, J. Phys. Chem., 77,427 (1973). (24) F. H. Dorer and B. S. Rabinovitch, J. fhys. Chem., 69, 1973 (1965). (25) G. W. Taylor and J. W. Simons, Int. J. Chem. Kinet., 3, 25 (1971). (26) L. M. Sverdiov and N. V. Tarasova, Opt. Spektrosk., 9, 160 (1960). (27) R. K. Harris, Spectrochlm. Acta., 20, 1129 (1964). (28) A. de w e r e and W. Luttke, Tetrahedron, 25, 2047 (1969). (29) S. E. Stein and B. S. Rabinovitch, J. Chem. Phys., 58, 2438 (1973). (30) S. S. Butcher and C. C. Costain, J. Mol. Spectrosc., 15, 40 (1965). (31) A. S. Rodgers and M. C. R. Wu, J. Amer. Chem. SOC.,95, 6913 (1973).
Further Mercury( 3P1) Quenching Cross Sections S. D. Gledltsch and J. V. Michael* Department of Chemistry, Carnegie-Melon University, Pittsburgh, Pennsylvania 752 13 (Received September IS, 1974) Publication costs assisted by the U S .Atomic Energy Commission
Quenching cross sections of mer~ury(6~P1) with Hz, HD, Dz, C3H6, l-C4H8, l-CbH10, CdHs, and CzHz have been determined from modified Stern-Volmer plots in a steady-state photolytic experiment. Low mercury atom concentrations were maintained to prevent imprisonment of 2537-A radiation. The cross sections which have been determined in this study are compared with those reported by other investigators. Preferred quenching cross sections are proposed for 11molecules.
Introduction The quenching of mercury 2537-A resonance radiation by a foreign gas is the most extensively investigated photochemical system and has been the subject of thorough re~ i e w . l -This ~ study has investigated the efficiency with which mercury atoms are quenched by a variety of molecules. T h e technique of the present study was recently presented along with measurements of cross sections for nine mole c u l e ~Phosphorescence .~ as a function of absorbed intensity is measured, and classical Stern-Volmer plots are obtained. From a knowledge of the Hg(3P1) emittive lifetime, bimolecular quenching rate constants can then be inferred from slope to intercept analysis, and cross sections are calculated from the usual kinetic theory of gases expression. This type of measurement has been known for over 50 years5 and has been used by numerous workers in mercury quenching e ~ p e r i m e n t s . l -The ~ feature of the earlier work4 and this work which is unique is that the measurements were taken at sufficiently low [Hg]1 that radiation imprisonment is negligible or nearly so. The problem of radia-
tion diffusion has been discussed previously,6-10 and its effects on quenching rate constants has been assessed.4 Eight cross sections have been measured including two which were previously measured. Analysis of all data has led to preferred cross sections for 11 molecules. Use of these preferred values will allow many more cross sections to be accurately inferred. Experimental Section The experimental apparatus which was used in this study is the same as that fully described by Michael and S u e ~ s Radiation .~ from a low-pressure mercury resonance lamp is cooled by a flow of compressed air to minimize reversal of the 2537-A resonance line. The radiation is collimated by a quartz lens after passing through an interference filter. The collimated beam passes through a quartz photolysis cell in which an observation window is oriented perpendicular to and midway between the end windows to facilitate detection of phosphorescence radiation emitted at right angles to the incident beam. The photolysis cell is separated from a conventional Pyrex high vacuum line by a The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
S.D. Gleditsch and J. V. Michael
41 0
stopcock and a cold trap which contains a drop of mercury. Transmitted radiation and phosphorescence radiation are detected by a phototube and photomultiplier, respectively, and are amplified by two Keithley 610 B electrometers. The signals are simultaneously recorded on two potentiometric recorders. The concentration of mercury in the photolysis cell is regulated in exactly the same way as reported previously. The principal difference in experimental technique in this paper and the earlier report is that the mercury resonance lamp is operated a t lower power and, therefore, at lower temperature. Thus, the sensitivity for atom absorption is greater due to less reversal of the resonance line. This can be seen in Figure 1which shows a typical quenching experiment and the measured [HgJl as a function of absorbed light intensity (compare right against left ordinates). Thus, the imprisonment of mercury radiation, which increases with increasing [HglI, is less in this experiment than that previously reported. Regulation and measurement of quenching gas pressures were the same as reported previously. Thus, all