1584
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
G. Levin
the reaction of OH radicals with ZC, alkanes, apart from cyclobutane (and by analogy, cyclopropane) for which no data presently exist.
(15) (16) (17) (18)
Acknowledgment. The authors gratefully acknowledge the assistance of F. R. Burleson and G. C. Vogelaar for carrying out the gas chromatographic analyses, and W. D. Long for valuable assistance in conducting the chamber experiments. This work was supported by the California Air Resources Board (ARB Contract No. A6-172-30) and by t h e National Science Foundation (Grant No. CHE76-10447).
(19) (20) (21) (22) (23) (24) (25)
References and Notes
(26)
(1) K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Environ. Sci. Techno/., 10, 692 (1976). (2) J. N. Pitts, Jr., A. C. Lloyd, A. M. Winer, K. R. Darnall, and G. J. Doyle, 69th Annual Air Pollution Control Association Meeting, Portland, Oreg., June 27-July 1, 1976, Paper No. 76-31.1. (3) J. N. Pitts, Jr., A. M. Winer, K. R. Darnall, A. C. Lloyd, and G. J. Doyle, "International Conference on Photochemical Oxidation Pollution and Its Control, Proceedings", Vol. 11, B. Dimitrlades, Ed., EPA-600/ 3-77-001b, p 687, Jan 1977. (4) W. E. Wilson and A. A. Westenberg, Symp. Int. Combust. [ f r o c . ] , I l t h , 1966, 1143 (1967). (5) N. R. Greiner, J. Chem. fhys., 46, 3389 (1967). (6) N. R. Greiner, J. Chem. fhys., 53, 1070 (1970). (7) E. D. Morris, Jr., and H. Niki, J . fhys. Chem., 75, 3640 (1971). (8) J. N. Bradley, W. Hack, K. Hoyermann, and H. Gg. Wagner, J. Chem. Soc., Faraday Trans. 1 , 69, 1889 (1973). (9) F. Stuhl, Z . Naturforsch. A , 28, 1383 (1973). (10) D. D. Davis, S. Fischer, and R. Schiff, J. Chem. fhys., 61, 2213 (1974). (1 1) J. J. Margitan, F. Kaufman, and J. G. Anderson, Geophys. Res. Lett ., 1, 80 (1974). (12) S. Gordon and W. A. Mulac, Int. J . Chem. Kinet., Symp. 1, 289 (1975). (13) A. B. Harker and C. S. Burton, Int. J . Chem. Kinet., 7, 907 (1975). (14) R. P. Overend, G. Paraskevopoulos, and R. J. Cvetanovic, Can. J. Chem., 53, 3374 (1975).
(27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41)
C. J. Howard and K. M. Evenson, J . Chem. fhys., 64, 197 (1976). R. Zellner and W. Steinert, Int. J. Chem. Kinet., 6, 397 (1976). C. J. Howard and K. M. Evenson, J . Chem. fhys., 64, 4303 (1976). R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J . Chem. fhys., 64, 5314 (1976). R. A. Gorse and D. H. Volman, J . fhotochem., 1, 1 (1972). R. A. Gorse and D. H. Volman, J . fhotochem., 3, 115 (1974). D. H. Volman, Int. J. Chem. Kinet., Symp. 1, 358 (1975). I. M. Campbell, B. J. Handy, and R. M. Kirby, J. Chem. SOC.,Faraday Trans. 1 . 71. 867 11975). A. C. Lloyd, K. R. Darnall,'A. M. Winer, and J. N. Pitts, Jr., J . fhys. Chem., 80, 789 (1976). R. Atkinson, K. R. Darnall, A. M. Wlner, and J. N. Pitts, Jr.. Final ReDort to E. I. duPont de Nemours and Co., Inc., Feb 1, 1976. I. M. Campbell, D. F. McLaughlin, and B. J. Handy, Chem. phys. Lett., 36, 362 (1976). K. R. Darnall, A. M. Wlner, A. C. Lloyd, and J. N. Pitts, Jr., Chem. Phys. Lett., 44, 415 (1976). C. H. Wu, S. M. Japar, and H. Niki, J . Environ. Sci. Health, A l l , 191 (1976). R. Butler, I. J. Solomon, and A. Snelson, Chem. fhys. Lett., 54, 19 (1978). R. Atkinson, K. R. Darnall, A. C. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Adv. fhotochem., in press. G. J. Doyle, A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., Environ. Sci. Techno/.,9, 237 (1975). A. C. Baldwin, J. R. Barker, D. M. Golden, and D. 0. Hendry, J. fhys. Chem., 81, 2483 (1977). W. P. L. Carter, A. C. Lloyd, J. L. Sprung, and J. N. Pitts, Jr., Int. J. Chem. Kinet., in press. C. C. Schubert and R. N. Pease, J . Chem. fhys., 24, 919 (1956). J. T. Herron and R. E. Huie, J. Fhys. Chem. Ref. Data, 2, 467 (1973). F. Kaufman and J. R. Kelso. J . Chem. fhvs.. 46. 4541 (1967). A. M. Winer, A. C. Lloyd, K. d. Darnall, and j .N: Pitts, Jr., J : fhys. Chem., 80, 1635 (1976). A. C. Lloyd, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., Chem. fhys. Lett., 42, 205 (1976). A. M. Winer, A. C. Lloyd, K. R. Darnall, R. Atkinson, and J. N. Pitts, Jr., Chem. fhys. Lett., 51, 221 (1977). L. G. Parratt, "Probability and Experimental Errors in Science", Wiley, New York, N.Y., 1961. R. A. Perry, R. Atkinson, and J. N. Pitts, Jr., J . Chem. fhys., 67, 5577 (1977), and references therein. R. Atkinson and J. N. Pitts, Jr., J . Chem. fhys., 63, 3591 (1975).
