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An ESR study of the stable radical in a γ-irradiated single crystal of 17α-dydroxy-progesterone
Journal of Molecular Structure, 240 (1990) 133-140 Elsevier Science Publishers B.V., Amsterdam
133
AN ESR STUDY OF THE STABLE RADICAL IN A y-IRRADIATED SINGLE CRYSTAL OF 17#-HYDROXY-PROGESTERONE
R. KRZYMINIEWSKI,
J. PIETRZAK
and R. KONOPKA
Solid State Spectroscopy Laboratory, Institute of Physics, A. Mickiewicz University, 60-780 Poznari (Poland) (Received 4 November
1989; in final form 12 March 1990)
ABSTRACT Electron spin resonance spectroscopy was used to investigate y-radiation damage of 17cY-hydroxy-progesterone molecules in a single crystal. Two types of radicals with different rates of recombination were observed and a definite structure was assigned to the specimen by analyzing the orientational variation of the spectra. The unpaired electron of the radical is delocalized in the 2p, orbitals of the C (6) ,C (4) and C (3) atoms, giving rise to a hyperfine spectrum by interaction with two equivalent cy-protons in positions 4 and 6 and with two non-equivalent P-protons attached to C (7). The hyperfine coupling tensors are reported, together with the g tensor of the radical. The presence of additional intermolecular interactions caused by hydrogen bonding between 0 (3) and HO (17) of two molecules does not change the type of radical (which is the same as the stable radical in a y-irradiated single crystal of progesterone) but does increase the hyperfine coupling anisotropy.
INTRODUCTION
Steroid hormones are among the most important biologically active natural compounds. Their structure is based on a hydrogenated derivative of phenanthrene coupled with a cyclopentane ring, which yields a four-membered system called cyclophenoperhydrophenanthrale. The effects of X-rays and y-radiation on such compounds has been studied both in solution [l-3] and in the solid phase [ 4-71. Rexroad and Gordy [ 81 were the first to study steroids using the ESR method, and detected time-stable radicals in X-irradiated powdered cholesterol. Single crystals of steroid compounds were also studied using the ESR method [g-12]. The analysis of single-crystal ESR allows a precise determination not only of the type of radical, but also of conformational changes in that part of the radical where hyperfine interactions between the unpaired electron and hydrogen atom nuclei occur. The ESR lines may be severely broadened due to unresolved hyperfine structure, and under such circumstances additional spectroscopic techniques, such as ENDOR, may be used to increase the resolution and resolve the ESR spectra [ 13,141. 17a-Hydroxy-progesterone is a starting material for derivatives such as an-
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
134
drostendione and testosterone. This compound is of great biological importance, since it is involved in biosynthesis of androgens and oestrogens, responsible for the development of male and female secondary sexual characteristics, respectively. The aim of the present study of y-irradiated single-crystal 17cr-hydroxyprogesterone is to identify the structure of the free radical formed and to explain the effect of intermolecular hydrogen bonds on the formation of this radical. EXPERIMENTAL
Single crystals of 17a-hydroxy-progesterone with well developed faces were grown from acetone solution by slow evaporation at room temperature. The crystal structure of the compound has been reported by Declercq et al. [ 151. The crystals are orthorhombic with space group P212121and a=9.831, b= 23.463 and c= 7.839 A. There are four molecules per unit cell, forming pairs of two magnetically equivalent molecules. The crystallographic axes were located by external morphology and identified by the Laue X-ray method. The external shape of the crystals and the structural formula, with the numbering system used, are presented in Fig. 1. For ESR measurements an orthogonal x,y,z system coinciding only with the crystallographic b-axis was chosen. The crystals were irradiated with y-rays in a “Co-source at 295 K. The total dose was up to 15 Mrad. ESR spectra were recorded at room temperature using a reflection spectrometer operating at 8.7 GHz with 100 kHz field modulation. The crystals were oriented in the cylindrical cavity by means of a goniometer which was rotated about the axis perpendicular to the magnetic field. The crystal could be oriented with an absolute accuracy + 0.5’. The magnetic field was measured with a Radiopan JTM-41 digital nuclear magnetometer. The diagonalized hyperfine tensors and the g tensor were calculated by standard procedures [ 161 using a Schneider 646 computer. The program calculated initial values of the
Fig. 1. Morphology of the 17a-hydroxy-progesterone Crystallographic
single crystal and the atom numbering scheme.