pressures are accurate to f 1 0 p. Each run was made from 100 to 80% transmitted intensity and took 2 min. Measurements of transmitted and emitted intensities were repeated three times for each sample for increased accuracy by means of statistical data analysis. Research grade H2 and research grade D2 from Air Products and Chemicals, Inc. were used without further purification. The reaction of LiAlH4 and 99.8% isotopic purity D2O in uucuo was used to generate HD. Mass spectral analysis showed the HD to be 99% pure with the principal impurity to be H2. CP propene and 1-butene together with instrument grade butadiene were obtained from Matheson Gas Products and were purified by bulb-to-bulb vacuum distillation. Technical grade acetylene from Linde Division, Union Carbide Co., and 1-pentene from American Petroleum Institute Research Project 6 were also purified by bulbto-bulb distillation.
Results A single quenching experiment consists of monitoring the changes in transmitted and emitted radiation intensities following removal of a low-temperature bath from the cold trap. Mercury atoms then diffuse into the photolysis cell which already contains a given pressure of quenching gas or is a t high vacuum (0 Torr quenching gas). The phototube output (it) is proportional to the transmitted intensity and decreases with an increase in mercury vapor pressure. The photomultiplier output (if) is proportional to the emitted intensity and increases with an increase in mercury vapor pressure due to the isotropic emission of photons by excited mercury atoms which are not quenched by molecules of the quenching species. The data collected in a single experiment were plotted as it us. if and a typical result is shown in Figure 1. The useful information obtained from this plot is the slope i&. In a series of quenching experiments a t different pressures of quenching gas, the slope becomes more negative with increasing quencher pressure due to the decrease in emission intensity attributable to collisional quenching of mercury(63Pl). The slope of the it us. if plot is linear at high transmittance. The slope curves a t higher mercury vapor pressure (low transmittance) due to an end-on effect together with the complications of radiation imprisonment as reported previ~usly.~ The Journalof Physical Chemistry, Vol. 79, No. 5, 1975
I
,
I
,
,
,
,
100
-.-
,
,I - -
,
0.12
2
45.00
;;
80
v)
c
c
ZJ
v
6 0
4.0
I
x
r--l
cn I
u
2.Q
4.0 6.0 8.0 i f ( R e l a t i v e Units)
Flgure 1. Plot of transmitted intensity vs. emitted intensity for two quenching experiments at 0 and 480 of hydrogen. The right-hand ordinate gives [Hg] I (I = 3 cm) which corresponds to the signal decrease given at the left-hand ordinate.
Only regions of it us. if plots with transmittance exceeding 90% were employed for determination of the slopes, and this is a variation over the earlier work where transmittance was allowed to drop to 85%.4 Because the intensity of the incident beam equals the sum of the absorbed and transmitted intensities, the slope idif of the data plots equals the negative of ia/if. The proportionality of the monitored photocurrents with the radiation intensities which holds at low mercury vapor pressure is given by
,k= cp” if If
(1)
where I , and If refer to absolute absorption and emission intensities. This relationship permits the slopes from data plots which are determined at a number of quencher pressures to be used in constructing a modified Stern-Volmer plot from
The ia/if values which were determined at various pressures of quenching gas for each of eight quenching species is the least-squares fit of the slope in the linear region (see Figure 1 for a typical result). The values of ia/if represent the average of the least-squares slopes from three or four experiments performed at the same pressure. The uncertainty of each reported &/if value represents the standard deviation obtained from the residuals from the mean. The ia/if values are least-squares fit to a straight line as a function of quencher pressure [Q] with each point weighted as the inverse of the square of the standard deviation. These Stern-Volmer plots are in Figures 2-4. The slopeto-intercept ratio provides the value kQ/kf. Jn calculating the bimolecular quenching rate constant kQ, a value of 1.14 X loe7 sec is used for the lifetime of m e r ~ u r y ( 6 ~ P lThe ).~~ quenching cross section is calculated as described previousIY.~ The experimentally determined quenching rate constants and cross sections are presented in Table I. The uncertainty reported for each value represents the standard deviation calculated with the standard deviations in the slope and intercept values from the least-squares fit.