Photooxidation of Aromatic Hydrocarbons by Europium(111) Salts G. Levin Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210 (Received January 12, 1978) Publication costs assisted by the State University of New York
Aromatic hydrocarbons such as tetracene (T), perylene (P),coronene (C), and naphthalene (N) are photooxidized in acetonitrile by Eu(II1) perchlorate or nitrate to their respective radical cations. The reversible reaction, , place in the dark period. The rate constants being 2.8 X lo6, 1.6 AH.+ + Eu(I1) AH Eu(III), M 0takes x lo7,1.3 x lo9, and 3.9 x IO9M-ls-l for perylene, tetracene, coronene, and naphthalene, respectively. These satisfactorily correlate with the reduction potential llEo. In addition, the rate constant of oxidation by Eu3+ of the excited singlets of tetracene and coronene were determined. The latter are diffusion controlled being 4.5 x 1Olo and 4.0 x 1O1O M-l s-l for tetracene and coronene, respectively. The triplets of tetracene and coronene are not oxidized by Eu3+ implying that the rate constant of these reactions are smaller than lo6 M-' s-'.
-
+
Introduction Photochemical reactions, and especially photooxidations and photoreductions, are attracting much attention as possible routes for harnessing solar energy. Since photooxidation or photoreduction resulting from electron transfer induced by light proceed in homogeneous solutions simultaneously with the reverse thermal electron transfer, the resulting products are rapidly destroyed. It is our 0022-3654/78/2082-1584$01.00/0
intention to control and, if possible, to slow down the undesirable thermal processes. With this in mind, we investigated some photooxidations and photoreductions and studied the kinetics of the respective reverse thermal electron-transfer processes using aromatic hydrocarbons as the reducing agents and europium salts as the oxidants. The results of our studies are reported in this communication. 0 1978 American
Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1585
Photooxidation of Aromatic Hydrocarbons
m 0.3
. AOD
I O0.1
'
I 3M
C
.
550
Figure 2. Spectra of the coronene radical cation
750
( t = 100 ps).
in bulb B added to the cell. Thus the desired degree of dilution was achieved.
Figure 1. Glass apparatus used in the flash photolysis.
