1308
J. Phys. Chem. 1983, 87, 1306-1308
Quantum Yields for the Photodissociation of CBr,F, in the 200-300-nm Region Lulsa T. Moiinat and Mario J. Molinat’ Bpartment of Chemistry, University of California, Irvine, California 9271 7 (Received: October 6, 1982: In Final Form: November 29, 1982)
The gas-phase photolysis of CBr2F2has been studied in the presence of 1atm of air at 206,248, and 302 nm. The only products formed are CF20and Br2,and within experimental error the measured quantum yields for the disappearance of CBr2Fzand for the appearance of CF20and Br2are unity. These results are in disagreement with earlier literature reports suggestingsignificantly smaller qutllitum yields and indicating that 265-nm radiation produces excited CBr2F2molecules long lived enough to permit collisional stabilization.
Introduction The release of brominated hydrocarbons to the atmosphere carries potential for removal of stratospheric ozone similar to that of chlorinated hydrocarbons due to the existence of the BrO, catalytic chain reactions,’ which are analogous to the well known C10, reactiom2 In a recent paper3 we reported measurements of the ultraviolet absorption spectra of several bromofluorocarbons together with estimates of their rates of interaction with solar radiation, in both the troposphere and stratosphere. These spectra are similar to the spectra of the corresponding chlorine-containing molecules except that the absorption bands are shifted toward longer wavelengths by several tens of nanometers. Absorption of light by chlorinated hydrocarbons leads to photodissociation with unit quantum yield, and in the lowest frequency band the process is interpreted as an n cr* transition involving excitation to a repulsive electronic state.4 The similarities in the smooth, continuous absorption spectra suggest that the nature of the electronic transitions is the same for the corresponding bands of the brominated hydrocarbonds, i.e., the quantum yield for photodissociation should be unity, and this has been found to be the case for molecules such as CH,Br and CH2Br2.5 The only set of studies on the quantum yields for photodissociation of hydrocarbons containing both Br and C1 or F atoms indicates a different behavior: photolysis of CBr2Fzat 265 nm6 and of CBrCl, at 366 nm’ is reported to generate electronically excited molecules with sufficiently long lifetimes to permit collisional stabilization, giving a corresponding pressure-dependent yield of Br atoms. If this interpretation is correct the photodissociation quantum yields for these brominated molecules under atmospheric conditions would be very much less than unity.3 An early study by Francis and Haszeldine8showed that CBr2F2photolyzes readily in the presence of oxygen at wavelengths longer than 220 nm, yielding CF20 and Br2 (as well as some C02 and SiF4 formed presumably by decomposition of CF20 on the glass surface); however, the quantum yield values were not measured in this study. We report here our measurements on the quantum yields for formation of CF20 and Br2 in the photolysis of CBr2F2,at 206,248, and 302 nm, in the presence of O2 and N2at atmospheric pressure. Our results indicate unit quantum yield for the formation of these products.
-
Experimental Section The photolysis of CBrzF2was carried out in a 10-cm glass cell fitted with a pair of 5-cm diameter sapphire windows. Pressent address: J e t Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.
Two UV radiation sources were used for the photolysis: (1)a 1OOO-W xenon-mercury arc lamp, coupled to a 1/4-m Bausch and Lomb monochromator for the 248- and 302nm photolysis (fwhm 4 and 3 nm, respectively), and (2) a microwave-discharge iodine lamp for the 206.2-nm transition of the I atom); photolysis ( ~ P ~ [ ~ P , , ,6s[2P3/2] ] in both cases the collimated beam cross section was -5 cm2. The incident light intensity at 248 and 206 nm was obtained by the rate of Hz production from HBr by assuming $(H2) = l.9 At 248 and 302 nm the intensity was determined by means of a calibrated thermophile (Scientech, 36-0001). A t 248 nm the results from the two methods agreed with each other within -10%. Quantum yield calculations in which the detailed line shape of the light source was taken into account together with the variation in absorption cross sections of CBr2F2and HBr yielded essentially the same results as the simple calculation assuming monochromatic photolysis light. The gas samples were prepared in a glass vacuum line by first filling the cell with CBr2F2and then pressurizing to 1atm with a 1502/N2mixture, using MKS capacitance manometers. The samples were allowed to stand for several minutes to permit complete mixing and subsequently UV-visible and IR spectra were recorcded at regular intervals during the photolysis. The infrared analyses were carried out with a Nicolet 7199 A Fourier-transform infrared spectrometer equipped with a standard data handling system and a liquid-N2 cooled HgCdTe detector. The spectra were taken at 1-cm-’ resolution, each one being computed from the average of 100 interferograms. The absorbances of the CF20 and CBr2F2samples were measured in the presence and absence of diluent gas (N2 and 02).Samples of CF20 (1-5 torr) were transferred through a monel-stainless steel vacuum line to the Pyrex cell or monel cell fitted with sapphire windows. Both cells gave the same extinction coefficient for the 1943-cm-’ absorption of CF20. A value
-
(1) Y. C. Yuna, J. P. Pinto. R. T. Watson, and S. P. Sander. J. Atmos.
scz., 37, 339 (1960).
