J . Phys. Chem. 1987, 91, 4370-4374
4370
2r I
0
3
e
00 . )
I
I
- '-3L -2 -1 AGo/eV
Figure 5. AGO dependence of reaction rates of the backward ET between ZnOEP" or Er'- and anion radicals of the quinones. The open circles represent ZnOEP'' in hexyl alcohol, and the closed circles represent Erain methanol.
The F values obtained for the quenching of triplet excited Er2in methanol change slightly with the redox potentials (-0.54-0.98 V vs. SCE in dimethylformamide) of the quinones (Table 111). The AGO dependence of kb/kdis for Era- in methanol (Figure 5) is intermediate to that of the backward ET of Cr(dp-phen)32+and Rh(dp-phen)32+. The weakest AGO dependence of kb/kdij was obtained for the backward ET between Zn(OEP)'+ and quinone'-
in hexyl alcohol (Figure 5). This weak AGO dependence of kb might be caused by the beginning of the inverted region of the exergonicity, which is calculated from the redox potential of the quinones in dimethylformamide and yields -AGO = 1.5 eV for the maximum kb value as Table 111shows. However, -AGO should be 1.2 eV because the redox potential of quinone in methanol is shifted by 0.3 V.36-38 In summary, the AGO dependence of k b in the normal region decreases in the following order: C r ( b ~ y ) ~ ~C+r ( d p - p h e ~ ~ ) , ~ + > Er'- > ZnOEP" Rh(dp-phen)?+. The order is in agreement with decreasing order of the backward ET rate with the same AGO involved (-1 eV). Registry No. MB, 100-66-3; TMB, 366-29-0; TMPD, 100-22-1; DPPD, 74-31-7; BQ, 106-51-4; DMBQ, 137-18-8; NQ, 130-15-4; TMBQ, 527-17-3; AQ, 84-65-1; 1,3,5-TMB,621-23-8; 1,2-DMB, 9116-7; 1,4-DMB, 150-78-7; 1,2,4-TMB, 135-77-3; 3,3'-DMB, 119-93-7; Na,Er, 16423-68-0; Zn(OEP), 17632-18-7; [Rh(dp-phen)3](C104)3, 108795-59-1; [Cr(bpy),](CIO,),, 23539-86-8; [Cr(dp-phen)J(ClO,),, 70657-60-2; diphenylamine, 122-39-4; 1,2-phenylenediamine, 95-54-5; 1,4-phenylenediamine, 106-50-3; triphenylamine, 603-34-9; N,N-dimethylaniline, 121-69-7; 1,4-anisidine, 104-94-9; 2-aminonaphthalene, 91-59-8; phenothiazine, 92-84-2.
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(36) Peover, M. E. J. Chem. SOC.1962, 4540. (37) Harriman, A,; Porter, G.; Wilowska, A. J . Chem. Soc., Faraday Trans. 2 1983, 79, 807. ( 3 8 ) Kamel, L. A,; Shams El Din, A. M. J . Electroanal. Chem. 1970, 24, 230.
Mass- Independent Oxygen Isotopic Fractionation in a Microwave Plasma Swroop K. Bains-Sahota and Mark H. Thiemens* Department of Chemistry, B-017, University of California, San Diego, La Jolla, California 92093 (Received: November 24, 1986; In Final Form: April 10, 1987)
The non-mass-dependent isotopic fractionation process in ozone synthesis in a 2450-MHz microwave discharge has been studied. The isotopic variation as a function of pressure of the ozone demonstrates that there are several isotopic fractionations occurring simultaneously. These processes are both mass-dependent and mass-independent and reflect homogeneous and heterogeneous processes. This study extends the electromagnetic regions which may produce the mass-independent isotopic fractionation. This is of considerable interest both to the early solar system chemistry and to the stratosphere where such mass-independent oxygen isotopic fractionations are observed.
