HAROLD C. Box
3426
Electron Spin Resonance and Endor Studies of
Radiation-Produced Radical Pairs by Harold C. Box Biophysics Department, Roswell Park Memorial Institute, Buffalo,New York 14803
(Received January 4, 1071)
Publication costs assisted by the Atomic Energy Commission
Esr and endor spectroscopy were used to study radical pair formation in several organic systems, namely (1) formation of pairs of phenyl radicals by elimination of two molecules of COZ from dibenzoyl peroxide in mixed crystals (solid solutions) subjected to ultraviolet irradiation at 4.2'K; (2) formation of pair of benzoyloxy radicals by dissociation of dibenzoyl peroxide in mixed crystals X-irradiated at 4.2'K; (3) formation of radical pairs by dehydrogenation of adjacent potassium hydrogen malonate molecules in single crystals Xirradiated at 4.2'K. Of the three processes for forming radical pairs, elimination, dissociation, and dehydrogenation, only the latter appears to have general significanceas a radiation damage process. The mechanism involved in the dehydrogenation of adjacent molecules is discussed in some detail.
Introduction
g H H = gfs
When two free radicals are formed in close proximity, there is an exchange interaction between electrons which separates the spin states into singlet and triplet states. The triplet levels are lower in energy than the singlet level and can be examined by esr and endor spectroscopy. In this presentation we shall be concerned with radical pairs produced in several organic solid systems by either ultraviolet' or ionizing radiation. Our principal objectives will be (1) to distinguish various radiation-induced chemical processes which generate radical pairs, and (2) to assess the significance of radical pair production as a radiation damage process.
+
(3)
XWiaHjAQtj il
DHH = & H Z ~ H ~ D I ~
(4)
t.i
The subscripts in (3) and (4) refer to orthogonal axes fixed with respect to the radical pair. The components of the tensors, D t j and gzl = grsSt5 Aylj, are defined with respect to these axes as is the direction of the applied field by means of the direction cosines aH1. When the distance between the unpaired spins is large compared to atomic dimensions, the spins may be regarded as point dipoles and an explicit expression for the dipole-dipole coupling is given by
+
Theoretical Considerations The spin Hamiltonian for a radical pair takes the form X
pH*g.(S1
+ Sz) + JS1.SZ + Si*D*SZ (1)
where S1 and S2 are the unpaired spins.' The g tensor defines the effective magnetic moment of the electrons and the D tensor, which is traceless, defines the dipoledipole coupling between electrons. The exchange coupling is measured by J . Assuming the g value is isotropic and equal to the free spin value, gf,, and ignoring the dipole-dipole coupling term, the eigenenergies of the Hamiltonian consisting of the Zeeman and exchange energies can be found. A first-order correction for the anisotropy in g and for thc dipole-dipole coupling energy can then be added. The resultant energies for the triplet states, EM,are given by
E1
=
Eo E-1
=
+ (J
Pg"H =
DHH)/~
J / 4 - D"/2
-bg"H
+ +DHH)/~ (J
where The Journal of Physical Chemistry, VoZ. 76, No. $8, 1071
(24 (2b) (2c)
where gfs = 2.0023, R is the distance between dipoles, and 6 is the angle between the vector R and the field H. Esr measurements allow us, through ( 3 ) and ( 4 ) , to evaluate experimentally the tensors g and D. When there are also hypeyfine couplings to be accounted for, another term X' =
-CPNg,In. n
(H -
A n * (SI
+ SZ))
(6)
must be added (1) where I, and g, refer to a nuclear spin and its g value and A, defines its hyperfine coupling. The energy levels of the entire spin system become EM,^^, ... = EM- zPNgnmnhMn n
(7 )
where m, is the magnetic quantum number of the nth nuclear spin. The quantity hv" is the magnitude of the effective field at the nth nucleus. The effective field is given by (1) J. Kurita, J . Chem. SOC.Jup., 85, 833 (1964).
ESRSYMPOSIUM.
