664
COMMUNICATIONS TO THE EDITOR
The Reaction of Chlorine Atoms with Acetylene and Its Possible Stratospheric Signlflcance Publication costs assisted by the U.S. Energy Research and Development Admlnistration
Sir: The only hydrocarbon considered to have major importance in current models of stratospheric chlorine chemistry is which acts as the primary reactant for conversion of C1 to HC1, as in (1). Since the troCl + CH, + HCl + CH, (11 pospheric mixing ratio of CH4 is about 1.5 X lo4, and at stratospheric temperatures reaction occurs for about 1 collision in lo4 of C1 with CH4? possible chlorine atom reactions with other species should be considered for any hydrocarbon present in 10-l' or larger mixing ratio. We consider here the reactions of chlorine atoms with acetylene, which has been tentatively identified at the 2-7 X 10-l' level between 15-21 km altitude, with the concentration decreasing with increasing altitude: The reaction rate for atomic chlorine with acetylene has apparently not previously been discussed in the scientific literature. The abstraction reaction (2), analogous to (l), cannot
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Figure 1. Reciprocal yields of C2H:%I for 38CIreactions in mixtures of C2H2and HI. (See footnote 9 for explanation of 0.97 in the numerator.)
initially "hot" 38Clatom before reactions 3 or 7 occur? The key measurement in this system is the yield of CzH28C1from the sequence (3)) (4),and (6)) as a function C1+ C,H, + HCl + C,H of HI concentration. The slope of a plot of reciprocal (2) CzH:8C1 yield vs. HI/CzH2 ratio is a measure of the occur because the extremely strong C-H bond in acetylene relative rates of reactions 3 and 7.' However, if reaction makes the reaction endothermic by about 9 k ~ a l / m o l . ~ 3 is followed by (5), the 38Clis not permanently removed Consequently, the only important reaction path available from the system and will once more undergo competition for atomic C1 with acetylene is addition, as shown in (3). between (3) and (7). This apparent "inefficiency" of C1+ C,H, + CHCl=CH* (3) reaction 3 in removing %C1can be experimentally detected by carrying out the experiments at widely differing The excited C2H2C1*radical can then undergo either pressures, as shown in Figure 1. The two sets of data are stabilization or decomposition in (4)or (5), the latter consistent with k 7 / k 3= 0.86 f 0.08 and P1 (pressure of CHC~=CH*t M + CHCI=CH + M (4) half stabilization, i.e., k& = k5) = 840 f 80 borr of CClF3, as shown by the calculated lines of Figure 1." The inCHC~=CH*+ C,H, + c i (5) tercept at 1.0 confirms that reaction 2 is not observed. releasing the free C1 atom again.6 No information is Reaction 7 is very fast, with a reaction rate estimated available about the fate of a stabilized CzHzClradical from as 10-l' cm3molecule-' s-', and the initial addition reaction (4),but probably the radical would react with 02 and lead 3 must therefore also have a rate coefficient that correeventually to the formation of a chlorine-containing sponds to addition in not more than 2-5 collisions. If the oxy-carbon compound. A previous study of CzH2Clrestratospheric mixing ratio of C2Hzis indeed lo-'', then the actions was carried out in the absence of any specific addition of C1 to C2H2should occur at a rate comparable scavenger molecule and consequently was very much to that of abstraction with CH4. However, the fraction of concerned with the polymerization produds resulting from CzHzC1*molecules which are stabilized is then given by C2HzClreaction with acetylene itself.' the stratospheric density divided by the density at which We have measured the relative rates of (4)and (5) in kJ4 = k5. At 30 km,for instance, the density is about 0.01 a system containing CClF3as the inert molecule, M, with times that at sea level, and only about 1%of the CZHzCl* HI present as a scavenger for the stabilized C2H2C1radical, radicals would be stabilized. A more accurate calculation as in (6). The inclusion of HI in the system necessarily would require the measurement of PllZfor N2 and 0 2 at CHCl=CH + H I + CHCl=CH, + I stratospheric temperatures. The value of k5 should be less (61 at 210 K by about a factor of 2, while the smaller size of permits the direct reaction of C1 with HI, as in (7). All N2and O2vs. CClF3may also reduce k4 by a comparable C1 t H I + HC1+ I (7) amount, leaving the room temperature measurements with CClF3 as a reasonable approximation to the actual of our experiments have utilized radioactive 38Clatoms stratospheric situation. formed by the thermal neutron reaction, 37Cl(n,y)38C1.The Accordingly, the overall conversion of C1 to stabilized CClF3 thus serves as the nuclear target, and then as a CzHzClhas a rate constant of roughly cm3molecule-' moderator to remove the excess kinetic energy of the The Journal of Physical Chemistv, Vol. 81, No. 7, 1977
685
Communications to the Editor
s-l at 30 km, and the removal of C1 through reaction with CzHzis very much less important (10.01) on a quantitative basis than the conversion of C1 to HC1 through reaction 1. Since Cl is cycled through HC1 many times, the amount of C1 which can be bound up in acetylenic compounds is also dependent upon the rate of return of C1 (or C10) to the ozone-removingcatalytic chain. The HC1 formed from (1)is returned to the catalytic cycle as C1 through reaction OH t HC1+ C1 t H,O (8)
8 in a time period of a few days at 30 km. The oxychlorocarbon molecule presumably formed by C2HzCl reaction with O2 are likely to be both chemically and photochemically reactive and release of C1 or C10 within a still shorter time seems likely. Nevertheless, a minor amount of stratospheric chlorine atoms may be bound into organic form following addition to acetylene, and the subsequent history of these molecules is of some scientific interest.