reference axes are a,b,c and ESR reference axes are n,y,z.
135
g and hyperfine coupling tensor data, then these were optimized by a leastsquares procedure to obtain the best fit to the data in the experimental spectrum. RESULTS AND DISCUSSION
The ESR spectrum of the stable radical shown in Fig. 2 consists of 12 separate lines. The main lines of the spectrum are easily interpreted in terms of two doublets, each being split into a slightly smaller triplet. The angular dependence of the hyperfine splittings of the triplet and doublets and of the g factor are shown in Fig. 3. The principal values of the g and hyperfine coupling tensors, and the direction cosines are listed in Table 1. Because of the small anisotropy of the hyperfine coupling, the doublet splittings arise from two protons in /3positions, while the triplet arises from the equivalence of two protons in cypositions. We can assume that the radical is formed by removal of the hydrogen atom from C (6) in ring B of the molecule. The unpaired electron is delocalized over the atoms C (4), C (6) and 0 (3) (see Fig. 4). The unpaired electron interacts with two non-equivalent p protons on C (7). The isotropic g value is 2.0039, higher than that for normal carbon-centred radicals, but typical for cases where a small amount of the spin density is located on an oxygen atom. The isotropic part of the hyperfine coupling, A::, arising from the cx-protons can be used to estimate the spin density pc at the neighbouring carbon atoms, C (4) and C (6)) by using McConnell’s relation [ 171,
where the constant Qn = - 2.62 mT [ 181. The calculated value, pc = 0.42, confirms the delocalization of the unpaired electron on to 0 (3). It should be noted that when the radical is formed by the loss of a hydrogen atom from the B ring, the bond lengths and the angles between atoms in the molecule change. These changes are caused by the change in the electronic structure of that part of the molecule where the unpaired electron is delocalized. This is the case for the A and B rings of the diamagnetic molecule which, after radiation, will tend to flatten. In this case the 2p, orbitals of C (6) and C(4) are expected to be normal to the plane C(3)-C(4)-C(5)-C(lO)-C(6)C (7) of this radical. This direction is expected to be close to that for the intermediate principal value of the a-proton hyperfine coupling tensor. Furthermore, the direction of the C (5)-C (10) bond of the radical should coincide with that of the minimum principal value of the cu-proton hyperfine coupling tensor. Comparison between the direction cosines of the cu-hyperfine coupling tensor and the crystallographic data for 17cu-hydroxy-progesterone [ 151 shows that the intermediate principal value of this tensor deviates from the direction of the normal to the plane C(3)-C(4)-C(5) by about 27” and that the minimum
136
I
I
I
I
I
III
I
CC) Fig. 2. ESR spectra of 17whydroxy-progesterone single crystal irradiated by y-rays at room temperature. (A) The magnetic field is oriented at 40” from the z axis in the zy plane, (B) the magnetic field is oriented at 20” from the x: axis in the xz plane, and (C) the magnetic field is oriented at 20 ’ from the y axis in the xy plane.