Introduction Mass-dependent isotopic effects result from differences in molecular velocities or from changes in vibrational frequencies of isotopically substituted molecules. The former is responsible for most of the physicochemical differences between isotopic species (diffusion rates, condensation, vapor pressure), and the latter for kinetic and equilibrium isotope effects. Both processes lead to a similar result which may be represented by using the conventional 6 notation' for isotopic composition
where S represents the variation, in parts per thousand of an of a sample isotopic ratio (I7R = I7O/I6O)and (18R= '80/160) relative to a standard material. In this notation, mass-dependent processes produce fractionations related by the following equation? S ( l 7 0 ) r 0.526('80) ( 1 ) Craig, H. Geochim. Cosmochim. Acta 1957, 12, 133
0022-3654/87/2091-4370$01.50/0
The mass-dependent effect is of great importance to cosmochemists and astrophysicists, since mass-independent variations in isotopic behavior have been considered as direct evidence for the presence of nuclear processes. Chemical processes had been thought incapable of producing mass-independent fractionation^.^ It has been shown that non-mass-dependent oxygen isotopic compositions are produced in the synthesis of ozone from molecular oxygen in a static system using a high-frequency d i ~ c h a r g e . ~ - ~ Ozone is produced in these experiments with S(I7O) "= S(I8O), similar to that observed in chondritic meteorite^.^ The nonmass-dependent isotope effect has been attributed to an isotopically selective stabilization of the excited ozone intermediate, resulting from the ability of the different isotopomers to exhibit different molecular symmetries6 Ozone production from O2 photolysis by UV light at liquid nitrogen tempeatures also produces isotopic fractionations with S(I8O) S ( ' 7 0 ) . 7 It is important to develop (2) Hulston, J. R.; Thode, H. G. J. Geophys. Res. 1965, 70, 3475. ( 3 ) Clayton, R. N.; Grossman, L.; Mayeda, T. K. Science 1973, 182, 485. (4) Heidenreich, J. E.; Thiemens, M. H. J . Chem. Phys. 1983, 78, 892. (5) Thiemens, M. H.; Heidenreich, J . E. Science 1983, 219, 1073. (6) Heidenreich,J. E.; Thiemens, M. H. J. Chem. Phys. 1986, 84, 2129. (7) Thiemens, M. H.; Jackson, T. Lunar Planet. Sci. 1985. 17. 889,
0 1987 American Chemical Society
Oxygen Isotopic Fractionation in a Microwave Plasma
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4371
a thorough understanding of chemically produced mass-independent isotope effects, particularly those which resemble nuclear fractionations. In addition to the necessity for mechanism determination, the effect of dissociation energy, frequency, and gas pressure are parameters which require characterization. The effect of varying excitation and dissociation sources is of particular interest, since there is a wide variety of energy sources available in the astrophysical environment. The present experiments document the mechanism and isotopic chemistry of ozone formation in flow and static regimes utilizing microwave and high-frequency (-0.5 MHz) electrical discharges. A major body of experimental data exists for microwave flow system ozone production; however, none employ isotopic measurements. The present work then allows comparison to the existing kinetic and fluid mechanical models and experiments. The flow system also provides a constant pressure and isotopic composition which permits quantitative measuement of the pressure dependency of the mass-independent isotopic fractionation process. The documentation of the pressure dependency is of current interest with the recent observation that stratospheric ozone is enriched in 170and I8O on a mass-independent basis* similar to that observed in our The present work uniquely characterizes a flowing discharge system. The use of precise stable isotopic ratio measurements permits observation of secondary effects such as concentration gradients due to diffusion, velocity, drag, and heterogeneous interactions. These processes are common to all flow systems employed for kinetic and spectroscopic measurements, yet they are generally uncharacterized. The present experiments provide further documentation of the relative importance of these secondary processes in flow discharge systems.
Experimental Section Two methods of molecular oxygen dissociation were used: microwave and radio-frequency discharges, which are separately described below. Microwave Discharge. The apparatus for production of ozone consisted of an all-glass flow system. The flow (30 L/min) maintained a constant pressure and isotopic composition of the molecular oxygen reservoir, which permits low-pressure (