91
Communications to t h e Editor
Spin Trapping of Hydrogen Atoms in y-Irradiated Liquid Alkanes Publication costs assisted by Wayne State University
Figure 3. Esr spectrum for the system tert-butylcyclooctatetraene-HMPA-K at r o o m temperature.
small quartet from the methylene protons of 0.26 G. These results are in complete agreement with those of Carrington.2 For this system, the nodal ring proton splitting of 4.33 G and the other ring proton splitting of 1.99 G yield values for Ci2 and Cz2 of 0.31 and 0.69, respectively. From eq 1 the splitting of the nonbonding orbitals is found to be 0.45 kcal/mol. This is in good agreement with the value predicted from the tetrasubstituted system. The reduction of tert-butoxycyclooctatetraene7 in the same manner also yields a well-resolved esr pattern, Figure 3. This spectrum consists of a quartet of 4.89 f: 0.02 G due to three equivalent protons and a pentet of 1.30 f 0.02 G due to four equivalent protons. For this system C12 = 0.21 and Cz2 = 0.79 and 6 = 0.78 kcal/mol. Alkoxy groups are known to be more electron releasing in character than alkyl groups. These results indicate that the splitting of the nonbonding orbitals due to the alkoxy group is about double that for the alkyl group.
Acknowledgment. We are grateful to Research Corporation for support of this work. References and Notes (1) J. R. Bolton and A. Carrington, Moi. Phys., 4, 497 (1961). (2) A. Carrington and P. F. Todd, Mol. Phys., 8, 299 (1964). (3) R . G. Lawler, J . R. Bolton, G. K. Fraenkel, and T. H. Brown, J. Amer. Chem. SOC.,88, 520 (1964). (4) (a) P. deMayo and R. W. Yip, Proc. Chem. SOC.,84 (1964);(b) F. A. Cotton, J. W. Feller, and A. Musco, J. Amer. Chem. Soo., 80, 1438 (1968). (5) G. R. Stevenson and J. G. Concepcion, J. Phys. Chem., 76, 2176 (1972). (6) A. C. Cope and H. 0. vanorden, J. Amer. Chem. SOC., 74, 175 (1952). j7) A. Krebs, Angew. Chem., 77,966 (1965). University of Puerto Rico Department of Chemistry Rio Piedras, Puerto Rico 0093 1
Gerald R. Stevenson* Jesus G. Concepcion
Received September 70, 7973
Sir: The only direct observation of hydrogen atoms in y irradiated alkanes has been in liquidi and solid2 methane by electron paramagnetic resonance (epr). In other alkanes no H atoms are detected by epr either in the liquid during continuous irradiation1 or in the solid at 4 K after i r r a d i a t i ~ n Trapped .~ H atoms have also not been detected in other organic systems, such as ethanol and methyltetrahydrofuran, after radiolysis at 4 K.4 These results imply that thermal H atoms may not be formed in irradiated organic systems, In contrast, thermal H atoms have been indirectly implicated as intermediates in the radiolysis of higher liquid alkanes, such as n-pentane, by scavenging ~ t u d i e s .In~ this work we report the indirect observation of thermal H atoms in several organic liquids, including alkanes, by a spin trapping technique.6 Spin trapping involves the addition of a short-lived reactive free radical to a nitroso or nitrone function to form a stable spin adduct nitroxide that can be observed by epr to identify the original radical. We use phenyl-tert-butyl nitrone (PBN) as the spin trap for which the epr spectra of various spin adducts have been discussed.6 The spin adduct of PBN with H atoms has a characteristic large /3proton splitting of -7 G which distinguishes it from the spin adducts with other radicals.? Typically, a 0.1 M solution of purified PBN, obtained from Dr. LeBel of this department, is made with the organic liquid of interest. Samples are degassed under vacuum and sealed in 2 mm i.d. quartz tubes. The samples are irradiated with 6OCo y rays at a dose rate of 0.2 Mrad hr-I at room temperature to a typical dose of 0.01 Mrad. Epr spectra are obtained at room temperature with a Varian E-4 spectrometer. The H atom spin adduct with PBN has been observed in n-hexane and 3-methylpentane in the neat liquid. A typical epr spectrum from n-hexane is shown in Figure l a . The prominent lines are a triplet of doublets with AN 14 G and AoH 2 G which is characteristic of alkyl radical spin adducts with PBN. These will not be discussed further here. In addition to the six intense lines, there are six weaker lines, which are shown more clearly in Figure l b , which comprise six of the nine lines expected for the H atom spin adduct. The other three H atom spin adduct lines are hidden under the strong lines. These weak lines can be analyzed to give A N = 14.8 f 0.1 G and AsH = 7.0 0.1 G which are unambigously characteristic of the H atom spin adduct of PBN.? The same splittings are obtained for the H atom spin adduct observed in 3-methylpentane. Other C, H, 0, N containing radicals give spin adducts with PBN with ABH = 1.5-4 G. The observed H atom spin adducts could be formed in at least the two ways given by reactions 1 and 2. Reaction
-
-
*
- -
H e,-
+
PBN
+
PBN
PBN
PBNH
hexane
PBNH
(1)
+
C,H,,
(2)
1 seems most probable to us. In alkane glasses we have found that PBN does not act as an efficient electron scavenger. It also seems doubtful that the proton affinity of C6H13- is greater than the proton affinity of PBN-. Finally, the radical anion of PBN would probably protonate on the oxygen and it is not clear that this adduct would The Journal of Physical Chemistry, Vol. 78, No. 7, 1974
Communicationsto the Editor
92
5 6 c _
Figure 1. (a) Epr spectrum of 0.1 M phenyl-tert-butylnitrone in n-hexane at room temperature after 0.008 Mrad 6oCoy-irradiation. (b) Weak lines of spectrum in a at tenfold higher sensitivi-
ty.