(2) F. S. Rowland and M. J. Molina. Reu. GeoDhvs. SDace Phvs.. 13. 1 (1975); WMO Global Ozone Research and Monkoring Project, Report
No. 11. 1981. (3) L. T. Molina, M. J. Molina, and F. S. Rowland, J . Phys. Chem., 86, 2672 (1982). (4) J. R. Majer and J. P. Simmons in “Advances in Photochemistry”, Vol. 2, J. N. Pitts, G. S. Hammond, and W. A. Noyes, Ed., Interscience, New York, 1964, pp 137-81. (5) J. G. Calvert and J. N. Pitts, “Photochemistry”, Wiley, New York, 1966, p 526. (6) J. C. Walton, J . Chem. SOC.,Faraday Trans. 1 , 68, 1559 (1972). (7) H. W. Sidebottom, J. M. Tedder, and J. C. Walton, Trans. Faraday SOC.,65, 755 (1969). (8) W. C. Francis and R. N. Haszeldine, J . Chem. SOC.,2151 (1955). (9) H. Okabe, “Photochemistry of Small Molecules”, Wiley-Interscience, New York, 1978, p 128.
0022-3654/83/2087-1306$01.50/0 Q 1983 American Chemical Society
Quantum Yields for the Photodissociation CBr,F,
The Journal of Physical Chemistty, Vol. 87, No. 8, 1983
TABLE I : Photolysis of CBr,F, in the Presence of 0, and N,
A , nm
206.2
photolysis quantum yields time, [CBr,F,], Zoa min torr (CF,O) (Br,) -(CBr,F,) 2.6 2.6 2.6 2.6
30 60 30 60
10.8 11.0 20.0 20.0
1.17 1.22 1.11 1.04 1.14
1.09 1.09 1.14 1.11 1.11
1.32 0.96 1.16 1.04 1.12
13 26 13 26 14 27 12 24
21.0 21.2 42.4 42.0 11.0 10.5 10.6 11.2 51.2 52.0
1.14 1.10 1.06 0.94 1.01 0.96 1.06 1.04 1.01 0.96 1.03
1.05 1.02 1.07 0.91 0.95 0.94 1.04 1.04 0.97 0.94 0.99
1.22 1.10 1.26 0.92 0.98 0.94 1.07
1.4 1.2 1.2 1.1 1.23
1.5 1.1 1.3 1.2 1.28
av 241.7
61 61 62 63 63 63 36 36 36 36
8 17
av 302.4
av a
1.6 1.6 1.6 1.6
120 250 182 302
300 305 308 308
1.05 1.01 0.97 1.05
Incident radiation intensity, 1014 photons cm-, s-'.
of 0.012 torr-' cm-' (base 10) was obtained for CF20 alone, which agrees well with that reported by Saunders and Heicklen;'O this value increased to 0.017 torr-' cm-l in the presence of 1 atm of Oz/N2. For CBr2F2,we monitored the absorption at 2231 cm-'; it has an extinction coefficient of 0.0051 torr-' cm-' and is independent of added gas. The UV-visible spectra were recorded with a Cary 219 spectrophotometer interfaced to a Data General NOVA 3 computer. The Br, UV cross section values reported by Seery and Britton'l were used to calculate the amount of bromine produced in the decomposition of CBr,F2. For the absorption measurements of HBr, a 10-cm quartz cell with fused Suprasil windows was used and the spectra were recorded with a 0.2-nm slit at pressures ranging from 5 to 500 torr. The results were in very good agreement with those reported by Hubert and Martin.', Dibromodifluoromethane was obtained from Chemical Procurement Laboratories and was used after bulb-to-bulb distillation a t -90 0C.3 Hydrogen bromide (Matheson, 99.8%), carbonyl fluoride (PCR Research Chemicals), and bromine (Mallinckrodt) were used after vacuum distillation. Oxygen (Matheson Research grade, 99.997%) and nitrogen (Air Products) were used without further purification.
Results and Discussion A summary of the CBr2F2photolysis results is presented in Table I. 206 and 248 nm. The quantum yields for CF20 and Br, production are all unity within experimental error. No exposure time or pressure dependence of the quantum yield was observed. 302 nm. At this wavelength the absorption cross section of CBr2F2is about 3 X cm2molecule-', so that only small amounts of products were formed. In addition to the infrared absorptions due to CFzO and to CBr2F2,we observed weak bands at 2100 (due to CO) and 1875 cm-' (possibly due to CFC10). The growth of these two bands (10) D. Saunders and J. Heicklen, J. Phys. Chem., 70, 1950 (1966). (11) D. J. Seery and D. Britton, J. Phys. Chem., 68, 2263 (1964). (12) B. J. Huebert and R. M. Martin, J. Phys. Chem., 72,3046 (1968).