Discussion As discussed above in connection with Figure 1, [Hgll in
H g ( 3 P ~ Quenching ) Cross Sections
41 1
TABLE I: Quenching Rate Constants a n d Cross Sections 1Oi0k,,
Q
0
0.2 0
0.40 0.60 Pa ( t o r r )
0.80
1.00
Figure 2. Stern-Volmer plots for HP, DP, and C4H6.
0 0
0.20
0.40 0.60 PQ (torr 1
0.80
1 .OO
Figure 3. Stern-Volmer plots for HD, C3H6, and C2H2.
I
01 0
I
0.20
I
I
0.40 0.60 Pa ( t o r r )
I
0.80
1.00
Figure 4. Stern-Volmer plots for 1-C4H8and 1-CSHlo.
these experiments is lower for the same transmitted intensity than in the previous r e p ~ r t Also . ~ quenching was only measured over a 10% decrease in light intensity. The effect of radiation imprisonment on the lifetime of mercury(63P1) a t low concentrations of mercury atoms was studied by Michael and Yeh12 who indicated that the Milne theory of radiation diffusion7 reproduces the effective experimental lifetime if Samson’s equivalent opacitp is used. In these experiments the concentration of mercury is 0.04 X 1013 atoms cc-l a t 90% transmittance. [Hgll is therefore 0.12 X 1013 atoms C C - ~ . The effective lifetime of m e r c ~ r y ( 6 ~ P is 1)
cc molecule-’ sec-’ 5.48 3.42 4.59 5.02 6.57 3.80 6.45 4.78
*
-
*
0.21 0.21
f
0.50
10.0
f
0.54
37.6 55.3 34.8 39.4 39.7
*
0.48
i 0.35
*
0.56 f 0.36
9.8 i 0.4 8.6 i 0.5
*
1.1
i 4.0 i 4.1 i
3.2
* 3.4 i 2.9
theoretically predicted to increase at the minimum transmittance by a factor of 1.10, therefore. Because data is collected over a range extending from 100 to 90% transmita tance, the effective lifetime of m e r c ~ r y ( 6 ~ P 1during ) quenching experiment differs from the natural lifetime by a factor ranging from 1.0 to 1.1.Michael and Suess4 report a change in effective lifetime by a factor ranging from 1.0 to 1.2. It is impossible to perform this type of experiment under the ideal conditions of zero mercury atom concentration. The values for cross sections reported in this study should, however, more nearly reflect the ideal values than those obtained in the earlier experiment, being high by a t most 5% while those reported earlier could be high by 10%. This possibility is confirmed in the present work. The cross sections for two molecules, H2 and propene, have been . ~ ratio of cross measured here (Table I) and p r e v i ~ u s l yThe sections for H2 and propene between the previous and present values are 1.13 f 0.09 and 1.29 f 0.26, respectively. The previous discussion indicates, therefore, that the lower values of this study should be more nearly correct. Table I1 presents a summary of both absolute and relative quenching cross sections reported in the literature for molecular hydrogen and isotopic hydrogen. If necessary, absolute values have been corrected to reflect the value for sec. The the natural lifetime of m e r c ~ r y ( 6 ~ P 11.14 ), X Zemansky I value for hydrogen represents that obtained from a Zemansky type experiment using the Milne theory of radiation diffusion to calculate imprisonment If the effects of imprisonment are assessed with Samson’s equivalent opacity in Milne’s theory, as in the Zemansky I1 value for molecular hydrogen, the difference between the absolute cross sections can be reconciled in light of the relative agreement reported by Michael and Suess4 between their quenching cross sections (obtained with the same apparatus as that used in this study) and the consistently lower Zemansky I1 values. The cross section for hydrogen reported by Yarwood, Strausz, and Gunning14 was obtained in a Zemansky type experiment with Holstein theoryg to correct for