- Email: [email protected]
Proton ENDOR at a microwave frequency of 97 GHz
JOURNAL
OF MAGNETIC
RESONANCE
80,383-388
(1988)
Proton ENDOR at a Microwave Frequency of 97 GHz OLAF BURGHAUS,
ANNA
TOTH-KISCHKAT,
ROBERT KLE’ITE,
AND KLAUS
MOBIUS*
Institutfir Molekiilphysik,Freie UniversitiitBerlin,Amimallee 14, D-1000Berlin 33, WestGermany Received June 27,1988
Most commercial EPR spectrometers work at microwave frequencies below 40 GHz; practical considerations such as sample handling, sensitivity, and equipment cost favor X-band (9- 10 GHz) spectrometers over S (2-4 GHz)- and Q-band (3436 GHz) extensions, which are normally used only for special purposes. There are, however, many unsolved problems in molecular spectroscopy, such as lack of resolution in disordered samples, which require taking advantage of higher microwave frequencies (hmf) and, correspondingly, higher magnetic fields, &. The first advantage of hmf EPR is the improvement of spectral resolution due to the larger Zeeman interaction. It brings about a more precise determination of g tensors of frozen solutions or polycrystalline samples (1-4). In spectra of y- or X-ray irradiated single crystals with different sites and small g-factor anisotropy a better separation of EPR lines becomes possible at hmf (5). An increasing field of investigations is mixed radicals in solution with small g-factor differences and anisotropic rotation of radicals in viscous solvents (3,4). In the slow-motion regime hmf EPR is distinguished by its increased spectral sensitivity to motional dynamics and important applications in this field are in progress (6). The second advantage of hmf EPR is the higher sensitivity. The two important factors that determine the sensitivity of an EPR spectrometer are the filling factor and the loaded Q of the resonator. Although sensitivity does not increase with a high power of B,, (the power ranging between f and g depending on experimental conditions (7)), there is a significant sensitivity improvement compared with X-band EPR in the investigation of small single crystals. Another advantage of hmf EPR spectrometers becomes apparent when investigating high-spin transition metal complexes with large zero-field splittings. X- or Q-band frequencies are often too low for measuring the zero-field splitting parameters D and E. Examples for such a situation occur for many compounds of biological interest (2). With these advantages in mind hmf EPR spectrometers have been constructed in a few laboratories, at 70 GHz (X = 4.3 mm) (8), at 94 GHz (X = 3.2 mm) (3, 9), at 150 GHz (X = 2 mm) (I, 4, 5) and, most recently, even at 250 GHz (X = 1.2 mm) (6). Additional advantages are envisaged when extending hmf EPR to hmf ENDQR spectroscopy. Due to the larger Zeeman interaction, selective saturation of EPR lines * To whom correspondence should be addressed.
383
0022-2364188$3.00 Copyright Q 1988 by Academic Pm, Inc. All rights of reproduction in any fom reserved.
384
COMMUNICATIONS
becomes possible even in the case of small g-factor anisotropy. Thus one can select ENDOR spectra of different radical sites in crystals or of different species in radical mixtures. Saturation of the EPR at the turning points,in the powder pattern of polycrystalline or amorphous samples or frozen solutions results in single-crystal-like ENDOR spectra provided the g and hyperfme tensors have a common axes system (IO). In view of analyzing complex ENDOR spectra, working at high magnetic fields might result in a considerable simplification, since even for large hyperfine couplings the hmf ENDOR spectra will be symmetric with respect to the Larmor frequency of the free nucleus. A schematic diagram of the EPR and ENDOR spectrometer is given in Fig. 1. An earlier design of the EPR part was described previously (3, 9). We use two different microwave sources. The first one is a low-noise solid-state Gunn oscillator. It is part of a microwave test apparatus (Millimeter Wave Sweeper Model 706, Micro Now Instrument Company, Inc.) and supplies 6.3 mW between 92 and 97 GHz. The second source is a 97 GHz klystron (Varian VRB 200 1) with a maximum output power of 300 mW. Because of the short lifetime of the klystron and its high cost, a sweepable microwave source turned out to be essential for developing and improving the microwave components, particularly the resonator, and performing EPR test experiments. Both mw sources are stabilized by a homebuilt AFC. The microwave sources are shielded from the spurious magnetic field of the cryomagnet by p-metal boxes. The microwave bridge consists of a Fabry-Perot resonator in reflection, a reference arm, and a low-noise preamplifier. To minimize the attenuation losses in W-band waveguides, oversized waveguides are used and their length is reduced to a minimum. Only for the last part to the cavity is a W-band waveguide used, since this determines the microwave mode configuration in the Fabry-Perot resonator. Still the microwave losses between the klystron’s isolator and the cavity are almost 5 dB (over a waveguide length of approximately 40 cm).
