J. Phys. Chem. 1983, 87, 1674-1675
1674
Chemically Induced Dynamic Electron Polarization of Dialkoxyphosphonyl Radicals' M. Anpo,2a K. U. Ingold,*2b and J. K. S. Wan* Division of Chemistty, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6 and Department of Chemistry, Oueen's Universitv. Kingston, Ontario, Canada K7L 3N6 (Received: February 17, 1983)
The EPR spectra of phosphonyl radicals, (RO),P=O, which are characterized by a well-spaced (-700 G) doublet, show an impressive E-A polarization when generated by photolysis of a 20% (v/v) solution of a dialkyl phosphite in di-tert-butyl peroxide.
Chemically induced dynamic electron polarization (CIDEP) is a firmly established phenomenon that has been extensively investigated and is now well under~tood.~ Most studies of CIDEP have involved organic radicals that have relatively complex EPR spectra with hyperfine splittings (hfs) by many protons. During an investigation of the kinetics of some reactions of diethoxyphosphonyl radicals, 1, by EPR spectroscopy4 we observed that this radical exhibited a rather fine example of "emissive-absorptive (E-A) polarizationn3 (see Figure 1). The simplicity of the EPR spectrum of 1, which is characterized by one extremely well-spaced doublet due to hfs by 31P(aP = 696 G ) with no other h f ~affords ,~ an almost unique advantage in studies of E-A polarization6 since the current theory3 shows that the magnitude of E-A polarization is dependent upon both the magnitude of the hfs and the concentration of the radicals. The polarization of 1 also represents the first example of CIDEP for a radical having the unpaired electron centered on phosphorus. Radical 1 was generated by flowing a 20% (v/v) solution of diethyl phosphite in di-tert-butyl peroxide through the cavity of a Varian E-104 EPR spectrometer and subjecting the solution to the UV irradiation from a 1-kW highpressure mercury lamp.4 Me3COOCMe3-k2Me3C0. Me3C0. + (EtO),P(O)H
-
Me3COH + (EtO),P=O 1
For a simple two-line spectrum the E-A polarization enhancement factor, VEA,is given by
VEA= ([HI - [LI)/([Hl
+ [LI)
where [HI and [L] are the amplitudes of the high- and low-field lines, respectively. In our radical-generating system there should be no "initial p~larization"~,~ and therefore the magnitude of PA should increase linearly with radical c o n ~ e n t r a t i o n s . ~ J ~The ' ~ radical concen(1) Issued as NRCC No. 21204. (2) (a) NRCC Research Associate 1981-82. (b) NRCC. (3) Wan, J. K. $4.; Elliot, A. J. Acc. Chem. Res. 1977, 10, 161-6, and references cited. (4) Anpo, M.; Sutcliffe, R.; Ingold, K. U. J. Am. Chem. SOC., in press. (5) Davies, A. G.; Griller, D.; Roberts, B. P. J . Am. Chem. SOC.1972, 94, 1782-3. (6) The only comparable species known to us is the hydrogen atom which is more easily studied in matrices9 than in solution. (7) Wong, S.K.; Wan, J. K. S. J . Chem. Phys. 1971,55, 494Cb7. (8)Initial polarization is mainly generated by the reaction of polarized excited triplets and its magnitude is therefore independent of the radical concentration. The first excited state of dialkyl peroxides (and H202) corresponds to a dissociative singlet state, there being no evidence for the involvement of predissociative excited state^.^ Initial polarization of 1 is therefore highly improbable in our system and was further ruled out by direct time-resolved experiments. An attempt to induce initial polarization by using photoexcited duroquinone as the H-atom abstracting reagent was unsuccessful.
0022-3654/83/2087-l674$0~ ,5010
Polarization Data for Some Dialkoxyphosphonyl Radicalsa TABLE I :
(Me0)2P=0
0.55 1.0
15.3 x 15.8 X
0.879 0.873
5.74 x l o 5 5.54 X l o 5
(EtO),P=O
0.30 0.55 1.0
13.9 X
14.8 X 16.5 X
0.690 0.724 0.790
4.95 X l o 5 4.93 X l o 5 4.79 X l o 5
(n-BuO)-P=O 0.55 1.0
17.6 X 17.6 X
0.403 0.397
2.29 X l o 5 2.26 X l o 5
(i-PrO)*P=O
2 4 . 7 X 10.' 24.3 X
0.411 0.441
1.67 X l o 5 1.82 X l o 5
0.55 1.0
a T = 2 9 5 K. Solution is (RO),P(O)H:Me,COOCMe,, 1 : 4 viv. Full,light intensity. ([HIt [ L ] ) is assumed to equal [ ( R O ) ? P = O ] .