?C0
30
60
90 9 [degl
120
150
180
30
60
90
120
M
180
I3[degl
Fig. 3. Angular variation of g factor and hyperfine splittings. (A) g factor; (B) A, hyperfine splittings resulting from interaction with two equivalent protons in (Ypositions; (C ) and (D ) ApI and A, hyperfine splittings resulting from interaction with two non-equivalent protons in j? positions. ( 0 ) , plane xy; ( 0 ) , plane 2.x; ( 0 ) , plane zy.
value is directed about 23’ from the C (5)-C (10) direction. The differences of 27” and 23’) greater than the average error of measurement, indicate bond distortion in the radical relative to the diamagnetic molecule. The two /?-protons at C( 7) are not equivalent as indicated in Fig. 4. The resulting hyperfine coupling tensor elements for both proton interactions are listed in Table 1. The /? hydrogen hyperfine coupling can be described by the equation [ 191
A, =A1 +A2 cos%
(1)
138 TABLE 1 ESR spectral parameters
g” and
Ab in mT for the 17cr-hydroxy-progesterone
Principal values
Tensor
Direction
cosines with respect to axes
a 2.0029
g
2.0033 2.0056
A,
p
0.51 1.00 1.79
radical
b
C
0.8188 0.2324 0.5249
0.5155 0.1049 - 0.8505
0.2528 -0.9669 0.0339
0.1523 0.8359 -0.5273
0.2630 0.4799 0.8369
-0.9527
0.2662 0.1468
1.10
a
A,
1.89 1.97 2.05
0.8858 0.3583 -0.2948
0.3613 -0.1338 0.9228
A,
2.34 2.58 2.82
0.1965 - 0.9761 0.0929
0.0663 0.1078 0.9920
Ais0
2.58
82
“Eigenvalues,
k 0.0002. bEigenvalues,
-0.2911 0.9239 0.2479
0.9782
0.1887 - 0.0859
f 0.03 mT.
0 H,
Ha
Fig. 4. The structure of the radical formed in a y-irradiated
molecule of 17a-hydroxy-progesterone.
where A, and A, are constants, the former usually small, and 8 is the angle between the plane containing the 2p, orbital and the C (6)-C (7) bond, and the calculated from experimental values, planeC(6)-C(7)-H.TheratioAB,/AD, was 0.76. The ratio of the squared cosines (the angles 8, and 0, were evaluated from crystallographic data [ 15]), calculated using eqn. 1 and neglecting the constant Al, was 0.078. These results indicate strong distortions in the 17cwhydroxy-progesterone molecule damaged by y-irradiation. If the hyperfine interaction anisotropy is defined as AA,, = A, - A,, where A, and A, are the principal hyperfine coupling tensor components, then the
139 TABLE 2 Comparison of isotropic hyperfine coupling values and fractional anisotropy values for the 17ahydroxy-progresterone radical with similar steroid radicals having no hydroxyl group Radical source
17&-Hydroxy-progesterone Progesterone* Androst-4-en-3,17-dione”
A’” (mT)
Ayk;
A&IA,
a
a
82
a
PI
Pz
1.10 1.18 1.18
1.97 1.82 1.77
2.58 2.55 2.59
0.71 0.45 0.65
0.08 0.13 0.17
0.17 0.08 0.09
0.76 0.71 0.68
“From ref. 11. bFrom ref. 12.
fractional value of anisotropy is AAJA,. Calculated values are given in Table 2. For 17a-hydroxy-progesterone the hyperfine coupling anisotropy of the aand&-protons are the greatest. However, very similar values of AA,,/A, (Table 2) are obtained for both androst-4-en-3,17-dione [ 121 and progesterone [ 91, the radicals from which have no hydrogen bonds. From X-ray data it follows [ 151 that the higher anisotropy for the radical from 17c+hydroxy-progesterone, compared with that observed in androst-4en-3,17-dione and other steroids, might be due to increased intermolecular interaction induced by the presence of a hydrogen bond between 0 (3) and OH (17) of two separate molecules. We conclude that the identification of the radical species can provide information about the differences in electronic structure between the radical and the diamagnetic molecule.