radiation imprisonment. Their value of 9.3 Az compares quite favorably with the value of 9.8 f 0.4 A2 obtained in this study. The values for the cross section of hydrogen obtained in the depolarization experiment of Barrat, et ul.,l6 the reactant depletion experiment of Thomas and Gwinn,17 the experiment based on the thermal conductivity of hydrogen by London, Vikis, and LeRoy,l8 and the Lyman a photometric experiment of Michael and Yeh12 all agree within reasonable limits of experimental uncertainty with the value reported in this study. The cross sections of H2 and HD reported by Hong and Mainslg are in excellent agreement with those obtained in this study. The Journal of Physical Chemistry, Vol. 79, No. 5. 1975
S. D. Gleditsch and J. V. Michael
41 2
TABLE 11: Summary of Experimental Quenching Cross ~ of Hydrogen a n d Isotopic Hydrogen Sections U Q (A2)
TABLE 111:Preferred Cross Sections
8
8
u2Q,
u'Q,
Absolute valuesa H,
This work Ref 4 Zemansky I'3 Zemansky I1 Ref 14 Ref 1 5 Ref 16 Ref 17 Ref 18 Ref 12 Ref 19 Ref 20 Ref 2 1
H2
HD
D2
HD D2
9.8 i 0.4 11.1 & 0.4 5.3 7.6 9.3 7.8 & 0.4 8 & 1 11.4 i 1.7 9.2 10.8 5 0.4 10.0 i 0.4 7.3 6.01
10.0 i 1.1
8.6 i. 0.5
02
N2O
7.2 i 0.4
10.9 i 1.2 8.41
Relative to H, H2
co
HD
D2
This work 1.oo 1.02 0.88 Ref 22 Chemical 1.o 1.1 0.98 Physic a1 1.oo 1.08 0.94 Ref 15 1.oo 0.92 Ref 19 1.oo 1.09 Ref 2 1 1.oo 1.40 a Quenching cross sections have been adjusted to a natural lifetime of 1.14 X 10-7 sec, where appropriate. The quenching cross sections reported by Evans21for Hz and D2 were suggested to be indicative of a reverse isotope effect. The values for H2 and D2 reported by Deech, et al.,15 for an experiment measuring the real time decay of mer~ury(6~P1) are slightly low in comparison to the values reported in this study, but they do not support the predicted reverse isotope effect. Yang22has conducted both chemical and physical experiments for the determination of relative quenching cross sections for hydrogen and the isotopic hydrogens. His values are also presented in Table 11. These values together with the other relative values calculated for the table lie well within the experimental uncertainty of the various determinations. The relative values of Evans21 provide the only discrepancy. No confirmation is found for any isotope effect. deB. Darwent and c o ~ o r k e r s ~have ~ - ~ ~determined quenching cross sections for a number of mono- and diolefins together with acetylene. The results of these experiments can be compared with those of this study. After adjustment to a natural lifetime of 1.14 X lo-' sec, the cross sections are 27.5 f 3.2 for CsHs, 30.0 f 3.6 for 1-C4&, 38.0 20.7 f 2.5 for CzHz, f 4.5 for l-C5H10, 31.8 f 3.3 for and 5.3 f 0.6 for Hz. The stated uncertainty in these determinations was between 10 and 15%.A value of 12% has arbitrarily been selected. If the cross section obtained in this study of 9.8 f 0.4 A2 is near the correct value, a comparison of the results of deB. Darwent and coworkers with the cross sections determined here should only be made after adjustment of the data on the basis of the 9.8 f 0.4 A2 cross section of hydrogen. If these data are normalized accordingly the values for l-C4H8 and CzHz agree quite well. Cross secThe Journal of Physicai Chemistry, Vol. 79, No. 5, 1975
10.2 10.9 9.5 21.2 6.6 22.1
i i i i i i
0.5 0.5 0.5 1.5 0.4 3.6
n-C4H10
C2H4
1-CdH, 2-C4H, C2H2
7.1 f 0.9 41.2 i 4.0 56.5 + 2.8 64.9 3.3 39.6 i 2.0
*
