Influence of Nuclear Spin Exchange
629
ron Double Resonance of Irradiated Single Crystals of Zinc aionic Acid. The Influence of Nuclear Spin Exchange' Lowell B. Kispert," Kichoon Chang, and Carolyn M. Bogan2 Deparimenl of Chemistry. The University of Aiabama. Tuscaioosa. Aiabama 35486 (Received November 78. 7972) Publication costs assisted by The University of Alabama
Electron-electron double resonance (eldor) spectra of the CHzCOO- radical in irradiated single crystals of zinc acetate and the CHzCOOH radical in irradiated malonic acid were obtained over a temperature range from -100 to +50". A maximum in the eldor intensity of the allowed lines was observed at -70" for CHzCOO- and at 0" for CHzCOOH. A maximum in the intensity of the forbidden eldor lines was observed approximately 50" below the maximum intensity of the allowed lines. The qualitative aspects of the temperature dependence of the eldor spectra of CHzCOO- and CHzCOOH can be explained in terms of the correlation times of the nonadiabatic spin exchange process.
Introduction The temperature dependence of the esr spectra of CH2COO- or W & O O H , found in irradiated crystals of malonic acid,3 sodium acetate trihydrate,6,7 and zinc iicetatr dihydrate,8s9 has been the subject of much interest. Marked temperature dependence of the proton coupling constants has been reported in addition to the coalescence ox"the inner peaks63839 at high temperature, and the shift ng of the outer peaks toward the center of the spectrum.8J0 Despite these changes with temperature, the g values remain constant6 and the direction of the carbon p orbital remains quenched in space.6 Recently, Hayes, et a1.,I0 proposed that a nonadiabatic spin exchainge is responsible for the observed temperature dependence of CHzCOO- and were able to reproduce many of the features of the spectrum using this model. Electron -electron double resonance (eldor) has been quite succcsslul in probing intra- and intermolecular relaxation p ~ o c e s s e s ~ .It~ ~has - ~ ~been possible in several cases to correlatle the magnitude of the various eldor R values with the correlation times of the intramolecular motion. Kispert, et propose& that the eldor signals observed above 18" in (CH&CCOOH were dependent on the rnagnitude of the correlation times of the dynamic process which made the twe methyl groups appear equivalent in the esr spectrum. The nonadiabatic spin exchange proposed by Hayes, et u1.,'* for CHzCOO is also expected to have some effects on the relaxation processes responsible for the observation of an eldor spectrum. It is of interest then to examine the temperature dependence of the eldor spectrum of CHzCOO- in the hope that more can be learned about the origin of the temperature dependence of the esr spectra.18
Experimental Seetican All spectra were run on a standard Varian E-SO0 eldor accessory coupled to a Varian E-12 esr spectrometer. The magnetic field was measured by a tracking nmr system. Single crystals of malonic acid and zinc acetate were gown from aqueous solution by slow evaporation. The crystals were X-irradiated at room temperature and subsequently investigated by esr over a range of tempera-
