Absolute Reaction Rates of 0(2'D2) with Halogenated Paraffins
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Absolute Reaction Rates of Oxygen(2'D2) with Halogenated Paraffins by Atomic Absorption Spectroscopy in the Vacuum Ultraviolet 1. S. Fletcher and D. Husain* The Department of Physical Chemistry, The University of Cambridge, Lensfield Road, Cambridge CB2 lEP, England (Received February 26, 1976)
A study of the collisional removal of electronically excited oxygen atoms, 0(2lD2), by a range of fluorinated hydrocarbons is presented. O(2lD2) was generated by the repetitive pulsed irradiation of ozone in the Hartley-band continuum and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance radiation at X 115.2 nm (0(31Dz0) O(2lDz)). The following absolute second-order rate constants (in units of cm3 molecule-' s-l, 300 K) for quenching are reported for the gases CF4, CF3C1, CF2C12, CFC13, CC14,CHF3, CHF2C1, CHFC12, CF2ClCF2C1, and CF2ClCFC12, namely, 0.30 f 0.04,2.5 f 0.3, 4.8f0.5,5.5f0.7,8.5f1.1,0.98f0.12,2.4f0.3,4.8f0.5,3.7f0.4,and5.3f0.6,respectively.Theseabsolute data are compared, where possible with previously reported relative rate data. In the cases of CFBC1, CF2C12, and CFC13, estimates of absolute rate constants into the reactive channel yielding C10 (X2n)are given. +-
Introduction The depletion of stratospheric ozone resulting from fluorocarbons released into the earth's atmosphere is a matter of considerable topical interest. This subject has been reviewed recently in detail by Rowland and Molinal who have described the roles played by the two major effluents in this context, CFzC12and CFC13, these species being manufactured at a level approaching 103 ktons per year and employed as propellants for aerosol sprays and as refrigerants. An aspect of this general subject is the reactions of electronically excited oxygen atoms, 0(21Dz), 1.967 eV above the 23P2 ground state,2 generated atmospherically by the photolysis of ozone. Previous determinations of data for O(2lD2) with these simple halogenated paraffins have been relative in nature ahd have arisen from steady photolysis measurements. Thus, Pitts et al.334report relative data for the chemical removal of O(2lD2) derived from the photolysis of N20 at X 228.8 nm. Jayanty et al.5 have obtained relative rate data for total quenching resulting from the photolysis a t X 253.7 nm of 03-N20-halogenated paraffin mixtures. In this paper, we describe absolute rate measurements for O(2lD2) with a range of halogenated molecules using time-resolved absorption spectroscopy in the vacuum ultraviolet in order to monitor the optically metastable oxygen atom.6 In the absence of previous absolute data, the resulting data obtained here are compared with those from previous relative rate measurements. Experimental Section The general experimental arrangement for monitoring 0(Z1D2) by time-resolved resonance absorption at X 115.2 nm (0(31Dz0) O(2lD2)) has been given hitherto7>*with recent modificationsgfor the inclusion of precision logarithmic amplification circuitry effectively permitting direct measurement of In (Io/ItF).Briefly, O(2lD2) was generated by the repetitive pulsed irradiation ( E = 64 J, repetition rate = 0.2 Hz) of ozone in the Hartley-band continuum and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance radiation at X 115.2 nm derived from a microwave-powered atomic flow lamp. The resulting photoelectric pulses, detected by means of a "solar blind" photomultiplier tube (EMR 541G-08-18) mounted on the exit slit of a 1-m
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concave grating vacuum ultraviolet monochromator (Hilger and Watts), were amplified with a current-to-voltage converter,1° fed into the precision logarithmic amplification circuitryl1J2 and subsequently digitized and stored in a 200channel signal averager (Datalab Model 102A). The resulting data were punched onto paper tape and analyzed kinetically by means of a computer using the numerical data smoothing procedure of Savitsky and G01ay.l~The transient photoelectric pulses were analyzed using the modified Beer-Lambert iaw:14