REFERENCES
10 11 12
B. Coleby, M. Keller and J. Weiss, J. Chem. Sot., (1954) 66. M. Keller and J. Weiss, J. Chem. Sot., (1951) 1247. M. Keller and J. Weiss, J. Chem. Sot., (1951) 25. M. Fazakerly, Int. J. Appl. Radiat. Isot., 9 (1960) 130. M.J. Kulig and L.L. Smith, J. Org. Chem., 39 (1974) 3398. L.L. Smith, J.I. Teng, M.J. Kulig and F.L. Hill, J. Org. Chem., 38 (1973) 1763. J.I. Teng, M.J. Kulig, L.L. Smith, G. Kan and J.E. van Lier, J. Org. Chem., 38 (1973) 119. H.N. Rexroad and W. Gordy, Proc. Nat. Acad. Sci. U.S.A., 45 (1959) 256. R. Krzyminiewski, J. Masiakowski, J. Pietrzak and A. Szyczewski, J. Magn. Reson., 46 (1982) 300. R. Krzyminiewski, A.M. Hafez, J. Pietrzak and A. Szyczewski, J. Magn. Reson., 51 (1983) 308. A.M. Hafez, R. Krzyminiewski, A. Szyczewski and J. Pietrzak, J. Mol. Struct., 130 (1985) 301. R. Krzyminiewski, A.M. Hafez, A. Szyczewski and J. Pietrzak, J. Mol. Struct., 160 (1987) 127.
140 13 14 15
16 17 18 19
T. Henriksen and E. Sagstuen, J. Magn. Reson., 63 (1985) 333. M.F. Andersen, E. Sagstuen and T. Henriksen, J. Magn. Reson., 71 (1987) 461. J.P. Declercq, G. Germain and M. Van Meersche, Cryst. Struct. Commun., 1 (1972) 9. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, Clarendon Press, Oxford, 1970. H.M. McConnell and D.B. Chesunt, J. Chem. Phys., 28 (1958) 107. R.W. Fessenden and R.H. Schuler, J. Chem. Phys., 39 (1963) 2147. C. Heller and H.M. McConnell, J. Chem. Phys., 32 (1960) 1535.
133
AN ESR STUDY OF THE STABLE RADICAL IN A y-IRRADIATED SINGLE CRYSTAL OF 17#-HYDROXY-PROGESTERONE
R. KRZYMINIEWSKI,
J. PIETRZAK
and R. KONOPKA
Solid State Spectroscopy Laboratory, Institute of Physics, A. Mickiewicz University, 60-780 Poznari (Poland) (Received 4 November
1989; in final form 12 March 1990)
ABSTRACT Electron spin resonance spectroscopy was used to investigate y-radiation damage of 17cY-hydroxy-progesterone molecules in a single crystal. Two types of radicals with different rates of recombination were observed and a definite structure was assigned to the specimen by analyzing the orientational variation of the spectra. The unpaired electron of the radical is delocalized in the 2p, orbitals of the C (6) ,C (4) and C (3) atoms, giving rise to a hyperfine spectrum by interaction with two equivalent cy-protons in positions 4 and 6 and with two non-equivalent P-protons attached to C (7). The hyperfine coupling tensors are reported, together with the g tensor of the radical. The presence of additional intermolecular interactions caused by hydrogen bonding between 0 (3) and HO (17) of two molecules does not change the type of radical (which is the same as the stable radical in a y-irradiated single crystal of progesterone) but does increase the hyperfine coupling anisotropy.
