J. Phys. Chem. A 1997, 101, 2153-2158
2153
Radiolytic Studies of the Mechanism of Autoxidation of Triphenylphosphine and Related Compounds Z. B. Alfassi† and P. Neta* Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Bruce Beaver Department of Biochemistry and Chemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282 ReceiVed: NoVember 8, 1996; In Final Form: January 21, 1997X
Triphenylphosphine (Ph3P) undergoes one-electron oxidation in irradiated CH2Cl2 solutions to yield the radical cation Ph3P•+. This short-lived species exhibits intense absorption with a maximum at 325 nm and an extended shoulder at 400-500 nm and decays by a second-order process in the absence of O2. In the presence of O2, however, the radical cation reacts with O2 with a rate constant of 7 × 106 L mol-1 s-1 to yield a peroxyl radical, Ph3P+OO•, which exhibits no significant absorbance at λ > 300 nm. Similar results, but with slightly different rate constants, were obtained also in CCl4 solutions and in CH3CN and cyclohexane solutions containing 1% CCl4. Tris(2,4,6-trimethoxyphenyl)phosphine in CH2Cl2 exhibits a behavior similar to that of Ph3P, but the reaction of its radical cation with O2 is an order of magnitude more rapid. On the other hand, the perfluorinated Ph3P•+ in CH2Cl2 reacts with O2 much more slowly, if at all. Diphenyl-2-pyridylphosphine yields a radical cation which exhibits a slightly narrower absorption band but reacts with O2 with the same rate constant as Ph3P•+. The peroxyl radicals propagate a chain reaction by further oxidation of phosphine molecules to Ph3P•+ either directly or indirectly. The final radiolysis product is the phosphine oxide, Ph3PO. The radiolytic yields for oxidation of the phosphines were much higher than the radiolytic yield of the solvent derived radicals, except for the case of the perfluoro derivative, and were dependent on the concentrations of phosphine and O2 and on the dose rate. At low dose rates and high solute concentrations the chain lengths exceeded 1000.
Introduction Triphenylphosphine is one of several compounds being evaluated as potential additives for enhancing the stability of future jet fuels toward thermal oxidation.1 It has been recently demonstrated to improve the thermal oxidative stability of one jet fuel by at least 50%. The mechanism of its action as an oxygen-removing agent was suggested1 to involve initial formation of the radical cation of triphenylphosphine, reaction of the radical cation with O2 to form a peroxyl radical, and chain propagation via reaction of this peroxyl radical with another molecule of Ph3P. The present study is aimed at elucidating the mechanism of autoxidation of Ph3P by using radiolytic methods to initiate the oxidation, to observe transient species, and to determine the kinetics of the reactions involved. Irradiation of triphenylphosphine on the surface of porous glass was suggested2 to lead to formation of the radical cation, which exhibits optical absorption peaks at 330, 345, and 438 nm. Irradiation of CCl4 glassy solutions at 77 K also indicated formation of Ph3P•+, but the spectrum was reported to have a broad peak around 530 nm and was not measured at λ < 400 nm.3 Pulse radiolysis permitted the observation of this transient species in solution; its spectrum in chlorocyclohexane was found to have a broad absorption around 500 nm and increasing absorption at λ < 420 nm, but the measurements were not continued for λ < 400 nm.4 No kinetic details were given in the previous articles. In the present study we examine the oxidation of Ph3P in CH2Cl2, CCl4, CH3CN, and cyclohexane * To whom correspondence should be addressed. † Permanent address: Ben-Gurion University, Beer Sheva, Israel.
