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Electron Spin Resonance Study of XIrradiated Single Crystals of DinbutylOxalate–Urea and Dinbutyl Malonate–Urea Inclusion CompoundsAllen A. Lai, G. Bruce Birrell, and O. Hayes Griffith Citation: The Journal of Chemical Physics 53, 4399 (1970); doi: 10.1063/1.1673956 View online: http://dx.doi.org/10.1063/1.1673956 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/53/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Single Crystal Electron Spin Resonance Spectra of XIrradiated Potassium Hydrogen Malonate J. Chem. Phys. 56, 1279 (1972); 10.1063/1.1677359 Electron Spin Resonance of RĊHCOR′ Radicals in XIrradiated Ketone—Urea Inclusion Compounds J. Chem. Phys. 42, 2644 (1965); 10.1063/1.1703218 Electron Spin Resonance and Molecular Motion of the RCH2ĊHCOOR′ Radicals in XIrradiatedEster—Urea Inclusion Compounds J. Chem. Phys. 41, 1093 (1964); 10.1063/1.1726011 Electron Spin Resonance of an Irradiated Single Crystal of Urea Oxalate J. Chem. Phys. 35, 362 (1961); 10.1063/1.1731917 Single Crystal Electron Spin Resonance Spectra for xIrradiated Glycine J. Chem. Phys. 31, 859 (1959); 10.1063/1.1730491
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J. CHEM. PHYS., VOL. 53, 1970 LETTERS TO THE EDITOR 4399
should be overwhelming favorites for molecular calculations.
* Work supported by the General Electric Foundation. t Samuel B. Silbert Graduate Fellow. 10. Sinanoglu, Advan. Chern. Phys. 6, 315 (1964). 2 R. E. Stanton, J., Chern. Phys. 42,2353 (1965). 3 F. W. Byron, Jr., and C. J. Joachain, Phys. Rev. 157, 1 (1967). 4F. W. Byron, Jr., and C. J. Joachain, Phys. Rev. 157, 7
(1967) . 6 J. Linderberg and H. Shull, J. Mol. Spectroscopy 5, 1 (1960). 6 (a) R. K. Nesbet, Advan. Chern. Phys.14, 1 (1969) j (b) R. K.
Nesbet, T. L. Barr, and E. R. Davison, Chern. Phys. Letters 4,203 (1969).
7 K. F. Freed, Phys. Rev. 173, 1 (1968). 8 H. F. King, J. Chern. Phys. 46, 705 (1967). 9 W. A. Lester, Jr., and M. Krauss, J. Chern. Phys. 41, 1407
(1964) j 42,2990 (1965). 10 F. W. Byron, Jr., and C. J. J oachain, Phys. Rev. 146, 1 (1966),
see Table 1.
Notes
Electron Spin Resonance Study of XIrradiated Single Crystals of Di-n-butyl Oxalate-Urea and Di-n-butyl MalonateUrea Inclusion Compounds*
ALLEN A. LA!, G. BRUCE BIRRELL, t AND O. HAYES GRlFFITHt
Institute of Molecular Biology and Department of Chemistry, University of Oregon, Eugene, Oregon 97403
(Received 24 July 1970)
Urea inclusion compounds have been used in numerous x-irradiation studies of aliphatic esters. l-4
Single crystals are easily prepared for ESR experiments, and the hexagonal urea matrix provides a common environment for the various guest molecules. To date all published work on saturated and unsaturated ester-urea inclusion compounds indicates that the x-ray produced stable free radicals occur near the carboxyl end of the fatty acid residue. In saturated ester molecules, the observed radicals simply correspond to removal of one hydrogen atom from the carbon atom adjacent to the carboxyl group.1.2 Thus, the radicals observed in saturated esters are essentially the same as the well-known radicals5 derived from the carboxylic acids. The purpose of this note is to point out that in
aliphatic esters the dominant radicals are not always produced adjacent to the carboxyl groups. As examples, we discuss below the free radicals observed in urea inclusion crystals of di-n-butyl oxalate and di-n-butyl malonate.
General experimental details concerning crystal growth, x-irradiation, and ESR measurements may be found elsewhere.4 All chemicals used were reagent grade. The crystals were x-irradiated at 77°K and ESR spectra were recorded on a Varian E-3 X-band ESR spectrometer. The spectral simulations were performed on a Varian 620/i computer. The results are summarized in Fig. 1. Spectra (a) and (b) are the first derivative ESR spectra recorded with the magnetic field along the needle axis (z axis) of a di-n-butyl oxalate-urea crystal [Fig. 1 (a)] and a di-n-butyl malonate-urea crystal [Fig. 1 (b)]. The proton hyperfine patterns of these two radicals are clearly the same and the computer simulation [Fig. l(c)] is in good agreement with the experimental results. We conclude that the long-lived radicals produced by x irradiation of di-n-butyl oxalate-urea and di-n-butyl malonate-urea crystals are, respectively,
and
The spectra are anisotropic except in the crystalline xy plane (the plane perpendicular to the needle axis). This behavior is characteristic of urea inclusion compounds and results from rapid motion of the long-chain guest molecules in the tubular cavities of the hexagonal urea matrix.1 The structure can be thought of as an extremely well-oriented liquid crystal. The coupling constants and g values observed in the two principal directions are given in Table I. One proton coupling constant (aa) is highly anisotropic, whereas the other five coupling constants are very nearly isotropic. An estimate of the isotropic component (aoa) of aa for both radicals, 22.1 G, is readily obtained from the relation ao"'= (a.a+2axy<» /3.1 By comparison with many known systems,I.5 radicals (1) and (2) are clearly 7r-electron
TABLE I. Proton coupling constants and g values.R,b
Di-n-butyl oxalate radical (1)
(1) a za =32.7
(4) a!=26.6 (1) a!=23.0
gz=2.0028
(1) axya =17.0 (4) ax,/=25.3 (1) axy~=22.1
gxy=2.0025
Di-n-butyl malonate radical (2)
(1) a za =32.8
(4) a!=26.7 (1) a!=23.0
gz=2.0029
(1) axya = 16.8 (4) ax,/=25,5 (1) axy~=22.1
gxy=2.0026
Ii All a;tll and a z values are believed accurate to within ±O.3 and ±1.S G, b The numbers in parentheses are the total number of protons with a respectively. The uncertainty in the g-value measurements is ±O.0002. given coupling constant.
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4400 LETTERS TO THE EDITOR ]. CHEM. PHYS., VOL. 53, 1970
(0)
(b)
I------li 40 GAUSS
}'IG. 1. ESR spectra of x-irradiated single crystals of (a) di-nbutyl oxalate at 263°K and (b) di-n-butyl malonate at 298°K. In both (a) and (b) the magnetic field is parallel to the z axis of the crystal. Spectrum (c) is a computer simulation calculated using four proton coupling constantsof26.6G, one proton coupling constant of 23.0 G, and one proton coupling constant of 32.7 G.
radicals, and aa arises from one proton bonded directly to the radical center. The five /3-proton splittings result from the adjacent -CHa and -CH2- groups. Apparently there is a small angle of twist causing one of the methylene proton coupling constants to be accidentally equivalent, within experimental error, to the three methyl proton coupling constants. This is not an unusual feature of trapped 1r-electron radicals.!
The surprising observation is that the dominant radical in both di-n-butyl oxalate-urea and di-n-butyl malonate-urea crystals results from hydrogen atom removal from carbon atom 3 of the butanol group.6 The dominant radicals are not related in any way to the radicals previously observed in numerous studies of oxalic acid7 ,8 or malonic acid.9-11 There are, of course, some weak unresolved lines (see Fig. 1) that may result from damage to the carboxylic acid portions of these esters. Other radicals may exist in higher concentrations for short time periods after x irradiation or at low temperatures. Nevertheless, the results presented here provide an interesting example of differences that can occur between x-ray damage in esters and the parent acids. The data also suggest a note of caution in generalizing results from a limited series of xirradiation studies.
We are indebted to Ulista Brooks and Margaret
McMakin for some of the early experimental work on oxalates and for useful discussions.
* Supported by the National Science Foundation under Grant. GP-16341.
t NIH Postdoctoral fellow (Fellowship 5 F03 CA42789-02 from the National Cancer Institute).
t Alfred P. Sloan fellow. 10. H. Griffith, J. Chern. Phys. 41, 1093 (1964). 2 O. H. Griffith, Proc. Nat!. Acad. Sci. U.S. 54, 1296 (1965). • O. H. Griffith and E. E. Wedum, J. Am. Chern. Soc. 89, 787
(1967) . . ! E. E. Wedurn and O. H. Griffith, Trans. Faraday Soc. 63, 819
(1967) . • A. Carrington and A. D. McLachlan, Introduction to Magnetic
Resonance (Harper and Row, New York, 1967), Chap. 7. 6 We have also examined the radicals formed by x irradiation
of di-n-butyl succinate-urea inclusion crystals. In this case the radical
CHaCH2CH2CH202CCHCH2C02CH2CH2CH2CH.
is the major species at 298°Kj however, the radical
CHaCHCH2CH202CCH2CH2C02CH2CH2CH2CHa
is also observed. 7 N. Rao and W. Gordy,]. Chern. Phys. 35, 362 (1961). 8 G. C. Moulton, M. P. Cernansky, and D. C. Straw, J. Chern.
Phys. 46,4292 (1967). 9 H. M. McConnell, C. Heller, T. Cole, and R. W. Fessenden,
]. Am. Chern. Soc. 82,766 (1960). 10 A. Horsfield,]. R. Morton, and D. H. Whiffen, Mol. Phys. 4,
327 (1961). 11 R. C. McCalley and A. L. Kwirarn, ]. Am. Chern. Soc. 92,
1441 (1970).
Gas-Phase Recombination Rates of Hydrogen and Deuterium Atoms
]. E. BENNETT AND D. R. BLACKMORE
Shell Research Limited, Thornton Research Centre, P.O. Box 1, Chester CHI 3SH, England
(Received 6 March 1970)
In a recent paperl we have reported the measurement in a flow system of the gas-phase recombination rate of hydrogen atoms in the presence of molecular hydrogen as a third body [Reaction (2H)]:
klH H+surface ---t tHz,
k2H H + H + H2 ---t H2+ H2.
(lH)
(2H)
In this letter we report the measurement of the corresponding recombination rate of deuterium atoms [Reaction (2D)] and also a value at room temperature of the ratio k2H/ k2D which has not been determined previously.
k2D D+ D+ D2 ---t D2+ D2. (2D)
The experimental technique has been described in detail earlier.l Briefly, the atoms were generated by passing molecular hydrogen or deuterium through a microwave discharge, and the concentrations of atoms at various points along the flow tube were determined
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