INTRODUCTION
Steroid hormones are among the most important biologically active natural compounds. Their structure is based on a hydrogenated derivative of phenanthrene coupled with a cyclopentane ring, which yields a four-membered system called cyclophenoperhydrophenanthrale. The effects of X-rays and y-radiation on such compounds has been studied both in solution [l-3] and in the solid phase [ 4-71. Rexroad and Gordy [ 81 were the first to study steroids using the ESR method, and detected time-stable radicals in X-irradiated powdered cholesterol. Single crystals of steroid compounds were also studied using the ESR method [g-12]. The analysis of single-crystal ESR allows a precise determination not only of the type of radical, but also of conformational changes in that part of the radical where hyperfine interactions between the unpaired electron and hydrogen atom nuclei occur. The ESR lines may be severely broadened due to unresolved hyperfine structure, and under such circumstances additional spectroscopic techniques, such as ENDOR, may be used to increase the resolution and resolve the ESR spectra [ 13,141. 17a-Hydroxy-progesterone is a starting material for derivatives such as an-
0022-2860/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
134
drostendione and testosterone. This compound is of great biological importance, since it is involved in biosynthesis of androgens and oestrogens, responsible for the development of male and female secondary sexual characteristics, respectively. The aim of the present study of y-irradiated single-crystal 17cr-hydroxyprogesterone is to identify the structure of the free radical formed and to explain the effect of intermolecular hydrogen bonds on the formation of this radical. EXPERIMENTAL
Single crystals of 17a-hydroxy-progesterone with well developed faces were grown from acetone solution by slow evaporation at room temperature. The crystal structure of the compound has been reported by Declercq et al. [ 151. The crystals are orthorhombic with space group P212121and a=9.831, b= 23.463 and c= 7.839 A. There are four molecules per unit cell, forming pairs of two magnetically equivalent molecules. The crystallographic axes were located by external morphology and identified by the Laue X-ray method. The external shape of the crystals and the structural formula, with the numbering system used, are presented in Fig. 1. For ESR measurements an orthogonal x,y,z system coinciding only with the crystallographic b-axis was chosen. The crystals were irradiated with y-rays in a “Co-source at 295 K. The total dose was up to 15 Mrad. ESR spectra were recorded at room temperature using a reflection spectrometer operating at 8.7 GHz with 100 kHz field modulation. The crystals were oriented in the cylindrical cavity by means of a goniometer which was rotated about the axis perpendicular to the magnetic field. The crystal could be oriented with an absolute accuracy + 0.5’. The magnetic field was measured with a Radiopan JTM-41 digital nuclear magnetometer. The diagonalized hyperfine tensors and the g tensor were calculated by standard procedures [ 161 using a Schneider 646 computer. The program calculated initial values of the
Fig. 1. Morphology of the 17a-hydroxy-progesterone Crystallographic
single crystal and the atom numbering scheme.
reference axes are a,b,c and ESR reference axes are n,y,z.
135
g and hyperfine coupling tensor data, then these were optimized by a leastsquares procedure to obtain the best fit to the data in the experimental spectrum. RESULTS AND DISCUSSION
The ESR spectrum of the stable radical shown in Fig. 2 consists of 12 separate lines. The main lines of the spectrum are easily interpreted in terms of two doublets, each being split into a slightly smaller triplet. The angular dependence of the hyperfine splittings of the triplet and doublets and of the g factor are shown in Fig. 3. The principal values of the g and hyperfine coupling tensors, and the direction cosines are listed in Table 1. Because of the small anisotropy of the hyperfine coupling, the doublet splittings arise from two protons in /3positions, while the triplet arises from the equivalence of two protons in cypositions. We can assume that the radical is formed by removal of the hydrogen atom from C (6) in ring B of the molecule. The unpaired electron is delocalized over the atoms C (4), C (6) and 0 (3) (see Fig. 4). The unpaired electron interacts with two non-equivalent p protons on C (7). The isotropic g value is 2.0039, higher than that for normal carbon-centred radicals, but typical for cases where a small amount of the spin density is located on an oxygen atom. The isotropic part of the hyperfine coupling, A::, arising from the cx-protons can be used to estimate the spin density pc at the neighbouring carbon atoms, C (4) and C (6)) by using McConnell’s relation [ 171,
where the constant Qn = - 2.62 mT [ 181. The calculated value, pc = 0.42, confirms the delocalization of the unpaired electron on to 0 (3). It should be noted that when the radical is formed by the loss of a hydrogen atom from the B ring, the bond lengths and the angles between atoms in the molecule change. These changes are caused by the change in the electronic structure of that part of the molecule where the unpaired electron is delocalized. This is the case for the A and B rings of the diamagnetic molecule which, after radiation, will tend to flatten. In this case the 2p, orbitals of C (6) and C(4) are expected to be normal to the plane C(3)-C(4)-C(5)-C(lO)-C(6)C (7) of this radical. This direction is expected to be close to that for the intermediate principal value of the a-proton hyperfine coupling tensor. Furthermore, the direction of the C (5)-C (10) bond of the radical should coincide with that of the minimum principal value of the cu-proton hyperfine coupling tensor. Comparison between the direction cosines of the cu-hyperfine coupling tensor and the crystallographic data for 17cu-hydroxy-progesterone [ 151 shows that the intermediate principal value of this tensor deviates from the direction of the normal to the plane C(3)-C(4)-C(5) by about 27” and that the minimum
136
I
I
I
I
I
III
I
CC) Fig. 2. ESR spectra of 17whydroxy-progesterone single crystal irradiated by y-rays at room temperature. (A) The magnetic field is oriented at 40” from the z axis in the zy plane, (B) the magnetic field is oriented at 20” from the x: axis in the xz plane, and (C) the magnetic field is oriented at 20 ’ from the y axis in the xy plane.
?C0
30
60
90 9 [degl
120
150
180
30
60
90
120
M
180
I3[degl
Fig. 3. Angular variation of g factor and hyperfine splittings. (A) g factor; (B) A, hyperfine splittings resulting from interaction with two equivalent protons in (Ypositions; (C ) and (D ) ApI and A, hyperfine splittings resulting from interaction with two non-equivalent protons in j? positions. ( 0 ) , plane xy; ( 0 ) , plane 2.x; ( 0 ) , plane zy.
value is directed about 23’ from the C (5)-C (10) direction. The differences of 27” and 23’) greater than the average error of measurement, indicate bond distortion in the radical relative to the diamagnetic molecule. The two /?-protons at C( 7) are not equivalent as indicated in Fig. 4. The resulting hyperfine coupling tensor elements for both proton interactions are listed in Table 1. The /? hydrogen hyperfine coupling can be described by the equation [ 191
A, =A1 +A2 cos%
(1)
138 TABLE 1 ESR spectral parameters
g” and
Ab in mT for the 17cr-hydroxy-progesterone
Principal values
Tensor
Direction
cosines with respect to axes
a 2.0029
g
2.0033 2.0056
A,
p
0.51 1.00 1.79
radical
b
C
0.8188 0.2324 0.5249
0.5155 0.1049 - 0.8505
0.2528 -0.9669 0.0339
0.1523 0.8359 -0.5273
0.2630 0.4799 0.8369
-0.9527
0.2662 0.1468
1.10
a
A,
1.89 1.97 2.05
0.8858 0.3583 -0.2948
0.3613 -0.1338 0.9228
A,
2.34 2.58 2.82
0.1965 - 0.9761 0.0929
0.0663 0.1078 0.9920
Ais0
2.58
82
“Eigenvalues,
k 0.0002. bEigenvalues,
-0.2911 0.9239 0.2479
0.9782
0.1887 - 0.0859
f 0.03 mT.