Experimental Section Material. Naphthalene crystallized from methyl alcohol was twice sublimed under high vacuum. High vacuum sublimation also was used in purification of the other hydrocarbons, perylene, tetracene, and coronene (Aldrich Co.). Acetonitrile (MVB) was stirred overnight with calcium hydride and then fractionated on a high vacuum line. Europium(II1) perchlorate, acquired commercially (K&K), was used without further purification. Apparatus. The conventional flash-photolysis apparatus utilized two parallel quartz lamps of 2.5 cm diameter and 25 cm length connected to 40-pF condenser charged to 6400 V. The lamps enclosed an area of 220 cm2and in this setup the half-lifetime of a flash was shorter than 25 ps. A 10-cm long cylindrical quartz cell was attached to the previously reported device5 (Figure l), modified and adapted for use in any chosen atmosphere. The cell, filled with the investigated solution, was placed between the lamps. The flash light was filtered through two flat cuvets 1 cm thick filled with solutions of CuS04 in water (which cut the light below 370 nm and above 440 nm) or biphenyl in acetonitrile (which cut the light below 300 nm). The transients were monitored by an interrogating beam which passed through a monochromator onto a photomultiplier and the output of the latter was displayed on an oscilloscope. Absorption spectra were recorded with a Beckman Acta VI spectrophotometer. Procedure. The device shown in Figure 1was attached to a vacuum line via opening A. After evacuation the teflon fitted valve C was closed and the apparatus detached from the line. Dry nitrogen was then slowly introduced through opening A and thereafter the aromatic hydrocarbon and Eu(II1) salt were introduced through opening D. The apparatus was attached again to the vacuum line and pumped out for several hours. Purified acetonitrile (50 cm3) was then distilled through the vacuum line into bulb B and frozen with liquid nitrogen. After reevacuation of the device valve C was closed, the unit detached from the line, and its contents mixed and collected in bulb B. The solvent from bulb B was distilled to the 10-cm long cell and a drop or two of the concentrated solution remaining
Results and Discussion Flash photolysis of the investigated aromatic hydrocarbons in acetonitrile yields the respective triplets. These decay by simultaneous first- and second-order processes?' the reaction being over in about 100-300 ps. On addition of europium(II1) perchlorate or nitrate, or ferric perchlorate: oxidation of the excited hydrocarbons takes place yielding radical cations and the salt of the reduced transition metal. The presence of the respective radical cations and the triplets was established spectrophotometrically by examining the optical spectra of the irradiated solution in the dark period following the flash. The absorbance of the long-lived radical cations was easily recorded at later stages of the reaction after decay of the short-lived triplets. The correction for the absorbance of Eu(II1) and Eu(I1) was minor because these salts only weakly absorb in the visible rangee9 Hence, the spectra of radical cations were obtained and these well agree with those reported in the literature.lOill For the sake of illustration, we show in Figure 2 the spectrum of the coronene radical cations resulting from the photooxidation of the parent hydrocarbon by Eu(II1). In the previous study,8ferric perchlorate was employed as the oxidant. However, in darkness this salt was found to slowly oxidize the investigated hydrocarbons. For example, even in the absence of light, naphthalene in acetonitrile is oxidized by ferric perchlorate to binaphthyl, trinaphthyl, and higher oligomers. The identity of these products was established by mass spectroscopy. One could expect the triplets of the irradiated hydrocarbons (3AH)to be the precursor of the radical cations. This is not the case. For example, inspection of Figure 3 shows that the concentration of tetracene radical cations formed immediately after a flash (Arnm 740 nm) did not increase during 400 p s whereas the concentration of triplets (A, 460 nm) decreased almost to 0. The concentration of Eu(II1) was as high as M, sufficient to ensure the oxidation of triplets had this reaction been possible. Similar results were obtained with coronene. Its triplet has extinction coefficient of 900 a t 700 nm whereas the molar absorbance of its radical cations at the same wavelength is 9000. However, although the cations were formed immediately after flashing a solution of coronene containing Eu(III), their concentration did not increase thereafter, while the absorbance at 460 nm (A, of the triplet, E 1.5 X lo4) decayed. We conclude therefore that
1588
G. Levin
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
Scheme I. Energy Diagram in kcal/mol of Tetracene Europium Salts and Their Excited State
SOD
T3n*46C.,
9
TET7dCEliE'
i Time
I
ms
Figure 3. Change in the concentrations of the tetracene triplet (T3) and the tetracene radical cation (T'.).
TABLE I: Free Energy of Oxidation of Excited Aromatic Hydrocarbons by Eu3+ aromatic hydrocarbons
A G ,kcal/mol
tetracene singlet* tetracene triplet* perylene singlet* perylene triplet* coronene singlet* coronene triplet* naphthalene singlet* naphthalene triplet*
-46.31 -15.15 -49.61 - 19.88 -44.28 - 29.70 -60.00 - 28.84
the excited singlets ('AH)* are the precursor of the radical cations. The following reactions account for our observations: ['AH] ['AH] *
% !
h". [ 'AH] * kF
['AH]*+ ['AH] *
+ Eu'+
['AH]*-
['AH] *
['AH]
kOXS
AH+-+ ELI*+
kisq
Eu
'+['AH]*
(11 (2) (3)
(4) (5)
The intersystem crossing, reaction 2, with a rate constant (kiJ of 108-109 s-l competes with fluorescence and radiationless transition (reaction 3). Reaction 5 represents collisional quenching of the singlet excited molecule by Eu3+ which results in (lAH)* (3AH)*. The oxidation of the singlet (reaction 4) results in formation of the radical cations from the parent hydrocarbons. The standard free energies of these oxidation reactions can be calculated from e l e c t r o ~ h e m i c a land ~ ~ ~photochemical ~~ data,14 i.e.