1307
was nonlinear with time and reached a maximum after 2 h of exposure. At the end of the first 30-min photolysis period the quantum yield values for CF20 and for Br, production were somewhat erratic, and significantly greater than unity. However, the two values dropped rapidly with increasing exposure, and the quantum yields for incremental CF,O and Br, production stabilized to a value of -1.1 after approximately 2 h. These results can be explained by assuming the presence of impurities in the CBr,F, samples which were decomposing during the initial photolysis period due to their much stronger absorption cross sections in the 300-nm region. These impurities probably consist of higher molecular weight brominated fluorocarbons which yield some CF,O, Br,, and possibly CFClO as photooxidation products. It appears then that the quantum yield for the photodissociation of CBr2F2is unity throughout the 310-200-nm region, as expected from the nature of the absorption spectrum. The primary photolysis reaction a t these wavelengths should be C-Br bond fission: CBr2F, + hv
-
CBrF,
+ Br
(1)
The 0, present in our system rapidly adds to the CBrF, radical preventing its recombination with Br atoms to regenerate the parent molecule. The exact nature of the subsequent reactions is not well established and heterogeneous processes probably play a significant role. It is clear, though, that the only important photooxidation products with sufficient stability to be observable in our system should be CFzO and Br,. Our results, as well as those of Francis and Haszeldine, confirm this expectation. Plausible reaction sequences leading to CF20 and Br, have been discussed, for example, by Hei~k1en.I~ Flash photolysis studies at shorter wavelengths14have shown that the primary photodissociation step produces the carbene and a bromine molecule: CBr,F,
+ hv
-
CF2
+ Br,
(2)
Our experiments provide little information on the relative importance of reactions 1 and 2 (both of which are energetically feasible): we found no evidence for the formation of CzF4, which would be a product if reaction 2 yields singlet CF,, a species which does not react readily with 0, at room temperat~re.'~However, if reaction 2 yields triplet CF2, the final photooxidation products are expected to be the same as in reaction 1. In a study of the photolysis of CBr2F2 at 265 nm, Walton6 suggested that the primary process involves the formation of an excited species, CBr2F2*,with a sufficiently long lifetime to be stabilized by collisions in competition to fragmentation:
- + + + + + -
CBr2F,
+ hv
CBr2F2*
CBr2F2*
CBrF,
CBr2F2* M CBrF, CBrF, Br
CBr2F,
CBrF,
Br (+M)
+ Br + M
Br
-
(3) (4)
M
(5)
C2BrzF4
(6)
CBr,F, (+M)
(7)
+M
(8)
Br,
Reactions 6 and 8 lead to the disappearance of the parent molecule while reaction 5 regenerates CBr2F2,decreasing (13) J. Heickien in "Advances in Photochemistry", Vol. 7, J. N. Pitts, G. S. Hammond, and W. A. Noyes, Ed., Interscience, New York, 1969,
pp 57-148. (14) D. E. Mann and B. A. Thrush, J. Chem. Phys., 33, 1732 (1960); J. P. Simmons and A. J. Yarwood,Trans. Faraday SOC.,57,2167 (1961).
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J. Phys. Chem. 1983,8 7 . 1308-1312
the apparent photolysis quantum yield. As mentioned above, in our experiments reaction 5 does not occur due to the fast addition of O2 to the CBrF2 radical. Walton's suggestion for the existence of CBr2F2*was based on an analysis of the quantum yield for the formation of C2Br2F, as a function of pressure, assuming that only homogeneous gas-phase processes were taking place (reactions 3-8). Extrapolation of Walton's quantum yield value as a function of pressure to our experimental conditions implies a net quantum yield for CBr2F2disappearance of since a t 1 atm most CBr2F2*molecules would be collisionally stabili~ed.~ One possible explanation for the large discrepancy with our observations is that the O2 present in our system reacts with CBr2F2*leading to dissociation instead of stabilization. We believe that a much more plausible explanation is that the photodissociation quantum yield is in fact unity a t all pressures, even in the absence of 02,and Walton did not interpret his results correctly. Specifically, the assumption that only homogeneous gas-phase reactions were taking place is probably in error: if the photolysis rate in Walton's experiments is considered together with upper limits on gas-phase radical recombination rates, one can estimate free-radical lifetimes in his photolysis cell ranging from a few seconds at the lower pressures (- 2 torr) to tens of milliseconds at
-
the higher pressures (-500 torr). Hence, collisions with the walls of the reaction vessel-and consequently heterogeneous reactions involving adsorbed radicals-almost surely played a dominant role in Walton's experiments, particularly a t the lower pressures. If one adds heterogeneous reactions, diffusion processes, etc. to the system of reactions 3-8 proposed by Walton, his results can no longer be interpreted quantitatively; they are then consistent with a CBr2F2photodecomposition quantum yield value of unity. In conclusion, our results show that CBr2F2photodissociates with unit quantum yield at 206,248, and 302 nm. Hence, CBr2F2behaves like all other halogenated hydrocarbons: absorption of light in the lowest frequency UV band involves excitation to a repulsive electronic state followed by the rupture of the weakest carbon-halogen bond. In the atmosphere CBr2F2will have a lifetime toward solar photodissociation of about 1 year3 due to absorption of radiation in the 290-310-nm range.