0 H,
Ha
Fig. 4. The structure of the radical formed in a y-irradiated
molecule of 17a-hydroxy-progesterone.
where A, and A, are constants, the former usually small, and 8 is the angle between the plane containing the 2p, orbital and the C (6)-C (7) bond, and the calculated from experimental values, planeC(6)-C(7)-H.TheratioAB,/AD, was 0.76. The ratio of the squared cosines (the angles 8, and 0, were evaluated from crystallographic data [ 15]), calculated using eqn. 1 and neglecting the constant Al, was 0.078. These results indicate strong distortions in the 17cwhydroxy-progesterone molecule damaged by y-irradiation. If the hyperfine interaction anisotropy is defined as AA,, = A, - A,, where A, and A, are the principal hyperfine coupling tensor components, then the
139 TABLE 2 Comparison of isotropic hyperfine coupling values and fractional anisotropy values for the 17ahydroxy-progresterone radical with similar steroid radicals having no hydroxyl group Radical source
17&-Hydroxy-progesterone Progesterone* Androst-4-en-3,17-dione”
A’” (mT)
Ayk;
A&IA,
a
a
82
a
PI
Pz
1.10 1.18 1.18
1.97 1.82 1.77
2.58 2.55 2.59
0.71 0.45 0.65
0.08 0.13 0.17
0.17 0.08 0.09
0.76 0.71 0.68
“From ref. 11. bFrom ref. 12.
fractional value of anisotropy is AAJA,. Calculated values are given in Table 2. For 17a-hydroxy-progesterone the hyperfine coupling anisotropy of the aand&-protons are the greatest. However, very similar values of AA,,/A, (Table 2) are obtained for both androst-4-en-3,17-dione [ 121 and progesterone [ 91, the radicals from which have no hydrogen bonds. From X-ray data it follows [ 151 that the higher anisotropy for the radical from 17c+hydroxy-progesterone, compared with that observed in androst-4en-3,17-dione and other steroids, might be due to increased intermolecular interaction induced by the presence of a hydrogen bond between 0 (3) and OH (17) of two separate molecules. We conclude that the identification of the radical species can provide information about the differences in electronic structure between the radical and the diamagnetic molecule.
REFERENCES
10 11 12
B. Coleby, M. Keller and J. Weiss, J. Chem. Sot., (1954) 66. M. Keller and J. Weiss, J. Chem. Sot., (1951) 1247. M. Keller and J. Weiss, J. Chem. Sot., (1951) 25. M. Fazakerly, Int. J. Appl. Radiat. Isot., 9 (1960) 130. M.J. Kulig and L.L. Smith, J. Org. Chem., 39 (1974) 3398. L.L. Smith, J.I. Teng, M.J. Kulig and F.L. Hill, J. Org. Chem., 38 (1973) 1763. J.I. Teng, M.J. Kulig, L.L. Smith, G. Kan and J.E. van Lier, J. Org. Chem., 38 (1973) 119. H.N. Rexroad and W. Gordy, Proc. Nat. Acad. Sci. U.S.A., 45 (1959) 256. R. Krzyminiewski, J. Masiakowski, J. Pietrzak and A. Szyczewski, J. Magn. Reson., 46 (1982) 300. R. Krzyminiewski, A.M. Hafez, J. Pietrzak and A. Szyczewski, J. Magn. Reson., 51 (1983) 308. A.M. Hafez, R. Krzyminiewski, A. Szyczewski and J. Pietrzak, J. Mol. Struct., 130 (1985) 301. R. Krzyminiewski, A.M. Hafez, A. Szyczewski and J. Pietrzak, J. Mol. Struct., 160 (1987) 127.
140 13 14 15
16 17 18 19
T. Henriksen and E. Sagstuen, J. Magn. Reson., 63 (1985) 333. M.F. Andersen, E. Sagstuen and T. Henriksen, J. Magn. Reson., 71 (1987) 461. J.P. Declercq, G. Germain and M. Van Meersche, Cryst. Struct. Commun., 1 (1972) 9. A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, Clarendon Press, Oxford, 1970. H.M. McConnell and D.B. Chesunt, J. Chem. Phys., 28 (1958) 107. R.W. Fessenden and R.H. Schuler, J. Chem. Phys., 39 (1963) 2147. C. Heller and H.M. McConnell, J. Chem. Phys., 32 (1960) 1535.