STUDIES OF
RADIATION-PRODUCED RADICAL PAIRS
3427
where the udare unit vectors. 97%
Experimental Section
O
Esr and endor spectroscopy a t K band (24 GHz), as well as esr spectroscopy a t V band (70 GHz), were used to examine radical pairs formed in single crystals irradiated a t 4.2"Ii. Mixed crystals (solid solutions) consisting of dibenaoyl peroxide (approximately 3%) in dibenaoyl disulfide were grown by evaporation from toluene solution. The morphology of the crystals was like that of pure crystals of dibenzoyl disulfide which belong to the space group P21/ca2 Single crystals of potassium hydrogen malonate3 (space group C2/m with four molecules per unit cell) and of hydroxyurea4 (space group P21/c with four molecules per unit cell) were grown by evaporation of aqueous solution. Single crystals of dimethylgly~xime~ (space group Pi with two molecules per unit cell) were grown from alcohol solution. The sample cavity of the spectrometer2was immersed in a liquid helium bath contained in a dewar equipped with either a beryllium or a sapphire window. Irmdiation of the peroxide-disulfide solid solution with radiation from a high-pressure Hg lamp produced radical pairs by eliminating COz from the peroxide molecules. Irradiation by X-rays produced radical pairs by dissociation of the peroxide molecules. X-Irradiation of potassium hydrogen malonate, hydroxyurea, and dimethylglyoxime crystals produced radical pairs by dehydrogenation of adjacent molecules. However, radical pairs were obtained in dimethylglyoxime only after warming considerably above 4.2"K.
0
II
0
I/
CeH&OOCC&,
22CeH5 -/-
2coz
D
O
Figure 1. The esr spectrum from an irradiated single crystal of deuterated dibenzoyl disulfide with approximately 3% of the molecules substituted with dibenzoyl peroxide. The ultraviolet irradiation and measurements were carried out a t 4.2'K. Arrow indicates resonance for DPPH.
dibenzoyl disulfide.2 I n this calculation R was taken as the vector between nuclei of the pair of phenyl carbon atoms linked by the O=CSSC=O group. These direction cosines are expected to define approximately the direction of a corresponding vector for the dibenzoyl peroxide guest molecules. This direction should coincide with the direction of maximum dipole-dipole coupling for the phenyl radical pair. Table I shows how well this expectation is fulfilled. Table I : The Principal Values, in Gauss, of the Dipole-Dipole Coupling Tensor for the Pair of Phenyl Radicals Produced in Dibenzoyl Disulfide-Dibenzoyl Peroxide Crystals by Ultraviolet Irradiation"
Results
Radical Pairs by Elimination. The esr absorption pattern obtained from a solid solution of dibensoyl peroxide in deuterated dibenzoyl disulfide following ultraviolet irradiation at 4.2"K is shown in Figure 1. The absorption is due to pairs of phenyl radicals produced from the peroxide by elimination of C02
D
- 203
+101 +103
R
a'
b
$0.86 -0.12 $0.47 f0.857
+0.23 f0.96 -0.17 0,269
G
-0.43 +0.26 +0.86
-0.438
a The direction cosines are given with respect to an orthogonal set of axes where a' = b X c. There is a second radical pair related in orientation to this one by a twofold rotation about the b axis. The direction cosines of R were calculated from the crystal structure of dibenaoyl disulfide as described in the text.
(9)
The doublet pattern arises from the allowed transitions (EO-+ E1 and E-1 + Eo)between energy levels defined by eq 2. The doublet splitting was measured for various orientations of the crystal with respect to the applied field, and the components of the dipole-dipole coupling tensor were deduced using eq 2 and 4. The principal values and the direction cosines of the principal axes of the tensor are given in Table I. Using ( 5 ) the distance between spins, R,was calculated to be 6.5 A. The direction cosines of the vector R, listed in Table I, were calculated from the crystal structure of
The resolution of the hyperfine pattern associated with the phenyl radical pair is somewhat improved if the host molecules are deuterated. However, the reso(2) H. C. Box, H. G. Freund, K. T. Lilga, and E. E. Budainski, J. Phys. Chem., 74,40 (1970). (3) G . Ferguson, J. G. Sime, J. C. Speakman, and R. Young, Chem. Commun., 162 (1968); R. Parthasarathy, J. G. Sime, and J. C. Speakmap, Acta CrystaEEogr., Sect. B , 2 5 , 1201 (1969). (4) I. Larsen and B. Jerslev, Acta Chem. Scand., 2 0 , 983 (1966). ( 5 ) L. L. Merritt, Jr., and E. Lauterman, Acta Crystallogr., 5 , 811 (1952).
The Journal of Physical Chemistry, Vol. 76, N o . $9, 1971
HAROLD C.Box
3428 Table I1 : Radical Pairs by Elimination Separation Parent molecule
Molecule eliminated
co
Diarylcarbonates
Radical produced
of radicals,
A
Experimental conditions
Referenoe
RO R = tolyl or
5.9
Polycrystal, y rays,
9
77°K
Azobisisobutyronitrile
Na
phenyl (CH,)aCCN
5.6
Dibenzoyl peroxide
2COa
CeH5
6.5
Dibenzoyl peroxide
2c02
C(IH~
6.5
lution is still incomplete for most crystal orientations. The width of the absorption shown in Figure 1, measured between outermost hyperfine peaks, is 43 G. This is somewhat less than the overall hyperfine splitting who found the sum of the reported by IZasai, et hyperfine couplings was 48 G in phenyl radicals produced in a glass by the photolysis of phenyl iodide. Attempts to get a complete specification of the hyperfine tensors for the phenyl radical using the endor technique have been unsuccessful thus far. The width of the doublet lines in Figure 1 limits the accuracy of g value determinations. The change in resonant field with crystal orientation due to g value variation is small compared with line width. The phenyl radicals generated by ultraviolet irradiation probably lie near the surface of the crystal. Prolonged irradiation or warming substantially above 4.2"K destroys the simple radical pair spectrum. Pairs of phenyl radicals can also be produced by ultraviolet light in pure crystals of dibenzoyl peroxide irradiated at 4.2"K.' Apparently the first observation of radical pairs formed by elimination was by Lebrdev8 who observed the photolytic decomposition of azobisisobutyronitrile at 77°K with the release of a nitrogen molecule. Radical pairs have also been obtained from X-irradiated organic carbonatesQ wherein elimination of CO occurs. Table I1 is a compilation of pertinent facts concerning radical pairs produced by the process of elimination. Radical Pairs by Dissociation. The peroxide molecules in the peroxide-disulfide mixed crystals of the preceding section can also be made to undergo simple dissociation. If the excitation energy is delivered via ionizing radiation, simple dissociation of the peroxide bond is achieved. Figure 2 shows the esr spectrum
0
0
II
/I
X-rays
/
\
(10)
0
from a mixed crystal X-irradiated at 4.2"K. The spec0 ',
RADICAL PAIRS
8
7 This report
BY DISSOCIATION
ESR: X-irradiated peroxide -disulfide crystal K band 4.2*K H II c
.'
.
SIP (AU88
Figure 2. The esr spectrum from an X-irradiated single crystal of dibenzoyl disulfide with 3% of the molecules substituted with dibenzoyl peroxide. The irradiation and measurements were carried out a t 4.2"K. Arrow indicates resonance for DPPH.
trum contains the expected components, labeled PO and (YO in Figure 2, due to oxidized and reduced disulfide molecules.2 In addition there are the y components which are attributed to pairs of dibenzoyloxy radicals. The size of the radical pair absorption indicates that excitation energy is efficiently transferred intermolccularly by the host molecules and deposited with the guest molecules which thereupon dissociate. The number of dissociated peroxide molecules is comparable with the number of oxidized and reduced disulfide molecules. Maximum separation of the y lines (K band) indicates a distance between spins, as calculated using ( 5 ) of 3.6 A. Presumably the disulfide matrix provides oversized spaces for the peroxide guest molecules so that upon dissociation of the peroxide bond the benzoyloxy radi-
0
CaHsCOOCCGHb -* 2C6HbC
The Journal
Polycrystal, photolysis, 77°K Single crystal, photolysis, 4.2% Crystal solid solution, photolysis, 4.2"K
Phyaical Chemistry, VGL76, No. dB, 1071
(6) P. H . Kasai, E. Hedaya, and E. B. Whipple, J. Ante?. C h e w SOC.,91, 4364 (1969). (7) H. C. Box, E. E. Budzinski, and H. G. Freund, {bid., 92, 5305 (1970). (8) Y. S. Lebedev, DOH.Alcad. Nauk SSSR, 171, 378 (1966). (9) A. Davis, J. H . Golden, J. A. McRae, and M. C. R. Symons, Chem. Commun., 398 (1967).