-
AH + Eu'+ ;r AH+. + Eu'+ A G l 'AH* AH AG, " A H , 'AH*
AH
AG, = A H ,
The contribution of enthalpy of excitation (-30 kcal/mol) to the free energy is much greater than that of the entropy
+
+
Ku*
I
change; therefore, the latter was neglected. The results listed in Table I show that the oxidation of singlets and triplets of the above hydrocarbons are both thermodynamically favored; however, no oxidation of tetracene triplet by Eu3+ was observed even though the free energy of this reaction is -15.15 kcal/mol. Apparently the electron transfer from the singlet hydrocarbon to Eu(II1) does not lead to Eu(I1) ground state but it forms instead the Eu(I1) excited state. The formation of Eu(I1) excited state resulting from electrochemical reduction of Eu(II1) was suggested by Vlcek15 who also estimated the reduction potential of Eu(III)/Eu(II),,,~~~ against SCE to be -0.85 to -0.90 V. Therefore such an electron transfer is endothermic and the estimated AG -6 kcal/mol. The diagram shown in Scheme I illustrates the free energy change of the intermediates involved in the tetracene oxidation. Oxidation of the coronene triplet by Eu3' to coronene radical cation does not take place even though the calculated free energy of this reaction, assuming excited Eu2+as the product, is approximately -9 kcal/mol. Since oxidation of triplets of tetracene is not observed even at concentration of Eu3+ as high as M, the maximum value of the rate constant of its hypothetical oxidation cannot be greater than lo6 M-l s-l. The rate constant of oxidation of N,N,N',N'-tetramethylbenzidine (TMB) triplets by E u ( N O ~was ) ~ reported by Alkaitis et ala3to be 6.4 X lo9 M-l d,Le., substantially higher than the maximum rate constant estimated for the hypothetical oxidation of triplets tetracene (lo6 M-' s-l ). The large difference in the energies of the respective triplets 1.27 eV for tetracene and 2.7 eV for N,N,N',N'tetramethylbenzidine justifies these findings. However, the energy of coronene triplet state lies below the energy of the triplet TMB by only 0.4 eV. It is possible that the formation of TMB+. is favorable due to the fact that the positive charge is partially localized on nitrogen allowing for better stabilization by solvation. On the other hand, the positive charge on the coronene radical cation is more delocalized, and hence the transition state is relatively less stabilized by solvation. The rate constants of oxidation of the singlet tetracene (kom)and of the Eu(II1) induced intersystem crossing (kh, reaction 5) may be calculated since reactions 2, 4, and 5 are competing for the excited singlet. The pertinent differential equation leads to
Plots of the ratio of concentrations of tetracene triplets to the concentration of tetracene radical cations (both calculated at 25 p s after a flash) against the reciprocal of Eu3+ concentration are shown in Figure 4. It gives a straight line with a slope of kisc/koxsand an intercept equal
The Journal of Physical Chemistv, Vol. 82, No. 14, 1978 1507
Photooxidation of Aromatic Hydrocarbons
TABLE 11: Electron Transfer from EuZ+t o Tetracene*. [tetracene] x 105
[Eu(CIO,), .3H,O J x 104
[ Eu*+,I x 10 (calcd)
5.05 3.75 2.71 2.43 1.43 0.57 0.27 0.52 0.82 2.0
9.67 7.17 5.19 4.65 2.75 1.1 0.51 48 75 180
5.7 4.2 3.0 2.7 1.6 0.6 0.3 171 187 249
k [Tt*12, s x 1& 9.3 6.8 4.4 4.2 1.6 0.61 0.19
/'
M-' x
kr s-'
10-7
106.0 88.1 55.7 47.7 20.3 9.9 5.4 3600 5400 9700
1.9 2.1 1.9 1.8 1.3 (?) 1.7
1.8 2.1 2.9 3.9
-6
-5
d 6
a
C
-4
-3
os
10
IS
-2
10
20 TIME, M SEC
30
Figure 4. Effect of Eu3+ concentration on the ratio of tetracene triplet to tetracene radical cation.