where y = 0.41 f 0.03 for the X 115.2 nm transition under the present condition^.^ Fuller experimental details can be found in the previous papers cited, especially ref 8. Materials. Os, He (buffer gas to ensure no significant temperature rise above the ambient temperature on photolysis), and Kr (for the photoflash lamp) were prepared as previously de~cribed.~>8 With the exception of CCl4 (BDH Analar), all the halogenated reactant materials employed were prepared by the Matheson Co. (>99% purity) and all these reactant species were thoroughly degassed by many freezepump-thaw cycles at liquid nitrogen temperature (-196 "C). With the exception of CF4, the materials were further purified subsequent to the freeze-pump-thaw cycles by fractional distillation to -196 "Cas follows: CF3Cl from -111 "C (CFC13 slush); CF2C12 from -23 "C ( C C 4 slush); CFCl3, CHFZC1, CHFC12, CF2ClCF2C1, and CF2ClCFC12 from -63 OC (CHCl3 slush); CHFs and CHF2Cl from -97 "C (CH2C12 slush), (CFC13 and CF2ClCFC12 are liquids at room temperature and atmospheric pressure. The remainder are gases.) Results a n d Discussion Figure 1 shows the digitized form for In (Itr/Io) a t h 115.2 nm, indicating the decay of resonance absorption by 0(Z1D2) in the presence of CHFC12. Figure 2 shows typical first-order kinetic plots for the decay of O(2lDz) at different pressures of CHFClz, derived from plots of the type given in Figure 1. The slopes of plots such as those shown in Figure 2 are given by -yh' where h' is the standard first-order decay coefficient The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
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I. S.Fletcher and D. Husain
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me
e
-1LO
0
LO
120
80
160
200
Channel
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Figure 1. Digitized time variation of In (ltr//0) at 115.2 nm (O(3’Dz0) 0(2’D2))indicating the decay of resonance absorption by O(2’Dz) in the presence of CHFC12 E = 64 J; repetition rate = 0.2 Hz; po3 =
0.025 N r r 2 , PCHFC,~ = 0.041 N rn-’,
fiotar with
ne = 4 . 0 kN m-2;
average of 32 experiments; 20% of actual point density represented; sweep rate = 5 pdchannel; delay = 100 ps.
Figure 3. Plots of pseudo-first-order rate coefficients (yk‘)for the decay of 0(2‘Dz) in the presence of (0) CF2CICF2CI, ( 0 )CHFCIz, (A) CFpCICFCIz, and (A)Cc14; ptotal with H~ = -2.0 kN m-2 (yk‘ for CC14 displaced by 1 X lo3 s-’).
0
002
0 06
pa
006 (Nm-2)
0 08
Figure 4. Plots of pseudo-first-orderrate coefficients ( y t )for the decay of 0(2’D2)in the presence of (0)CHF2CI, ( 0 )CF3CI, (A) CFpCIz, and (A)CFCI3: ptotalWith He = -2.0 kN (displacements of yk‘: CHFzCI by -1 X IO3 s-‘; CFC13 by 1 X IO3 s-’).
Time (tis) Figure 2. Typical pseudo-first-order plots obtained by monitoring the absorption of light at A 115.2 nm (0(3’D2°)c0(21D2)) in the presence of CHFClp E = 64 J; po3 = 0.025 N w 2&tal , with H~ = -2.0 kN m-’; ~ C H F C (N I ~ nr2): 0 , 0.0; W , 0.022; 0, 0.041; 0 , 0.056.
of O(2lD2) in a particular experiment. k’ is then expressed in the usual form k’ = K
+k ~ [ $ ]
(ii)
where k Q is the absolute collisional quenching coefficient for O(2ID2) by the gas, Q.K is a constant in a given series of experiments and the various components of which it is comprised have been described in detail p r e v i o u ~ l y . ~Figures -~ 3, 4, and 5 show k’ (yk’) in the form of eq ii for the quenching of 0(2’D2) by the various halogenated paraffins, the slopes of these plots from weighted least-squares analysis, together with the above value of y (0.41 & 0.03),yielding the absolute values of kQ. We have reported preliminary data for CF2C12 and CFC13I5 on account of their special importance in atmospheric studies. Here we present absolute collisional quenching data on a wide range of halogenated paraffins. The measurement for CFC13 has been repeated here as it transpired that the preliminary experimental5 employed this quenching agent at The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
0
01
02
03
04
pa (Nm2) Figure 5. Plots of pseudo-first-orderrate coefficients (yk‘)for the decay of 0(2’D2)in the presence of (A)CF4 and (A)CHF3: ptotalWith H~ -2.0 kN m-2 (yk‘ for CH3F displaced by 1 X IO3 s-‘).