1
\
LFP-resonator
•II
magnet rf
coil
(
pick-up coil /
oversized
d digital attenu.
waveguide amplifier
--
MW bridge , t
1
t
computer with I/O
1 FIG. 1. Block
diagram
of the 3 mm ENDOR
-
spectrometer.
ff-‘ock
385
COMMUNICATIONS
The magnetic field is produced by a cryomagnet (Oxford Instruments 250/89 NMR system) with a warm bore of 88 mm diameter. A homebuilt field regulator (9) allows a linear sweep of the magnetic field from 0 to the maximum of 5.87 T. The sweep rate can be set from 0 to 500 G/min. The cooling system gives stable temperatures in the range of 30-300 K. It is constructed as a heat exchanger and liquid helium or liquid nitrogen can be used as a coolant. The resonator stays in an Nz or He gas atmosphere and is cooled by convection. The B,-, field modulation coils are mechanically separated from the resonator and the sample holder to avoid microphonics. This allows use of modulation rates up to 40 Gpp in a frequency range of 100 Hz to 100 kHz without baseline problems. The resonator presently used is of the Fabry-Perot type with two concave mirrors of 13 mm diameter in almost confocal separation. Resonators of this biconcave type or of plane-concave type have been employed also in other hmf EPR spectrometer designs (3, 6, II, 12). Variable coupling to the waveguide is achieved by a ceramic sheet ( 1.4 X 0.4 X 5 mm) which can be shifted directly behind the iris ofthe removable Fabry-Perot mirror. The special design of the complex Fabry-Perot structure will be described in a forthcoming paper. Figure 2 shows schematically the Fabry-Perot resonator with the two-turn ENDOR coil. All the magnetic fields (&, B,, &) are perpendicular to each other to minimize undesirable cross couplings. In this respect it is also necessary to make the
I /
-
0
‘is
Sample
\I0’
, Endor
position
1
coil
‘1
13mm
u!J E
Capacitor 1 RF power
amplifier
FIG. 2. The Fabry-Perot ENDOR resonator. The ceramic coupling sheet is placed directly behind the iris with a X/4 choke. The diameter of the shown quartz capillary for liquid samples lies between 0.4 and 0.8 mm with a wall thickness of about 0.02 mm. In the measurements described the capillary in sample position 1 is replaced by the single crystal mounted on a quartz thread. Sample position 2 lies in the center of the resonator perpendicular to the drawing plane. The fundamental resonator mode is used with eight nodes of the BI field on the resonator axis. The ENDOR coil is made of the inner wire of a semirigid coaxial cable.
386
COMMUNICATIONS
ENDOR coil diameter larger than the mirror diameter. This large diameter of the coil ( 14 mm) and its high impedance at frequencies around 150 MHz require a resonance circuit to produce high enough B2 fields with the available power amplifier of only 50 W (EN1 550L). But unlike the situation in a similar X-band design there is no need to tune the resonance frequency of the LC circuit while sweeping the frequency of the B2field. A Q of 15 for the resonance circuit resulted in a B2field strength of at least 4 Gmt (rotating frame) in a frequency range between 140 and 155 MHz without retuning the RF circuit. This configuration allowed ENDOR measurements to be made of hyperfine interactions up to 10 MHz at a proton Larmor frequency of 147 MHz. By using a tuned circuit with higher Q it is, of course, possible to produce higher B2 fields, but in this case the full frequency range can only be swept in segments. A pickup coil near the ENDOR coil allows continuous detection of the B2field strength. A digital attenuator is set by a computer-controlled regulation circuit reacting on the B2 field strength changes which are detected by the pickup coil, thus keeping the B2 field at an almost constant level. Sample access to the Fabry-Perot resonator is either along the axis of the cryomagnet or perpendicular to it. In the first option exchange of samples is simple and can be done at any temperature without removing the resonator unit from the bore of the cryomagnet. However, no angular variation