The Journal of
Physical Chemistry
Registered in U.S. Patent Office 0 Copyright, 1981, by the American Chemical Society
VOLUME 85, NUMBER 18
SEPTEMBER 3,1981
LETTERS ESR Evidence for the Cation Radicals of Tetrahydrofurans and Dimethyl Ether Produced in a y-Irradiated Frozen Matrix of Trichlorofluoromethane Hldeo Kubodera, Tadamasa Shida,’ Department of Chemistry, Faculty of Science, Kyoto Unlverslty, Kyoto, Japan
and Karuo Shimokoshi Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan (Received: February 25, 198 1; I n Final Form: June 18, 1981)
Cation radicals of tetrahydrofuran, metyltetrahydrofuran,cis- and trans-dimethyltetrahydrofuran,and dimethyl ether have been produced for the fiit time in y-irradiated solid solutions of trichlorofluoromethane. The hyperfine splitting constants of axial protons adjacent to the ethereal oxygen are found to be as large as 90-100 G for the cyclic ethers. The spectral changes observed on temperature elevation toward the melting point of the freon matrix are accounted for in terms of ring puckering, and the activation energy is estimated as 1.65 kcal/mol. The cation radicals undergo ion-molecule reactions to produce neutral radicals of the parent ether molecules.
Introduction One of the conspicuous missing links in the ESR study of simple molecules is spectral data for cation radicals of aliphatic molecules. Except for the recent reports on the cation radicals of unsaturated aliphatic hydrocarbons,’P2 he~amethylethane?~ and he~amethyldisilane~ there seems (1) Y.Takemura and T. Shida, J. Chem. Phys., 73, 4133 (1980). (2) (a) T. Shida, Y. Epawa, H. Kubodera, and T. Kato, J.Chem. Phys., 73, 5963 (1980); (b) T. Ichikawa, N. Ohta, and H. Kajioka, J. Phys. Chem., 83, 284 (1979). (3) (a) J. T. Wang and F. Williams, J . Phys. Chem., 84, 3156 (1980); (b) M.C. R. Symons, Chem. Phys. Lett., 69, 198 (1980). (4) T. Shida, H. Kubodera, and Y. Egawa, Chem. Phys. Lett., 79,179 (1981). 0022-3654/81/2085-2583$01.25/0
very little ESR evidence for the cation radicals stated above.6 However, the freon matrix method developed by the present authors is found to be capable of producing cation radicals of solute molecules whose ionization potential is lower than that of CC13F of 11.9 eV. This is because the solute molecules are ionized to their cation radicals via positive charge migration induced by ionization of the (5) Recently M.Iwasaki and his co-workers have made an extensive study of the cation radicals of simple alkanes (M.Iwasaki, K. Toriyama, and K. Nunome, J. Am. Chem. SOC.,in press). Previously, H. Kubodera and T. Shida presented a relevant paper, “ESR Studies on Cation radicals of Some Cycloalkanes”,to the 23rd Symposium of Radiation Chemistry, Kyoto, Oct, 1980.
0 1981 American Chemical Society
2504
The Journal of Physical Chemistty, Vol. 85, No. 18, 1981
Letters
10
1 -
Flgure 1. Temperature dependence of the ESR spectra of the THF cation radical.
matrix molecules upon y irradiati~n.l*~*~ Another advantage of the matrix is its relatively high solvancy as the solvent and the fact that the concomitantly produced paramagnetic by-products such as “trapped holes”, (CC13F+)$,and “electron adducts”, (CC1,F. .e-), or their decomposition product, (CC12F+ Cl-), give practically no interfering ESR signals owing to the extreme dipolar ESR spectra of a broadening of hyperfine intera~tion.~>~ number of cation radicals of saturated molecules have been measured in our laboratory. In this paper a typical result for some ethers will be presented.
Flgure 3. Temperature dependence of the ESR spectra of cation radicals of CIS- and tran~-2,2’dirnethyitetrahydrofuran.