Figure 5. Electron transfer from Eu2+ to tetracene radical cation.
to kisp/hoxs. (See insert in Figure 4.) The intercept is distinguished from 0 and found to be 0.5. The rate constant of singlet triplet intersystem crossing for tetracene in benzene is given in ref 14, hi, = 1.18 X lo8 s-'. Assuming that this rate constant is independent of solvent, we calculated the following data: h , = 4.5 X 1O'O M-l s-l and hisq = 2.3 X 10" M-' S- The rate constants for the oxidation of the coronene singlet was also found to be similar to the rate constant for oxidation of singlet tetracene, i.e., h,,, = 4.0 X 1O1O M-' s-'. On the other hand, the spectra of naphthalene triplet and naphthalene radical cation are considerably overlapped and therefore cannot be studied. The oxidation of excited singlet perylene was attempted; however the quantum yield of triplet formation is 4TM= 0.01 as compared to tetracene triplet formation dTM= 0.63. Therefore, the amount of perylene triplet formed is extremely small and becomes even smaller on addition of Eu(II1) salt. It is the accuracy of measuring the perylene triplet that prevents us from calculating the oxidation of the excited singlet. Electron Transfer f r o m Eu2+Salts t o Radical Cations of Tetracene, Perylene, Coronene, and Naphthalene. The results reported in the preceding section have shown that the excited singlets of the investigated hydrocarbons are
oxidized by Eu3+to the corresponding radical cations, viz. lHY*+ Eu3++ Hy+. + E u Z t(excited) (4a) In the dark period following a flash electron transfer from the reduced Eu3+ to a radical cation regenerates the hydrocarbon and Eu3+, i.e. Hy++ Eu'+-+H, + Eu3* (6) Regeneration of the hydrocarbon is quantitative, even after 50 flashes; its concentration is indistinguishable from its initial value. The progress of reaction 6 was monitored a t the absorption maximum of the parent hydrocarbon or its radical cations. The ratios of the pertinent absorptions remained constant during the investigated process implying that each H,+* was converted into the corresponding H . No observable intermediates are formed because on& the transients that participate in reaction 6 were observed. The disappearance of tetracene radical cation obeyed first-order kinetics (Figure 5). The pseudo-first-order rate constant was proportional to the concentration of Eu3+ (Table 11). Apparently, the solution of Eu3+contained some Eu2+. This indeed was the case. Addition of Fe(C104), in acetonitrile to E U ( C ~ Oresulted ~)~ in the oxidation of Eu2+by Fe3+, i.e.
'.
1588
P. Maruthamuthu and J. C. Scaiano
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
TABLE 111: Electron Transfer from Euz+to Pe+.
[PeI6X 10 5.30 8.30
[Eu3+]x [Eu''] 104 107 1.42 8.3 2.22 1.3
remain the same. Therefore one would expect to correlate the rate constant (6) and the reduction potential AE" (Br6nsted's equation). The results of such a reaction are given in Table IV. The data listed in Table IV fit a reasonable linear relation between log k and AE". In conclusion we have shown that the rate of the back-reaction 6 is related to its reduction potential. Storage of chemical energy for a limited time might be achieved by choosing an appropriate combination of hydrocarbon and transition metal salts. We also have shown that in the case of coronene and tetracene only the singlet and not the triplet states are oxidized. This reduces the efficiency of light conversion into chemical energy.
k , s-'
x
M-'X k , s-' 2.25 3.8
2.7 2.9
rate constant k , M-'s"
log k,
AE",V
2.8 X l o 6 1.6 X l o 7 1.3 x 1 0 9 3.9 X lo9
6.44 7.20 9.11 9.59
0.70 0.62 1.08 1.38
TABLE IV radical cation perylene
tetracene coronene
naphthalene
EuZ+t Fe3+-t Eu3+t Fez+
Acknowledgment. The author thanks Professor M. Szwarc for helpful discussions and Mr. Wade Potter for his technical assistant. This study was supported by the National Science Foundation.