of the sample with respect to the B. field can be performed in this option. In the second option the sample can be rotated perpendicular to the B. field, but in order to change the sample the resonator unit must be removed completely. For ENDOR experiments we use a double-modulation scheme, decoding first the 20 kHz RF frequency modulation and then the 950 Hz field modulation. The magnetic field is stabilized with a field-frequency (ff) lock. Because of the double decoding the ff lock works on the second derivative of the EPR line. Figure 3 shows a 3 mm EPR spectrum of a single crystal of y-irradiated cu-aminoisobutyric acid (13) at 100 K together with the structural formula of the radical. The dimensions of the crystal are 0.3 X 2.1 X 2.3 mm. The arrow indicates the EPR transition on which ENDOR was performed at the same temperature. The 3 mm ENDOR spectrum is depicted in Figure 4. The spectrum consists of two frequency regions, the ENDOR lines appearing at frequencies (14)
(vu = free proton Larmor frequency, Aj = hypertine coupling constant). The resonance frequency of the RF circuit was not changed during accumulation of the ENDOR spectra. The first region from 114 to 118 MHz with the RF resonance fixed at 115 MHz with a Q of 10 shows the low-frequency ENDOR transitions of the two groups of methyl protons. The spectrum was scanned four times with a time constant of 1.25 s of the field modulation lock-in decoding. Accumulation took 15 min. The B2 field strength was about 3.5 Grot. The second frequency region between 145 and 149.5 MHz shows the small hyperhne couplings around the proton Larmor frequency Vn = 147.25 MHz. The B2field strength reached values of about 5 Gmt when the resonance frequency of the RF circuit with a Q of 15 was set at 147 MHz. Accumulation of this spectrum took 60 min for 16 scans. The 20 kHz fm modulation was set at +-40 kHz deviation in both plots. Because of the limited variation range of the
COMMUNICATIONS
387
OH
FIG. 3. EPR spectrum of y-irradiated a-aminoisobutyric acid with crystal axis a I&, at 100 K and structural formula of the radical. The arrow indicates the EPR transition on which ENDOR is performed.
capacitor in the RF resonance circuit, up to now we are not able to measure the highfrequency lines of the large hyperfine couplings. In this communication no analysis of the measured hyperfme couplings in terms of molecular conformations of the free radical in the irradiated amino acid will be given (see, for instance, Ref. (13)).
mm
114 115 116 117 MHz
145
146
147
148
149 MHz
FIG. 4. ENDOR spectrum of +-radiated cu-aminoisobutyric acid at 100 K. From the line positions the hypetine coupling constants, Ai, can be deduced (e.g., A, = 1.55 MHz, A2 = 2.32 MHz, A, = 3.08 MHz, Ad = 63.46 MHz).
388
COMMUNICATIONS
In summary, the 3 mm EPR spectrometer with a Fabry-Perot resonator was successfully extended to an ENDOR spectrometer with the free proton Larmor frequency as high as 147 MHz. The open structure and the large dimensions of the Fabry-Perot resonator as compared with a cylindrical cavity offer a wide field of new applications. As a first test example, a single crystal of y-irradiated a-aminoisobutyric acid has been investigated. For the Fabry-Perot resonator light irradiation with light pipes or directly focused laser sources poses no problem. The first 3 mm EPR measurements on the triplet state of pyrene-&, in single crystals of fluorene at room temperature were already successful. ACKNOWLEDGMENTS It is a pleasure to acknowledge the important contributions by Dr. E. Haindl (now at Thyssen, Munich) who was involved in earlier stages of our 3 mm EPR/ENDOR project. The expert machine shop work by Mr. J. Claus was essential for the final successof this endeavor, which is highly appreciated. We thank Drs. F. Lendzian, M. Plato, and C. J. Winscom for many helpful discussions. We also thank Dr. A. L. Maniero (University of Padova) for the single crystals of r-irradiated cY-aminoisobutyric acid and Dr. C. v. Borczyskowski (FUB) for the pyrene-&,/tluorene-mixed crystals. Support of this work by the Deutsche Forschungsgemeinschah is gratefully acknowledged. REFERENCES 1. 