tions for C3H6 and C4H6 are both higher than the present results, but the discrepancy is not severe considering the combined errors in both determinations. The cross section for 1-CbH10 differs from the present result by about a factor of 2. This discrepancy cannot be readily explained on the basis of radiation imprisonment and is outside the estimated statistical uncertainties. Preferred Cross Sections. Independently determined values reported for the cross section of a single quenching gas often differ significantly even in the instances in which two investigators have employed the same method of determination. Because of the reported discrepancies, a best estimate of the values for certain cross sections would be desirable. Such an estimate should be based on a systematic consideration of available data. The number of reported determinations for certain quenching gases is often quite large, as in the case of hydrogen. We feel that the availability of repeated determinations permits the proposal of preferred values for the cross sections of a number of quenching gases. Inspection of the cross sections for hydrogen presented in Table I1 has led to the selection of a preferred value for the hydrogen cross section of 10.2 f 0.5 Az. This value is statistically indistinguishable from the 9.8 f 0.4 Az cross section determined in this study. Because a 10% increase in the effective lifetime of m e r ~ u r y ( 6 ~ P 1was ) neglected by Michael and Suess, the value for hydrogen should be considered 10% larger than the value which would be determined a t conditions of negligible radiation imprisonment. Appropriate reduction of the value reported by Michael and Suess provides excellent agreement with the preferred value. This value for hydrogen has been used in the calculation of values for the cross sections of hydrogen deuteride and deuterium through the relative values in Table 11. The average relative cross section for both HD and D2 has been multiplied by the preferred value for H2 to obtain the values presented in Table 111. The uncertainties given in each case reflect the uncertainties reported for the individual determinations. Table IV presents values reported in the literature for the quenching cross sections of a number of molecules. All of the values listed for Michael and Suess4 have been adjusted to reflect the 10.2 A2 cross section of hydrogen. This adjustment was performed to provide the cross sections which should be obtained a t conditions of negligible radiation imprisonment. The Zemansky I1 values have also been adjusted to reflect the preferred hydrogen cross section. Because the values of other investigators presented in the table were obtained in absolute determinations, no adjustments were made of the reported values. The preferred cross sections for these quenching species were calculated as the average of the values in Table IV. The uncertainty of each value represents the standard deviation unless the calculated uncertainty is not sufficiently large to reflect a
Hg ( 3 Pi) Quenching Cross Sections
413
TABLE IV: Literature Values of Quenching Cross Sectionsayb
--
ZemanRef 4" sky IIc Ref 14 Ref 15 Ref 16 Others
C2H4
10.2 22.0 6.8 19.5 6.3 36.4
l-CdH8 2-C4H8
64.0
H2 0 2
co N2O n-C4H10
c 2H2
10.2d 22.8d 6.8d 6.V 43.0e 57.P 65.8" 39.8e
21.8 24.6 8.4 39.8
19.2 6.9
20. 6.5
5.gf 7.1g 45.v 55.3i 39.4i
a Quenching cross sections have been adjusted to a natural lifetime of 1.14 X 10-7 sec, where appropriate. All values are in &. e Values have been adjusted to reflect the 10.2 preferred cross section for Hz. d Reference 13. e References 23-25. f Reference 18. g Reference 26. Reference 27. This work.