too high a concentration, with accompanying adsorption at the walls. Table I lists the results for the present investigation and includes the absolute quenching rate constants for ten halo-
Absolute Reaction Rates of 0(2'D2) with Halogenated Paraffins TABLE I: Absolute Rate Constants (kg,om3 molecule-' ~ ~ 3 K) 0 for 0 the Collisional Removal of O(2lDz) by Various Halogenated Paraffins
CF4 CF3Cl CFzClz CFC13 CCl* CHF3 CHFzCl CHFClz CFzClCFzCl CFZClCFCl:, 02
coz a
0.12 f 0.01
0.30 f 0.04 2.5 f 0.3 4.8 f 0.5 5.5 f 0.7 8.5 f 1.1 0.98 f 0.12 2.4 f 0.3 4.8 f 0.5 3.7 f 0.4 5.3 f 0.6 0.53 f 0.06 0.7 f 0.05" 1.7 f 0.2 2.1 f 0.2"
1.0 f 0.1
2.0 f 0.1 2.3 f 0.2 3.5 f 0.3 0.40 f 0.03 0.98 f 0.06 2.0 f 0.1 1.5 f 0.1 2.2 f 0.1 0.22 f 0.02 0.29 f 0.02a 0.70 f 0.08 0.86 f 0.08"
Reference 8.
TABLE 11: Rate Data for the Collisional Quenching of O(2lDz) by Halogenated Paraffins Relative to NzO
Q
This worka
CF4 CF3C1 CF2Clz CFC13 cc14 CHF3 CHFPCl CHFClz CFZClCF2Cl CFzClCFCl2
0.14 f 0.01 1.1f 0.1 2.2 f 0.1 2.5 f 0.2 3.9 f 0.4 0.44 f 0.04 1.1f 0.1 2.2 f 0.1 1.7 f 0.1 2.4 f 0.2
Ref 4
Ref 5
1.4 X 108 M-l Mn3+ ions are unstable in near neutral ~ o l u t i o n s .In ~ , acid ~ solutions they tend to disproportionate to Mn2+ and Mn4+, and can be stabilized by the addition of Mn2+. In a previous paper we reported the use of the pulse radiolysis technique for the investigation of Mn E. Coli superoxide dismutase? first discovered by Forman and Fridovich.7 The role of Mn(I1) in catalyzing the 0 2 - dismutation, either as Mn2+ or in complexes other than E. Coli dismutase, is not known. The purpose of the present work is to report new data which will contribute toward a better understanding of the radiation chemistry of Mn2+aqueous solutions. This may help in the understanding of catalytic reactions of Mn compounds, such as the dismutation of 0 2 - and other redox reactions in which Mn ions are involved.
Experimental Section The pulse radiolysis apparatus and the optical detection system have been described previously.8 We used 0.05-1.5-ps pulses of 5 MeV and 200 mA. The light sources were 200-W Xe-Hg and 150-W Xe lamps. Scattered light (less than 10%) was ignored. The optical path was 12.3 cm, unless otherwise stated. B and L monochromator, 1P28 photomultiplier, and TekThe Journal of Physical Chemistry, Vol. 80, No. 17, 1976
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tronix oscilloscopes (556 dual beam and 549 memory) were employed. Triply distilled water was used for the preparation of solutions. Chemicals of high purity grade were used as received. The p H was adjusted with solutions of NaOH or HC104. 0 2 , N 2 0 , and Ar, used for saturating the solutions, were Matheson's ultra pure. N2O was purified from traces of 0 2 by bubbling through solutions of acid V2+ ions in contact with zinc amalgam. M I I ( C ~ O ~product ) ~ , of Fluka, was usually used as received. The same results were obtained in some experiments where the Mn(C104)2 was recrystallized. Unless otherwise stated, the pH was 6.7. The temperature was 24 f 2 "C. Dosimetry was carried out using 0.1 M HC02Na solutions saturated 0 2 . Measurements were carried out a t 275 nm, taking 6 = 1100 M-l cm-l 8,9 for Oz-. The G value of radicals in the dosimeter was assumed to be equal to that in the test solutions.
Results and Discussion The Reaction of Mn2+ with Oz-. When an oxygenated aqueous solution containing enough formate ions is irradiated, 02-radical ions (in equilibrium with HO2) are produced as the only radical species within less than 1 ks. The reactions taking place are the following:
