H. C. Box, H. G. FREUND, K. T. LILQA,AND E. E. BUDZINSKI
40
Magnetic Resonance Studies of the Oxidation and Reduction of Organic Molecules by Ionizing Radiations by Harold C. Box, Harold .G. Freund, Kenneth T. Lilga, and Edwin E. Budzinski Biophysics Department, Rowell Park Memorial Institute, Bug&, New York
(Received May W6, 1060)
By maintaining organic compounds at low temperature (4.2"K) it is often possible to stabilize oxidation and reduction products produced by ionizing radiation. The products were observed by electron spin resonance (esr) and in some cases by electron-nuclear double resonance (endor) spectroscopy. The oxidized and reduced species observed in irradiated single crystals of thiourea derivatives, several organic disulfides, thiodiglycolic acid, glycine, glycine HCl, and two dicarboxylic acids are enumerated. Some G values for ion production at 4.2"K are also given. The results are discussed in terms of the following processes: electron attachment, dissociative electron attachment, intramolecular transfer of holes, and anion degradation.
Introduction When an electron is stripped away from a neutral diamagnetic organic molecule the molecule becomes a paramagnetic radical cation. Another neutral diamagnetic molecule may attach the dissociated electron to become a paramagnetic radical anion. Both the anion and cation should be detectable by electron spin resonance spectroscopy. However, radical ions created in this fashion are usually unstable and have only a very transient existence at ordinary temperatures. In order to study these species many investigators have worked at 77°K (boiling point of liquid nitrogen a t atmospheric pressure) which is sometimes sufFiciently cold to stabilize and make possible the identification of the anions produced by ionizing radiation. In this laboratory it has proven more advantageous to work a t 4.2"K (boiling point of liquid helium at atmospheric pressure) in order to stabilize a more primitive state of the radiation damage process. In this report we survey the results of magnetic resonance measurements made on a series of organic compounds irradiated in the form of single crystals a t 4.2"K. Frequently it has been possible to stabilize both the anion and the cation formed in these compounds. The investigation utilized electron spin resonance (esr) spectroscopy primarily, but some electron nuclear double resonance (endor) measurements were made. It will usually be satisfactory for the description of our results to assume a hamiltonian which neglects components of spin (both electron and nuclear) which are perpendicular to the applied magnetic field.' This leads to a system of energy levels for the electron and N interacting nuclei given by
netic quantum number of the electron (lt1/2),ml is the magnetic quantum number of the ith interacting nucleus, and gr is its nuclear g value. The quantities g and A r define the g value and the hyperfine splittings, respectively. In esr spectroscopy, electronic transi-, tions (AM = 1, Am = 0) are observed, whereas in endor spectroscopy2the transitions ( A M = 0, Am = 1) are detected. The g value and the hyperfine coupling which are anisotropic with respect to crystal orientation can be described by tensor quantities. In this report the principal axes of tensors are defined with respect to crystal axes designated by letters such as u, b, and c' or a', b, and c. Theoretical arguments are most succinctly made using a frame of reference defined with respect to the structural geometry of a molecule. Axes defined in this manner are designated by subscripts 2, y, and x . It is often useful to compare the directions of principal axes deduced from theoretical arguments with the directions of principal axes determined experimentally. A theoretical estimate of the g tensor is given by
(1)
where g f s = 2.0023, X is the spin-orbit coupling parameter and L3refers to the operator for the component of angular momentum in the j direction. The wave functions $0 and may be regarded as the molecular orbitals occupied by the unpaired electron in the ground state and in the nth excited state, respectively, Eo is the difference in energy between these and E, states. Molecular orbitals are linear combinations of atomic orbitals. The operator XL,, for example, is understood to mean ZkXkL& where the subscript k refers to a particular atom.
where H is the magnetic field intensity, B is the Bohr magneton, Pn is the nuclear magneton, M is the mag-
(1) A. Carrington and A. D. McLachlan, "Introduction to Magnetio Resonance," Harper and Row, New York, N. Y., 1967,p 72. ( 2 ) C.Feher, Phys. Rev., 103,600 (1966).
N
E(rnf,M) = gPHM
N
- c gfBnHmf + ci A"fM i
The Journal of Physical Chemistry
-
OXIDATION AND REDUCTION OF ORGANIC MOLECULES BY RADIATION The use of single crystals was particularly important in this investigation since we invariably dealt with a t least two paramagnetic species, usually in the form of a cation and an anion. The chance of interpreting overlapping esr spectra is of course much better having single crystal data. Furthermore, for certain crystal orientations there may be sufficient difference in g value between two absorptions to allow complete separation of the spectra. Most organic free radicals have g values close to the free-spin value (2.0023) but exceptions occur in radicals in which the unpaired electron is largely localized on a heavy atom. I n these radicals, spin-orbit coupling can cause significant g value variation. Considerable use was made of this effect in this investigation and the reader will note that many of the compounds studied contain sulfur or halogen atoms.