realistic uncertainty (approximately 5%) in which case a f 5 % uncertainty was selected arbitrarily. These preferred values are also presented in Table 111. These preferred cross sections for Hz, HD, Dz, 0 2 , CO, n-C4Hlo, CzH4, l-CdH8, 2-C4H8, and CzHz have an uncertainty of less than 10% which is rather good for a gas-phase reaction rate constant. Because the preferred cross section for N20 was calculated on the basis of two determinations, the uncertainty of 16% was considered sufficiently small to permit the proposal of a preferred value. Similar calculations for propene on the basis of three determinations, for butadiene on the basis of two determinations, and for 1pentene on the basis of two determinations were found to have uncertainties of approximately 20, 30, and 50%, re-
spectively. The uncertainty in these calculated values was considered too large to permit the proposal of preferred cross sections. Achnowledgrnent. The authors gratefully acknowledge the support of the US.Atomic Energy Commission under Contract No. AT(ll-1)-3242 for this work. References and Notes (1) R. J. Cvetanovic, Progr. React. Kinet., 2, 39 (1964). (2) H. E. Gunning and 0. P. Strausz, Advan. Photochem;:1, 209 (1963). (3) J. G. Calvert and J. N. Pitts, Jr., "Photochemistry, Wiley, New York, N.Y., 1966. (4) J. V. Michael and G. N. Suess, J. Phys. Chern., 78,482 (1974). (5) 0. Stern and M. Volmer, Phys. Zeit., 20, 183 (1919). (6) A. C. G. Mitchell and M. W. Zemansky, "Resonance Radiation and Excited Atoms," Cambridge University Press, New York, N.Y., 1971. (7) E. A. Miine, J. London Math. SOC.,1, 1 (1926). (8) E. W. Samson, Phys. Rev., 40, 940 (1932). (9) T. Holstein, Phys. Rev., 72, 1212 (1947). (IO) L. M. Biberman, Zh. Eksp. Teor. Fiz., 17, 416 (1947). (11) Experimental lifetimes compiled by A. Lurio, Phys. Rev., 140, A1505 (1965). (12) J. V. Michaeland C. Yeh, J. Chem. Phys.53, 59 (1970). (13) M. W. Zemansky, Phys. Rev., 36,919 (1930). (14) A. J. Yarwood, 0. P. Strausz, and H. E. Gunning, J. Chem. Phys., 41, 1705 (1964). (15) J. S. Deech, J. Pitre, and L. Krause, Can. J. Phys.,49, 1976 (1971). (16) J. P. Barrat, D. Casaita, J. L. Cojan, and J. Hamel, J. Phys., 27, 608 (1966). (17) L. B. Thomas and W. D. Gwinn, J. Amer. Chem. SOC.,70, 2643 (1948). (18) G. London, A. C. Vikis, and D. J. LeRoy, Can. J. Chem., 46, 1420 (1970). (19) J. Hong and G. J. Mains, J. Photochem., 1,463 (1972-1973). (20) J. B. Farmer and K. Shimokoshi, Bull. Chem. SOC. Jap., 45, 2269 (1972). (21) M. G. Evans, J. Chem. Phys., 2,445 (1934). (22) K. Yang, J. Amer. Chem. SOC.,87, 5294 (1965). (23) B. deB. Darwent, J. Chem. Phys., 18, 1532 (1950). (24) B. deB. Darwent and M. K. Phibbs, J. Chem. Phys., 22, 110 (1954). (25) B. deB. Darwent, M. K. Phibbs, and F. G. Hurtubise. J. Chem. Phys.. 22, 859 (1954). (26) H. E. Gunning, S. Penzes, H. S. Sandhu, and 0. P. Strausz, J. Amer. Chem. SOC.,91, 7684 (1969). (27) K. Yang, J. Amer. Chem. Soc.,88, 4575 (1966).
The Journal of Physical Chemistry, Vol. 79, No. 5, 1975