Acknowledgment. This research was supported by the National Aeronautics and Space Administration under Grant No. NSG-7208. Registry No. CBr2F2,75-61-6; CF20,353-50-4; Br,, 7726-95-6; 02,
7782-44-7; N2, 7727-37-9.
Theoretical Study of Conformational Processes in Sulfur Diimides Krlshnan Raghavacharl" and Robert C. Haddon" Bell Laboratories, Murray HIII, New Jersey 07974 (Recelved: April 26, 1982; I n Final Form: November 29, 1982)
Ab initio molecular orbital calculations with the inclusion of electron correlation are used to investigate the conformational processes in sulfur diimides (R-N=S=N-R). The cis,cis isomer is found to be the most stable form of the parent sulfur diimide and the cis,trans isomer is the lowest energy form of dimethylsulfur diimide. The trans,trans isomer for both compounds is considerably higher in energy. Our results rationalize the temperature-dependentNMR spectra of dimethylsulfur diimide. The isomerization pathway for interconversion between the isomers is found to be intermediate between rigid rotation and in-plane inversion. d functions on S are extremely important in determining the geometries of the transition states involved. The cis,trans to trans,cis interconversioninvolves the cis,cis isomer as an intermediate and the barrier for the interconversion is 18.0 kcal/mol for sulfur diimide and 16.0 kcal/mol for dimethylsulfur diimide. The geometries and the reaction barriers obtained are in good agreement with the available experimental values.
Introduction The discovery of the superconducting polymer (SN), has stimulated interest in the study of compounds with N-S bonding. Sulfur diimides, R-N=S==N-R, have received a lot of attention recently due to their interesting electrochemical and photochemical proper tie^.'-^ These compounds are also important in coordination chemistry and form complexes with Pt or Pd.4,5 (1) M. L. Kaplan, R. C. Haddon, K. Raghavachari, S. Menezes, F. C. Schilling, J. J. Hauser and J. H. Marshall, Mol. Cryst. Liq. Cryst., 80,51 (1982). (2)(a) R. H.Findlay, H. H. Palmer, A. J. Downs, R. G. Edgell, and R. Evans, Inorg. Chem., 19, 1307 (1980);(b) R. C. Haddon, S. R. Wasserman, F. Wudl, and G. R. J. Williams, J. Am. Chem. Soc., 102, 5070 (1980);J. P.Boutique, J. Riga, J. J. Verbist, J. Delhalle, J. G. Fripiat, J. M. Andre, R. C. Haddon, and M. L. Kaplan, ibid., in press. (3)F. P.Olsen and J. C. Barrick, Inorg. Chem., 12, 1353 (1973). (4)J. Kuyper and K. Vrieze, J. Organomet. Chem., 74,289 (1974). 0022-3654183f2087-1308$01.5010
Sulfur diimides exhibit geometrical isomerism and three isomers, cis,cis ( l ) ,cis,trans (2), and trans,trans (31, are possible. The parent sulfur diimide has not been studied experimentally, but there is strong evidence from substituted sulfur diimides which indicates that the cis,trans isomer 2 is the most stable Temperature dependence of the NMR spectra of substituted sulfur diimides indicates the presence of a more symmetric isomer also and it has been assumed until now that it is the trans,trans isomer 3 which is present and that this is the (5) R. T. Kops, E. V. Aken, and H. Schank, Acta Crystallogr., Sect. B, 29, 913 (1973). (6)(a) G. Leandri, V. Busetti, G. Valle, and M. Mammi, J. Chem. SOC. D,413 (1970);(b) V. Busetti, Acta Crystallogr., Sect. B , 38,665 (1982). (7)J. Kuyper, P. H. Isselmann, F. C. Mijlhoff, A. Spelbos, and G. Renes, J. Mol. Struct., 29, 247 (1975). (8)J. R. Grunwell, C. F. Hoyng, and J. A. Rieck, Tetrahedron Lett.. 26, 2421 (1973).
0 1983 American Chemical Society