trations were varied by attenuating the intensity of the UV light by inserting wire mesh screens between the lamp and the cavity. As would be expected,1°-13a plot of VEA against ([HI + [L]),which serves as a measure of the concentration of 1, gives a straight line with zero intercept (see Figure 2). This serves to confirm that E-A polarization is being observed. This polarization arises from the diffusive encounters of radicals to form radical pairs in which a nonBoltzmann distribution of spin states is induced by mixing of the S and To states.14 Some values found for VEA and P/[ (RO)$=O] at full light intensity for 1 and for three other dialkoxyphosphonyl radicals are given in Table I. The radical concentrations were determined by summing the double integrals of the H and L peaks and comparing this quantity with the double integral for a known concentration of diphenylpicrylhydrazyl (DPPH). Since phosphonyl radicals appear to decay at, or close to, the diffusion:controlled limit15we attribute the decrease in VEA/ [ (RO),P=O] along the series R = Me > E t > n-Bu > LPr, to decreasing rates of diffusion, i.e., to a decrease in the radical encounter rate.1°-13 Further evidence that the polarization was dependent on the diffusive encounter of radicals was obtained by measuring the temperature dependence of VEAat a con(9) Calvert, J. G.; Pith, J. N., Jr. "Photochemistry"; Wiley: New York, 1966, pp 172-4,443-9. Dorer, F. H.; Johnson, S.N. J. Phys. Chem. 1971, 75,3651-5. Volman, D. H. Adu. Photochem. 1963,1,43-82. Evleth, E. M. J . Am. Chem. SOC.1976, 98, 1637-9. (10) Fessenden, R. W. J . Chem. Phys. 1973, 58, 2489-2500. (11) Ayscough, P. B.; English, T. H.; Lambert, G.; Elliot, A. J. Chem. Phys. Lett. 1975, 34, 557-60. (12) Verma, N. C.; Fessenden, R. W. J. Chem. Phys. 1976,65,2139-55. (13) Ayscough, P. B.; Lambert, G.; Elliot, A. J. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 177&81. (14) Adrian, F. J. J . Chem. Phys. 1971,54,3918-23. 1972,57,5107-13. (15) Griller, D.; Roberts, B. P.; Davies, A. G.; Ingold, K. U. J . Am. Chem. SOC.1974, 96, 554-6.
Published 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983 1675
Letters
T A B L E 11: Effect of Ti3+a n d CuZ+on t h e Apparent Concentrations of t h e H a n d L Lines f r o m ( E t O ) , P = O a 105 x [Ti3+],
107 x [HI,
0 0.05 0.5 1.4 2.0 2.5 3.3
6.4 5.7 5.5 5.5 5.1 3.6 3.2
M
200G 4
696G
I
Flgure 1. EPR spectrum of (EtO),+O zation.
I
-
showing strong E-A polari-
0.7 0.6 0.5
a
0.4
> 0.3 0.2 0.I 0 IO7 (CHI + C L I ) / M
Flgure 2. Plot of VEAvs. ([HI -t [L]) for (EtO),*O.
stant concentration of 1. In the temperature range 216-308 K with [l] (= [HI + [L]) = 4.0 X lo-' M values of VEAranged from 0.043 to 0.27. A plot of VEAvs. 1/T was linear and yielded an apparent activation energy of 2.7 kcal/mol. An activation energy of this magnitude is comparable to the activation energy for diffusion in this solvent mixture.16 The magnitude of PA for 1 and the other phosphonyl radicals was independent of the microwave power (0.1-20
a
M
1 0 7 ~
[LI,
M
2.5 2.3 2.0 1.5 1.1 0.64 0.36
105 x [Cuz+l,
10' x [HI,
10'X [LI,
0 0.7 2.0 3.3 10 17 30 41 80 170
6.4 5.6 4.8 3.7 2.4 2.0 1.4 1.1
2.5 2.2 2.0 1.4 1.2 1.0 0.74 0.67 0.57 0.46
M
M
0.90 0.64
M
T = 2 9 5 K. F l o w r a t e = 1.0 m L / m i n . Solution is
(EtO),P(O)H:Me,COOCMe,, 1:4 v/v. Light intensity was lower t h a n t h a t used to obtain t h e d a t a reported in Table I.
mW). This shows that the origin of the intensity difference between the H and L lines is not the same as that which causes H-atoms trapped in acidic glasses to have more intense high-field than low-field lines.' For the H-atoms this phenomenon is due to differential saturation of the two lines and cross relaxation effects cause the intensity difference to be enhanced by added Ti3+ ions (the resonance for which overlaps the high-field line) and reduced or even reversed by added Cu2+ions (the resonance for which overlaps the low-field line).7 In contrast, in the present system the major effect of added Ti3+ (as TiC13) and Cu2+ (as CuCl,) a t constant light intensity was to reduce both the H and L lines (see Table 11). This reduction in the 1 concentration is presumably due to light absorbtion by the added metal ions. However, in the presence of either ion, measured values of VEAat any particular ([HI + [L]) concentration were greater than the value of VEA determined in the absence of metal ion. This we attribute to strong light absorption by the metal ions which causes a nonhomogeneous distribution of radicals through the sample. That is, while the total concentration of 1 in the ESR tube is drastically decreased the actual concentration in the immediate vicinity of the side of the tube facing the light will not be appreciably altered. This localized, relatively high concentration of radicals will be normally polarized thus causing an apparently enhanced polarization relative to the measured total radical concentration. Registry No. 1, 31682-65-2;. (Me0)2P=0, 31682-64-1; (nBUO)~P=O,31682-66-3; (i-PrO)2P=0, 35716-26-8; (EtO),P(O)H, 598-02-7; Ti3+,22541-75-9; Cu2+, 15158-11-9. (16) This solvent mixture and n-decane have almost identical viscosities at 298 K.4 The diffusion-controlled bimolecular self-reaction of tert-butyl radicals has an apparent activation energy of 2.57 k 0.13 kcal/mol in n-decane.I7 (17) Schuh, H.; Fischer, H. Int. J. Chem. Kinet. 1976, 8, 341-56.