41
a-
n
s--
i
1
Experimental Procedures The esr spectrometers used in this investigation operated a t a microwave frequency of 24.00 and 70.10 Gc/sec and utilized superheterodyne detection. The use of superheterodyne detection makes it possible to measure paramagnetic resonance absorption at nanowatt levels of microwave power. This capability is essential for the study of radiation-induced radicals in organic systems at 4.2"K since spin-lattice relaxation times are generally long and saturation of the esr absorption occurs readily a t higher power levels. The sample cavity of the spectrometer was a cylindrical transmission type which resonated in the TE 011 mode. The dimensions of the cavity, and hence its resonant frequency, change drastically as the temperature is lowered. I n order to avoid the necessity of changing the frequency of the pair of klystrons used in the superheterodyne detection system the sample cavity was made mechanically tunable as shown in Figure 1. The cavity assembly was immersed in a liquid helium dewar which was sufficiently vacuum tight so that if necessary the helium gas pressure could be reduced to the X point. At the X point or below microphonics generated by gas bubbles in the sample cavity can be eliminated, but this device was seldom employed, the signal-to-noise ratio being sufficiently good a t 4.2"K. The whole assembly consisting of the dewar and enclosed microwave apparatus could be removed from the spectrometer and transported to an X-ray facility. During irradiations the sample was pushed out through an axial hole in the bottom of the cavity where it was exposed to radiation passing through beryllium windows fixed in the walls of the dewar. The usual exposure time using a 250-1c.V, 30-mA X-ray source was 10 min. The esr absorption spectra could be recorded directly from an oscilloscope display of absorption us. magnetic field strength. Whenever necessary the signalto-noise ratio of the signal could be enhanced using a
P 0
4
k
Figure 1. Apparatus for irradiation and measurement of crystals at 4.2'K. Cavity and dewar shown in cross section. Liquid nitrogen reservoir not shown. Legend: (a) sample rod, (b) waveguide, (0) mechanical tuning for cavity, (d) fill port, (e) discharge tube, (f) glass tubing sample guide, (g) dewar flange, (h) dewar inner wall, (i) dewar outer wall, (j) cold shield, (k)beryllium windows, (1) sample, (m) quartz fiber, (n) polyethylene mount, (0)TE 011 tunable cavity, (p) iris, (9) plunger, (r) sample during irradiation, (9) evacuation port, (t) helium.
computer of average transients. Field intensity measurements were made using a proton oscillator. Our adaption of Feher's electron nuclear double resonance (endor) technique has been described elsewhere.3 One minor modification was required for studying samples irradiated a t 4.2"K. The loop which carries the radio-frequency energy was extended through the axial hole in the bottom of the sample cavity so as not to interfere with the sample irradiation procedure. Endor is used to measure hyperfine couplings accurately when satisfactory measurements of bhe couplings directly from the esr spectra are impossible due either to line widths or overlapping of spectra. Determination of G values for radical production (the number of free radicals produced per 100 electron volts of energy deposited in the sample) a t 4.2"K (3) H.C. Box, H. G. Freund, and K. T.Lilga, J . Chem. Phya., 46, 2130 (1967).
Volume 74, Number 1
January 8, 1970
H. C. Box, H. G. FREUND, K. T. LILCA,AND E. E. BUDZINSKI
42 Table I
Molecule
Formula
Reduction product
Oxidation product
G, 4.2OK
G after warming
S Methylthiourea
II
CH3-NH-C-NHz
>c+-s
>c-s-
>c +-s
>c-s -
-(S-S)+-(S-S)+-
-(S-S)--(S-S)--(S-S)--(S-S)--
2.8 3.6 3.8
0.6 0.9 2.4
S Dibu tylthiourea
II
C~HS-NH-C-NH-CIH~
00 Benzoyldisulfide Cystine HC1 Dithiodiglycolic acid Cystine HBr Thiodiglycolic acid Succinic acid
/ \
C6Hj-C-S-S-C-CaHs
(COOH-CH(NHsC1)-CHz-S-)z COOH-CHz-S-S-CHz-C00H
(COOH-CH(NH~B~)-CHZ-S-)Z
Br
COOH-CHz-S-CHz-COOH
-s +-
0-'