a
Experimental Section All the tetrahydrofurans were purified by removing stabilizers and the purity was checked by gas chromatography. 2,2’-Dimethyltetrahydrofuran from Aldrich Chemical Co. was an admixture of cis and trans isomers which were separated by preparative gas chromatography. Trichlorofluoromethane from Daikin Kogyo was used as received. The solutions of various concentrations were degassed and sealed off in a Suprasil cell and were irradiated at 77 K to a dose of about 5 X 1019 eVJg. ESR spectra were recorded on a JEOL JES-PE-3X spectrometer and a Varian E-112 spectrometer at temperatures of 77 to a(He). Thus, if the cation radical is formed and if puckering takes place as in THF, all three protons will tend to have an approximately equal average coupling constant. In fact, the spectrum at 77 K can be interpreted as a double triplet with a = 83 G and 42 G which changes reversibly into another double triplet of a = 78 G and 49 G upon warming, e.g., to 146 K (see the lower spectrum). The result can be accounted for by assuming that the cation at 77 K is in conformation I and that the cation is subject to limited puckering at higher temperatures, that is, all three protons retain their axial or equatorial character but a limited wagging of the carbon atoms adjacent to the oxygen leads to a partial averaging of the coupling constants. This is an intermediate situation between THF and DMTHF systems. I t is concluded that in solutions, where all positive charges are scavenged by the ethers and yet the initial concentration of the ethers is low enough to isolate each cation radical from unreacted ether molecules, ESR spectra
-
Y
Flgure 5. Concentration dependence of the ESR spectra of Irradiated solutions of THF. (a-c) As the initial concentration is increased, the spectrum of the cation radlcal is replaced wtth a new spectrum. (d-f) The new spectrum shows a reversible change on warming. The spectrum is ascribed to the neutral THF radicals.’ The smaii peaks at the extremities of the spectrum are due to the residual cation radical of THF at the elevated temperatures (cf. lower spectra of Figure 1). The total spin concentrations for Figures 5a-c should be of the same order of magnitude if the radiation dose and the instrument settings are properly normalized. This fact implies that ail positive charges are scavenged even at the lowest solute concentration (5a) and that most of the solute molecules at the higher concentration (5c) are probably present as dlmeric aggregates within which ion-molecule reaction 1 in the text occurs. Similar situations were found for the cation radicals of 1,4-~yclohexadieneand other olefins.’ MTHF
c is DMTHF
trans DMTHF
-
Figure 8. Concentration dependence of the ESR spectra of irradlated solutions of MTHF and DMTHF in CCiBFmeasured at 77 K. As the initial concentration is increased, the spectra of the cation radicals are replaced with spectra of neutral radicals.8se
of the cation radicals are observed exclusively. The reversible spectral changes observed for THF and MTHF are associated with the ring puckering while such a motion is not apparent for the DMTHF’s.
2588
The Journal of Physical Chemistty, Vol. 85, No. 18, 1981
Letters
tration studied is 1:49.4,the energy absorbed by the segregated solute aggregates should be one to two orders of magnitude smaller than the energy absorbed by the solvent. Despite this the observed spin concentrations are almost the same for dilute and concentrated solutions if the radiation dose and the detector settings are properly normalized. Thus, if the G value for ionization in the matrix and that for direct radiolysis in the THF aggregates are of the same order of magnitude, as is plausible, the abundant formation of the neutral THF radicals is most probably attributed to the ion-molecule reaction RH+ + RH R. + RHg+ (1) The failure to detect any signal due to the possible radical pair (R...R) also argues against direct radiolysis. Similar ion-molecule reactions also account for the results of concentrated solutions of MTHF and DMTHF's shown in Figure 6. The spectrum for the MTHF solution with the highest concentration (1:9 in mole ratio) agrees well with the MTHF neutral radical studied by previous workers. The ion-molecule reaction has been assumed in radiation chemistry of MTHF but direct evidence has not been presented. Dimethyl Ether. As an example of our extensive studies on other ethers the results of a solution of dimethyl ether in CC13F is demonstrated in Figure 7. It is evident that the methyl groups are rotating much faster than the ESR time scale. Pertinent discussion on the rotation of methyl groups was made in our previous paper: The splitting of the septet is 42.3 G. It is noted that the total splitting, 42.3 G times 6, is close to that of the THF cation, (89 t 40 G) times 2. In conclusion, the present work exemplifies again the potential of the freon matrix for ESR study of cation radicals of various molecules. Such a study is of general interest in connection with the change of molecular geometry upon ionization, the spin density distribution over various u bonds, and secondary reactions of cation radicals. Acknowledgment. The authors thank Dr. Tadashi Sugawara for this technical assistance in the experiment at The Institute for Molecular Science. The research was supported in part by Grants for Fundamental Research in Chemistry, Japan Society for the Promotion of Science.
-
1U ,
100G
,
Figure 7. ESR spectra of the cation radical of dimethyl ether measured at 77 K (above) and at 120 K (below).
Effect of Concentration. Ion-Molecule Reactions. Spectra in Figure 5a-c demonstrate representatively a continuous change of spectrum with initial concentration of THF. Figure 5a is essentially the same as Figure 1 (top). The triple triplet decreases with concentration and a spectrum with a smaller hfs constant appears as shown in Figure 5, b and c. Upon warming the sample of Figure 5c a reversible change shown in Figure 5c-f was observed. The spectrum of Figure 5f may be identified with neutral THF radicals studied previously.' The neutral THF radicals may be produced by direct radiolysis, RH R. + H followed by RH + H R. + Hz (RH = THF), if THF molecules form aggregates at higher concentrations. However, since even the highest concen-
-
-
(7)W.T.Dixon and R. 0. C. Norman, J. Chem. Soc., 1087 (1964).
(8) F. S. Dainbn and G. A. Salmon, Proc. R.SOC.London, Ser. A, 285, 319 (1965). (9)A. C.Ling and L. Kevan, J. Phys. Chem., SO, 592 (1976).