3429
ESRSYMPOSIUM. STUDIES OF RADIATION-PRODUCED RADICAL PAIRS
c H malonate
ENDOR: Radical pair absorption in K H malonate
Figure 4. The endor lines obtained from the triplet state esr absorpt)ion in potassium hydrogen malonate.
I
180 POW8
I
Figure 3. (a) The spectrum for an X-irradiated single crystal of potassium hydrogen malonate at 4.2'K. (b) The spectrum enlarged to show the y lines associated with a radical pair. Arrow indicat>esresonance for DPPH.
cals can separate somewhat and thereby gain a measure of stability. Warming a few degrees above 4.2OIi causes the radical pair absorption t o disappear, undoubtedly because of recombination. Henzoyloxy radical pairs are not observed in single crystals of pure dibenzoyl peroxide crystals irradiated a t 4.2"K. The esr absorption indicates two sets of radical pairs related by a tnofold axis. This is consistcmt with the P21/cspace group of dibenzoyl disulfide.2 Howcver, there is a perplexing featurr of the absorption in that the intensities of the two sets of y spectra (which can be distinguished for certain crystal orientations where the magnotic field is neithcr parallrl or prrpendicular to the twofold axis) are usually unequal. Also an interesting dcpc>ndencc. of the esr absorption on magnetic field strength has been observed and is still being invrstigated. A t least one othcr example of radical pair production by dissociation exists in tho literature. Gordy and RIorehousP produced pairs of methyl radicals and hydrogen atoms by y irradiation of methane a t 4.2"K. Radical P a i r s by Dehydrogenation of Ad-jacent M d e cules. Figure 3 shows an esr spectrum obtained from a single crystal of potassium hydrogen malonate Xirradiated at 4.2"K. Thtl p1 and a. components of the spclctrum are due to para mag no tic^ species produced by oxidat ion and reduction proccsses, respectivcly. l 1 The smaller componmts labeled y in Figure 3 are due to radical pairs produwd by drhydrogcwation of adjacent molecules. The radivals in each pair are formed with X-rays
2KOOCCH&OOH -+ ZKOOCCHCOOH
+ Hz
(11)
the C H bond along the b axis of the crystal which is a twofold axis of rotation. The two radicals in each pair are related by a mirror plane perpendicular t,o the twofold axis. The hyperfine structure sssociat)ed with the y absorption in Figure 3 is particularly interesting. One of the y components has a nine-line hyperfine pattern; the other has a three-line pattern as is shown more clearly in Figure 3b. The entire hyperfine pattern arises from the two C H protons and can be accounted for quantit,atively by calculating t'he transition probabilities associated with the allowed and "forbidden" transitions.' Figure 4 shows the endor spectrum obtained from the y components of the esr absorption. The endor frequencies arc obtained from (7) for transitions between IcveIs for which Am = l, M = *l. From the hv =
(12)
PNgnhhl"
definition of hu" givcn in ( 8 ) it is clear that the higher endor frequency corresponds to transit$ions of the proton in the case where the component of effective magnetic field at the prot'on due to the unpaired electron augments the applied field; the lower frequency corresponds to thc case where the t,wo fields are opposed.
Table I11 : Principal Values in Gauss and Direction Cosines of the Principal Axes for the Hyperfine Coupling Fonnd by Endor for the Radical Pair Produced in X-Irradiated Potassium Hydrogen Malonate a'
-4.5
-10.0 -1R.X
-0.69 0.72 0
b
C
0 0
0.72 0.69
1
0
(10) W. Gordy and R. Morehouse, Phye. Rev., 151, 207 (1966). (11) H. C. Box, E. E. Budzinski, and W. Potter, J . Chem. Phys., 55, 315 (1971).
The Journal of Physical Chemistry, Vol. 76,N o . 88? 1971
HAROLD C. Box
3430 Table IV : Radical Pairs by Dehydrogenation of Adjacent Molecules“ Separation Parent molecule
Radical produced
Dimethylglyoxime* Glyoxime Methylglyoxime p-Chlorobenzaldoxime Hydroquinoned Oxalic acid Hydroxyurea Potassium hydrogenB malonate
of radicals,
ii
HONC(CHa)C(CHa)NO HONCHCHNO HONCHC(CH8)NO ClCsH4CHNO HO(I7)O HOOCCOO NHICONHO KOOCCHCOOH
5.6