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solutions both by pulse radiolysis to follow the formation and reactions of the radical cation and by γ-radiolysis to study the presumed chain reaction, and we compare the behavior of several phosphine derivatives. Experimental Section Triphenylphosphine and its derivatives were obtained from Aldrich5 and the solvents from Mallinckrodt. They were obtained in the highest available purity and were used as received. Solutions were prepared immediately before irradiation and were protected from light to prevent the UV photolytic oxidation of the phosphines by O2. γ-radiolysis experiments were carried out mostly under air in a Gammacell 220 60Co source with a dose rate of 0.79 Gy s-1. A number of experiments were carried out in deoxygenated (Ar-purged) and in O2-saturated solutions and also in another 60Co source with a dose rate variable between 0.029 and 0.0066 Gy s-1. The radiolytic yield of oxidation of the triphenylphosphines to the corresponding phosphine oxides was determined from the changes in absorbance following irradiation with various doses. The observation of transient species and the determination of their kinetic behavior were performed with the NIST pulse radiolysis apparatus. Solutions were irradiated with a 50 ns pulse of 2 MeV electrons, and the formation and decay of transient species were followed by kinetic spectrophotometry at various wavelengths. The dose per pulse was generally between 5 and 50 Gy. There are two major sources of uncertainty in the derived rate constants: the statistical uncertainties in the first-order fits of the kinetic traces and in the plots of kobs vs concentration, which were generally between 5 © 1997 American Chemical Society
2154 J. Phys. Chem. A, Vol. 101, No. 11, 1997
Figure 1. Optical absorption spectrum of triphenylphosphine radical cation. Recorded by pulse radiolysis of CH2Cl2 solutions containing 1 × 10-3 mol L-1 Ph3P under air, 10 µs after the pulse. The insert shows a plot of absorbance at 330 nm vs time: 20 µs for the left half and 300 µs overall for the right half.
Figure 2. First-order rate constant for formation of Ph3P•+, monitored by growth of absorbance at 340 nm, as a function of triphenylphosphine concentration, in aerated CH2Cl2 solutions.
and 10%, and uncertainties in the measurements of volumes, weights, and in the concentration of O2, which we estimate as e10%. The overall estimated standard uncertainties are given along with the rate constants. Other details of the apparatus were given before.6 All experiments were performed at room temperature, 20 ( 2 °C. Results and Discussion Radiolysis of many organic compounds in CH2Cl2 and in CCl4 solutions has been shown to lead to the production of oneelectron-oxidized species.6,7 Oxidation may occur in several steps. The strongly oxidizing species formed upon radiolysis of these solvents, mainly solvent radical cations and chlorine atoms, are relatively short-lived (in the nanosecond range or less) and thus react with solutes only when these are present at sufficiently high concentrations to scavenge these species before they decay. In addition, peroxyl radicals are formed in these solvents under air. These radicals are weaker oxidants and longer-lived than the Cl atoms and solvent radical cations, but they may oxidize certain solutes, albeit more slowly. To produce the radical cations of the various triphenylphosphines, we irradiated CH2Cl2 and CCl4 solutions of these
Alfassi et al. compounds. Kinetic spectrophotometric pulse radiolysis experiments with 1 mmol L-1 Ph3P solutions under air showed the formation of absorbance in the UV and visible range. The spectrum recorded 10 µs after the pulse exhibits a strong peak at 330 nm (with molar absorptivity of about 6 × 103 L moL-1 cm-1)8 and a substantial shoulder at 400-500 nm (Figure 1) and is ascribed to the radical cation, Ph3P•+. This spectrum is in agreement with earlier results; the main peak and the shoulder are in agreement with those obtained in irradiated glass,2 and the shoulder was also reported in a pulse radiolytic study in solution,4 where the spectrum at λ > 420 nm was recorded. The absorbance was formed in two steps; a substantial fraction was seen immediately after the pulse (within [O2], the oxidation nearly stops after a certain irradiation dose, indicating that diffusion of oxygen into the solution from the air space above it is quite slow. However, when the solution is resatuarated with air, the chain oxidation continues upon further radiolysis. The concentration of Ph3P was found to decrease exponentially with irradiation time (Figure 3), i.e.
[Ph3P] ) [Ph3P]0e-kt
(7)
This equation permits us to calculate the initial radiolytic yields (G) even when the measurements were done after substantial decrease in concentration (which was unavoidable at low [Ph3P]). The actual measurements provide the yield from the change in [Ph3P], the dose rate I, and the time of irradiation t, i.e., G ) ([Ph3P]0 - [Ph3P])/It. From eq 7, [Ph3P]0 - [Ph3P] ) [Ph3P]0(1 - e-kt). For kt