Acknowledgment. This research was supported by US. ERDA Contract No. AT(04-3)-34, P.A. 126. References and Notes (1) F. S.Rowland and M. J. Molina, Rev. Geophys. Space Phys., 13, 1 (1975). (2) “Halocarbons: Effects on Stratospheric Ozone”, Panel on Atmospherlc Chemistry, National Academy of Sciences, Sept 1976. (3) Recent measurements indicate a rate constant of about 2 X cm3 molecule-‘ s-’ at 210 K for reaction 1. See ref 2 or R. T. Watson, J. Phys. Chem. Ref. Data, in press. A summary of such measurements is also given in F. S.C. Lee and F. S.Rowland, J. fhys. Chem., 81, 86 (1977). (4) P. Hanst, private communication. (5) With the following heats of formation, the endothermicity is 9 kcallmol. AHA298 K, in kcallmol), as given in JANAF Thermochemical Tables, NatL Stand Ref. Data Ser., NafL Bur. Stand., No. 37 (1971); C2H2, 54.2; HCI, -22.1; CI, 28.9; CpH, 114; CpHCI, 51 10; H, 52.1. (6) The loss of H from CpH2CI”is also endothermic by about 20 kcallmol from the heats of formation in ref 5. (7) M. H. J. Wijnen, J. Chem. Phys., 36, 1672 (1962). (8) F. S. C. Lee, Ph.D. Thesis, University of California, Irvine, 1975. (9) Approximately 3% of the 38CIatoms react with CCIF3 while still “hot” and are found as CmCIF3,CC138CIF,, or CHmCIF2in this system. These correspond, respectively, to the hot substitution reactions of 38CI/CI, %I/F, and to the HJ-scavenged CmCIF2radical from the decomposition of CC138CIFp.The data are therefore plotted with 97% of the total 38CIavailable for thermal reaction. Details are given in ref 8, and in an article by F. S. C. Lee and F. S. Rowland, J. Phys. Chem., submitted for publication. (10) If a single collision (i.e., “strong” collision assumption) is sufficient to stabilize C2H2CIin (4), then the fraction of stabilized CpH,CI is given by [ k 4 M l ( k 4 M k 5 ) ] ,or by its equivalent, 8 = [P/(P PI,,)], in which Pis the actual pressure, and f i I 2 is the pressure for which h M = /(5. The ratio of rates of formatlon of HCI from (7) and CpHpCI” from (3) is k7(HI)lk3(C2H2)and of HCI and stabilized C2HpCIis k7(HI)Ik3e(CpHp).The ratio of total thermal chlorine reacting to that forming stabilized CpH2CIis given by (1 [k,(HI)/k3B(C2H2)]) and the slope of the graph in Figure 1 for pressure Pis given by k,lk& or (k,/ka)(l ( P i / 2 / P ) ) .The ratio for k7/k3and the value of f i , 2 are obtained from slopes of 2.17 and 1.03 for 550 and 4200 Torr, respectively.
Figure 1. Upfield (-) and downfield (-)traces of high-gain, fast-sweep signal of HMD: (A) signal showing dispersion characteristics; (B) pure absorption mode.
F. S. C. Lee S. Rowland”
equipped with nuclear magnetic resonance (NMR) spectrometers which can be used for such purposes, after minor modification. The dielectric constant of liquid samples can be determined easily and quickly by measuring the phase control adjustment of a high-resolution NMR spectrometer required to produce a pure absorption mode for the proton signal of an external standard. This is possible because the phase of the NMR signal is dependent upon the dielectric constant of a sample placed in the probe region. Because this dependency is a complicated function of the electrical circuitry involved, special calibration techniques were developed. An external standard was employed for several reasons: (1)to eliminate the need for the sample to provide the proton signal necessary for observation, (2) to eliminate
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Department of Chemistry University of California Irvine, California 927 17
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Received December 3, 1976
Dielectric Constants of Liquids from Nuclear Magnetic Resonance Phase Control Studies’
Sir: Most chemical laboratories are not equipped with specialized apparatus for measuring dielectric constants of liquids. Today, however, many laboratories are
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Figure 2. Plot of ARas a function of dielectric constant, E: 0 , points used for calibration; 0, points used to obtain E for unknowns from NMH phase control data.
The Journal of Physical Chemistw, Vol. 61, No. 7, 7977