(7 1
The formation of Fez+ was confirmed by the analysis of the products using 1,lO-phenanthroline as complexing agent. Also on mixing the ferric and Eu3+salts the optical density of the former salt decreases at its maximum absorption (X 350 nm), presumably due to the formation of the corresponding ferrous salt. The above analysis shows that the Eu3+was contaminated by approximately 0.6% of EuZf. This small concentration of Eu2+was sufficient to impart a pseudo-first-order kinetics. Deliberate addition of E u ( C ~ Oto~ )the ~ flashed solution (Table 11) led to the expected increase in the pseudo-first-order rate constant. The second-order rate constant (krr) appears to increase with increasing concentration of Eu(C10J3. The increase in (k1J might be due to the increase in the ionic strength of the solution (primary salt effect). Pseudo-first-order kinetics was also observed in the case of electron transfer from Eu2+to perylene radical cation. Table I11 summarizes the results. The rate constant of reaction 6 for different radical cations is extended in our case to more than three orders of magnitude, being 2.8 X lo6 M-ls-l for perylene and 3.9 X lo9 M-' s-l for naphthalene. These results require some comments. The reduction of the radical cations can be considered as a similar reaction where probably the entropy factors
References and Notes (1) International Conference on the Photochemical Conversion and Storage of Solar Energy. Aug 24-28, 1976, University of Western Ontario, London, Ontario, Canada. (2) S. A. Alkaitis, G. Beck, and M. Gratzel, J . Am. Chem. Soc., 97, 5723 (1975). (3) S. A. Alkaitis and M. Gratzel, J . Am. Cbem. Soc., 98, 3549 (1976). (4) (a) V. Balzani, L. Moggi, M. F. Manfrin, F. Bolietta, and M. Gleria, Science, 189, 852 (1975); (b) R. C . Young, T. M. Meyer, and D. G. Whitten, J . Am. Cbem. Soc., 97, 4781 (1975). (5) G. Ramme, M. Fisher, S. Claesson, and M. Szwarc, Proc. R . SOC. London, Ser. A , 327, 467 (1972). (6) G. Porter and M. W. Windsor, Proc. R. SOC.London, Ser. A , 245, 238 (1958). (7) 0. Levin and M. Szwarc, Cbem. Phys. Lett., 30, 1 116 (1975). (8) G. Levin, J. Cbem. Soc., Chem. Commun., 768 (1976). (9) P. K. Gallagher, J. Cbem. Pbys., 41, 10 3061 (1964). (10) "DMS uv Atlas of Organic Compounds", Butterworths, London, 1966. (11) (a) K. H. Grellmann, A. R. Watkins, and A. Weller, J . Pbys. Chem., 76, 469 (1972); (b) W. Ij. Aalbersberg, G. J. Hoijtink, E. L. Mackor, and W. P. Weijland, J . Chem. SOC.,3049 (1959). 12) (a) E. S. Fysh and N. C. Yang, J . Am. Cbem. Soc., 85, 2125 (1963); (b) I.M. Kolthoff and J. F. Coetzee, ibid., 79, 1852 (1957). 13) (a) G. J. Janz and R. P. T. Tomkins, "Nonaqueous Electrolytes Handbook", Vol. 11, Academic Press, New York, N.Y., 1973; (b) I. M. Kolthoff and J. F. Coetzee, J. Am. Cbem. Soc., 79, 1852 (1957). 14) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, N.Y., 1970. 15) A. A. Vlcek Collect. Czech. Cbem. Commun., 24, 181 (1959).
Biradical Double Trapping by Nitric Oxide. An Electron Spin Resonance Study P. Maruthamuthu and J. C. Scaiano" Radiation Laboratory,' University of Noire Dame, Notre Dame, Indiana 46556 (Received February 13, 1978) Publication costs assisted by the U.S. Department of Energy
The biradicals produced in the type I photocleavage of cycloalkanones in solution can be trapped by nitric oxide. The reaction results in the formation of relatively long-lived cyclic nitroxide radicals which contain structural information on the nature of the two biradical ends. The reaction is likely to proceed in a stepwise manner. In the case of 2-methylcyclohexanonethe radical produced is I, for which g = 2.0066, aN = 7.07 G, and aH = 1.91 G. Transient biradicals are not uncommon reaction intermediates; however, their detection and/or trapping has not been frequent. This reflects mainly the lack of suitable techniques for the study of this type of reaction intermediates. Recent developments in the field have led to a number of examples of direct d e t e ~ t i o n ~and - ~ trap~ing,~-lO and some of these techniques can be expected to find rather general application. For example, Wagner has shown that while mercaptans are inefficient triplet 0022-3654/78/2082-1588$01 .OO/O
quenchers,ll they are very efficient traps for the biradicals generated in the Norrish type I1 r e a ~ t i o n .The ~ method has also found application in the photochemistry of cycloalkanones8 and offers the possibility of carrying out deuterium labeling experiment^.^ Another example is the trapping by paraquat d i c a t i o n ~ which , ~ ~ ~ ~allows the measurement of the electron donor properties of the biradicals and their lifetime; the technique has been used for Norrish type I1 biradicals and in the photoenolization 0 1978 American
Chemical Society