rapidly isomerize to form the observed H atom spin adduct. It should be mentioned that addition of an efficient electron scavenger such as N20 does not unambiguously distinguish between (1) and (2) because H atoms can be formed by combination of the parent molecular cations with electrons. Recerft work suggests that PBN excited by uv irradiation in alcohols may abstract hydrogen to form the H atom spin adduct.8 Direct excitation of 0.1 M PBN in alkanes by y-irradiation is negligible because excitation is proportional to electron fraction, and there is little evidence for indirect excitation for low concentration of solutes in y-irradiated alkanes. So H abstraction by PBN* is not expected to contribute to the observed spin adduct in this work. Although the intensity of the H atom spin adducts is low compared to that of the other radical spin adducts, this does not necessarily imply that the H atom is low compared to other radical yields. The intensity of a certain spin adduct depends on the spin trapping efficiency, the stability of the spin adduct, and other factors. However, we can conclude that the trapped H atoms are thermal since the PBN concentration is only 0.1 M . Preliminary attempts have also been made to trap H atoms in *(-irradiated solid alkanes a t 77 K, but the re-
The Journal of Physical Chemistry, Vol. 78, No. 1, 1974
sults are inconclusive a t present. In liquid alkanes the H atom spin adduct intensity is below our detection sensitivity at PBN concentrations below -0.05 M . PBN concentrations of 0.1 M in 3-methylpentane crystallize out when the solution is rapidly frozen to 77 K. The most concentrated PBN solution that appears to freeze to give a solid solution in either 3-methylpentane or n-hexane is about 4 x M . When this sample is y-irradiated at 77 K and then warmed to room temperature, an epr spectrum is obtained which is identical with that from a similar sample irradiated at room temperature. Of course the PBN concentration is too low to observe the H atom spin adduct; however, it seems probable that H atoms may be spin trapped in the solid also. Finally, we have also observed H atom spin adducts with PBN in y-irradiated liquid methanol, and propionitrile, and in benzene solutions of succinonitrile but not in liquid benzene itself. The formation of the spin adduct in these various systems may well involve more than one mechanism. Further work is in progress.
Acknowledgment. This work was supported by the Air Force Office of, Scientific Research in its early stages and by the Atomic Energy Commission in its later stages. We thank Dr. N. LeBel for providing the nitrone and Dr. E. Janzen for helpful discussions. References and Notes (1) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 39, 2147 (1963). (2) W. Gordyand R. Morehouse, Phys. Rev., 151, 207 (1966). (3) D.Timm and J. E. Willard, J. Phys. Chem., 73, 2403 (1969). (4) D. R. Smith and J J. Pieroni, Can. J. Chem., 45, 2723 (1967). (5) R. A. Holroyd, J. Phys. Chem., 70, 1341 (1966). (6) E. G. Janzen,Accounts Chem. Res., 4, 31 (1971). (7) E. G . Janzen and B. J. Blackburn, J. Amer. Chem. SOC., 91, 4481 (1969). (8) E. G. Janzen and D. E. Nutter, private communication.
Department of Chemistry Wayne State University Detroit, Michigan 48202 Received August 10, 1973
S. W. Mao Larry Kevan*
