2158 J. Phys. Chem. A, Vol. 101, No. 11, 1997
Alfassi et al.
pp 241-254. Also see: Beaver, B. D.; De Munshi, R.; Heneghan, S. P.;
Whitacre, S. D.; Neta, P. Energy Fuels, in press.
(2) Wong, P. K.; Allen, A. O. J. Phys. Chem. 1970, 74, 774.
(3) Shida, T.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2369.
(4) Burrows, H. D.; Greatorex, D.; Kemp, T. J. J. Phys. Chem. 1972,
76, 20.
(5) The mention of commercial equipment or material does not imply
recognition or endorsement by the National Institute of Standards and
Technology, nor does it imply that the material or equipment identified are
necessarily the best available for the purpose.
For comparison with the above findings with triphenylphos-
phine, we carried out several experiments with triphenylamine.
First, triphenylphosphine is found to be oxidized relatively
rapidly by several peroxyl radicals whereas triphenylamine was
not.23 Second, γ-radiolysis of CH2Cl2 solutions of Ph3N under
air indicated disappearance of the amine with a radiolytic yield
of only 0.17 µmol J-1, i.e., less than half the yield of the initial
oxidizing species (0.75 µmol J-1), and no chain reaction. Third,
the radical cation of triphenylamine did not react with O2 in
the pulse radiolysis experiment. Pulse radiolysis of Ph3N in
CH2Cl2 produced the radical cation, Ph3N•+, which has an
optical absorption peak at 640 nm as observed before.4 This
absorbance was relatively long-lived at low amine concentrations
but decayed at higher concentrations to produce another species.
From this decay we estimate an upper limit for the rate constant
(6) Alfassi, Z. B.; Mosseri, S.; Neta, P. J. Phys. Chem. 1989, 93, 1380.
(7) Grodkowski, J.; Neta, P. J. Phys. Chem. 1984, 88, 1205 and
references therein.
(8) The molar absorptivity was estimated from experiments at higher
concentrations, where full scavenging of the oxidizing radicals is achieved,
and taking oxidation yield in CH2Cl2 as 0.75 µmol J-1, ref 6.
(9) Murow, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993; p 290.
(10) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data
1996, 25, 709. Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data
1990, 19, 413.
of the reaction of Ph3N•+ with O2 of <5 × 106 L mol-1 s-1 24
.
The above three differences in the behavior of triphenylamine
as compared with triphenylphosphine enable the latter com-
pound, but not the former, to enhance the thermal oxidative
stability of fuels.
(11) Vieira, A. J. C. S.; Steenken, S. J. Am Chem. Soc. 1987, 109, 7441.
(12) O’Neill, P.; Schulte-Frohlinde, D.; Steenken, S. Faraday Discuss.
Chem. Soc. 1977, 63, 141 and references therein.
The finding that Ph3N•+ exhibits a strong absorbance with a
maximum at 640 nm whereas Ph3P•+ exhibits a peak at 320
nm and only a shoulder at 400-500 nm is related to the
difference in the structures of these two radical cations. Ph3N•+
attains a flattened structure, and thus the unpaired spin is
delocalized on the rings; this provides the lower energy
transitions observed. On the other hand, although oxidation of
Ph3P to Ph3P•+ results in considerable flattening of the molecule,
the latter radical cation retains an equilibrium geometry that is
still pyramidal.25 In this geometry, about 85% of the unpaired
spin is localized on the P atom.26 This minimal delocalization
results in the absorption band being at a higher energy than
that for Ph3N•+. Furthermore, the localized triphenylphosphine
radical cations can form dimers with P-P bonding whereas the
delocalized triphenylamine radical cations dimerize through
C-C bonding mainly in their para positions.
The results presented above provide kinetic and mechanistic
information to explain why triarylphosphines, but not triaryl-
amines, might serve as additives for enhancing the stability of
future jet fuels toward thermal oxidation. The results further
indicate how changes in the arylphosphine structure influence
the rate constants for the reaction of the arylphosphine radical
cation with O2 and the reaction of the resulting peroxyl radical
with another phosphine molecule, i.e., in the rates of the chain
propagation steps. Continuation of our study of the effect of
phosphine structure on these rate constants may facilitate the
design of a phosphine structure for optimal fuel stabilization.
(13) McEwen, W. E.; Lau, K. W. J. Org. Chem. 1982, 47, 3595. Keldsen,
G. L.; McEwen, W. E. J. Am. Chem. Soc. 1978, 100, 7312.
(14) Mosseri, S.; Neta, P.; Meisel, D. Radiat. Phys. Chem. 1990, 36,
683.
(15) Baptista, J. L.; Burrows, H. D. J. Chem. Soc., Faraday Trans. 1
1974, 70, 2066.
(16) Dainton, F. S.; Janovsky, I. V.; Salmon, G. A. J. Chem. Soc., Chem.
Commun. 1969, 335. Ellison, D. H.; Salmon, G. A.; Wilkinson, F. Proc. R.
Soc. London A 1972, 328, 23.
(17) Neta, P.; Huie, R. E.; Mosseri, S.; Shastri, L. V.; Mittal, J. P.;
Maruthamuthu, P.; Steenken, S. J. Phys. Chem. 1989, 93, 4099.
(18) Furimski, E.; Howard, J. A. J. Am. Chem. Soc. 1973, 95, 369.
(19) Scho¨neich, C.; Aced, A.; Asmus, K.-D. J. Am. Chem. Soc. 1991,
113, 375.
(20) Engman, L.; Persson, J.; Merenyi, G.; Lind, J. Organometallics
1995, 14, 3641.
(21) Spinks, J. W. T.; Woods, R. J. Introduction to Radiation Chemistry,
3rd ed.; Wiley: New York, 1990; pp 186-190.
(22) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry;
Wiley: New York, 1995; pp 73-76.
(23) Shoute, L. C. T.; Neta, P. J. Phys. Chem. 1990, 94, 2447.
(24) The secondary product formed upon decay of the initial radical
cation exhibits a very broad absorption with a maximum at 820-850 nm,
as observed by Sumiyoshi: Sumiyoshi, T. Chem. Lett. 1995, 645. The rate
of formation of this secondary product was dependent on [Ph3N], and the
process was ascribed to a reaction of Ph3N•+ with another molecule of Ph3N.
In our experiments, the first-order rate constant of the decay of Ph3N•+
was also found to be linearly dependent on [Ph3N], with an intercept of 1
× 104 s-1. This intercept is due to self-reactions of the radical cations and
their reactions with impurities in the solvent and with O2. If we ascribe the
intercept totally to their reaction with O2, we derive an upper limit for the
rate constant of this reaction of k < 5 × 106 L mol-1 s-1. Further studies
are underway to clarify this system, but the upper limit for this rate constant
is valid independently of the mechanism involved.
References and Notes
(25) Tordo, P. In The Chemistry of Organophosphorus Compounds;
Hartley, F. R., Ed.; Wiley: New York, 1990; Vol. 1, Chapter 6, p 137.
(26) Berclaz, T.; Geoffroy, M. Mol. Phys. 1975, 30, 549.
(1) Beaver, B. D.; De Munshi, R.; Sharief, V.; Tian, D.; Teng, Y. 5th
Int. Conf. Stability Handling Liq. Fuels, Rotterdam, the Netherlands, 1995;