tures. The crystals were aged a few days to eliminate any unstable radicals initially present at room temperature. The eldor spectra of CHzCOOH in irradiated triclinic malonic acid crystals were taken in the ab plane, nearly parallel to the b axis where a four-line (equally intense) esr (1) This research was supported by the Atomic Energy Commission under Contract No. AT-(40-1)-4062 and this is AEC Document No. 0R0-4062-8. (2) AAUW Fellowship holder 1970-1971. (3) A. Horsfield, J. R. Morton, and D. H. Whiffen, Moi. Phys.. 4. 327 (1961). (4) J. R. Morton, J . Amer. Chem. Soc.. 86, 2325 (1964). ( 5 ) R. F. Weiner and W. S . Koski, J . Amer. Chem Soc.. 85, 873 (1963); H. C. Box, H. G. Freund, and E. E. Eudzinski, ibid., 88, 658 (1966). (6) M. Fujimoto and J. Janecka, J . Chem. Phys.. 55, 5 (19'71). (7) M. T. Rogers and L. D. Kispert, Advan. Chem Ser.. No. 82, 327 (1968), (8) W. M. Tolles, L. P. Crawford. and J. L. Valenti, J. Chem. P h y s . . 49, 4745 (1968). (9) H. Ohigashi and Y. Kurita, Bull. Chem. SOC.Jap.. 41, 275 (1968). (10) R. G . Hayes, D. J. Steible, W. M. Tolles, and J. W. Hunt, J . Chem Phys.. 53, 4466 (1970). (11) J. S. Hyde, J. C. W. Chien, and J. H. Freed, J . Chem. Phys.. 48, 4211 (1968). References to earlier work cited: J. S. Hyde, L. D. Kispert, R . C. Sneed, and J. C. W. Chien, J. Chem. Phys.. 48, 3824 (1968); J. S . Hyde, R. C. Sneed, Jr., and G . H. Rist, ibid.. 51, 1404 (1969); V . A. Benderskii, L. A. Blumenfeld, P. A. Stunkas, and E. A. Sokalov, Nature (London), 220, 365 (1968); P. A. Stunkas, V. A. Benderskii, L. A. Blumenfeld, and E. A. Sokalov, Opt. SpekfrOSk.. 28, 278 (1970); P. A. Stunkas, V. A. Benderskii, and E. A. Solalov, ibid. 28, 487 (1970); P. A. Stunkas and V. A. Benderskii, ibid.. 30, 559 (1971); M. P. Eastman. Ci. V . Bruno, and J. H. Freed, J. Chem. Phys.. 52, 321 (1970); M. Nechtschein and J. S . Hyde, Phys. Rev. Lett.. 24, 672 (1970): V. A. Benderskii, P. A. Stunkas, and A. I . Rakoed, Mol. P h y s . . 24, 449 (1972). Eldor restilts reported by L. D. Kispert, C. M. Bogan, and K. Chang at the Varian EPR Double-Resonance Workshop, Jan. 26-28, 1972, Palo Alto, Calif., and the 23rd Southwestern Regional Meeting of the American Chemical Society, Nashville, Tenn., Nov 4-5, 1971, Abstract No. 54: H. M. Vieth, H. Brunner, and ti. H. Hausscr, Z. Naturforsch. A, 26, 167 (1971); T. S. Kuau, D. S. Tinti, and M. A. El-Sayed, Chem. Phys. Lett.. 4, 507 (1970); M. Leung and M. A. El-Sayed, ibid., 16,454 (1972). (12) L. D. Kispert, K. Chang, and C. M. Dogan, J. Chem. Phys., in press (1973); results given at the 24th Southeastern Regional Meeting of the American Chemical Society, Nov 2-4, 1972, Physical Chemistry Abstract No. 75, and at the SE Magnetic Resonance Conference held at Athens, Ga., Oct 12-13, 1972. (13) L. D. Kispert, K. Chang, and C. M. Dogan, Chem. Phys. Lett,, 17, 592 (1972); L. D. Kispert and M. T. Rogers, J . Chem. Phys., in press. The effect of chlorine quadrupole relaxation, (14) G. Rist and J. H. Freed, personal communication; eldor investigations of .CH(COOH)2. (15) J. H. Freed, D. S. Leniart, and H. C. Connor, J. Chem. Phys., in press. (16) Results reported by ti. Chang and L.. D. Kispert at the 24th Southeastern Regional Meeting of the American Chemical Society, Nov 2-4, 1972, Physical Chemistry Abstract No. 76. The effect of p orbital anisotropy. The Journal of Physical Chemistry, Vol. 77, No,5, 1973
L. D. Kispert, K.
630
I.
c
Chang, and 6.M. Bogan
i
0.mI
a 0
0
A
Freshly krodioted crystal 2-3 weeks old
R
0.40 Q.20
0.00 T
'C
Figure 1. Eldor reduction factors for CHPCOO- (in zinc acetate) vs. temperature. A and B are the "allowed" eldor lines: both the
observing and the pumping esr transitions are allowed with AMJ = 0. C and [I are the "forbidden" eldor lines: the observing esr transition is allowed but the pumped esr line is forbidden with AMJ = 1. For all curves, the M J = -1 esr line is the observing esr line. Rexpt = eldor line height/esr line height. No corrections were mads for the difference in the eldor (5.0 MHz) and esr (6.5 M H z ) line widths. The M J = -1 esr and eldor line widths did not vary significantly (