0. YA. GRINBERG, A. A. DUBINSKII, ANDYA. S. LEBEDEV, Russ. Chem. Rev. (Engl Transl.) 52,850 (1983). 2. R. L. BELFORD, R. B. CLARKSON, J. B. CORNELIUS, K. S. ROTHENBERGER, M. J. NILGES, AND M. D. TIMKEN, in “Electronic Magnetic Resonance of the Solid State” (J. A. Weil, Ed.), p. 2 1, The Canadian Society for Chemistry, Ottawa, 1987. 3. Yu. D. TSVETKOV, V. I. GULIN, S. A. DIKANOV, AND I. A. GRIGOR’EV, in “Electronic Magnetic Resonance of the Solid State” (J. A. We& Ed.), p. 45, The Canadian Society for Chemistry, Ottawa, 1987. 4. YA. S. LEBEDEV, J. Chem. Sot. D. I. Mendelejev 30,187 (1985). 5. E. HAINDL, K. MC~BIUS,AND H. OLOFF, Z. Naturforsch. A 40, 169 (1985). 6. B. LYNCH, K. A. EARLE, AND J. H. FREED, Rev. Sci. Instrum., in press. 7. C. P. POOLE, JR., “Electron Spin Resonance,” 2nd ed., p. 404, Wiley, New York, 1983. 8. E. E. BUDZINSKI, W. R. POTTER, AND H. C. Box, J. Chem. Phys. 70,132O (1979). 9. 0. BURGHAUS, E. HAINDL, M. PLATO, AND K. M~~BIUS,J. Phys. E 18,294 (1985). 10. G. H. RIST AND J. S. HYDE, J. Chem. Phys. 49,2449 (1968); SO, 4532 (1969); 52,4633 (1970). Il. I. AMITY, Rev. Sci. Instrum. 41,1492 (1970). 12. U. HARBARTH, J. KOWALSKI, R. NEUMANN, S. NOEHTE, K. SCHEFFZEK, AND G. zu PUTLITZ, J. Phys. E 20,409 (I 987). 13. J. W. WELLS AND H. C. Box, J. Chem. Phys. 46,2935 (1967). 14. G. FEHER, Phys. Rev. 103,834 (1956).
OF MAGNETIC
RESONANCE
80,383-388
(1988)
Proton ENDOR at a Microwave Frequency of 97 GHz OLAF BURGHAUS,
ANNA
TOTH-KISCHKAT,
ROBERT KLE’ITE,
AND KLAUS
MOBIUS*
Institutfir Molekiilphysik,Freie UniversitiitBerlin,Amimallee 14, D-1000Berlin 33, WestGermany Received June 27,1988
Most commercial EPR spectrometers work at microwave frequencies below 40 GHz; practical considerations such as sample handling, sensitivity, and equipment cost favor X-band (9- 10 GHz) spectrometers over S (2-4 GHz)- and Q-band (3436 GHz) extensions, which are normally used only for special purposes. There are, however, many unsolved problems in molecular spectroscopy, such as lack of resolution in disordered samples, which require taking advantage of higher microwave frequencies (hmf) and, correspondingly, higher magnetic fields, &. The first advantage of hmf EPR is the improvement of spectral resolution due to the larger Zeeman interaction. It brings about a more precise determination of g tensors of frozen solutions or polycrystalline samples (1-4). In spectra of y- or X-ray irradiated single crystals with different sites and small g-factor anisotropy a better separation of EPR lines becomes possible at hmf (5). An increasing field of investigations is mixed radicals in solution with small g-factor differences and anisotropic rotation of radicals in viscous solvents (3,4). In the slow-motion regime hmf EPR is distinguished by its increased spectral sensitivity to motional dynamics and important applications in this field are in progress (6). The second advantage of hmf EPR is the higher sensitivity. The two important factors that determine the sensitivity of an EPR spectrometer are the filling factor and the loaded Q of the resonator. Although sensitivity does not increase with a high power of B,, (the power ranging between f and g depending on experimental conditions (7)), there is a significant sensitivity improvement compared with X-band EPR in the investigation of small single crystals. Another advantage of hmf EPR spectrometers becomes apparent when investigating high-spin transition metal complexes with large zero-field splittings. X- or Q-band frequencies are often too low for measuring the zero-field splitting parameters D and E. Examples for such a situation occur for many compounds of biological interest (2). With these advantages in mind hmf EPR spectrometers have been constructed in a few laboratories, at 70 GHz (X = 4.3 mm) (8), at 94 GHz (X = 3.2 mm) (3, 9), at 150 GHz (X = 2 mm) (I, 4, 5) and, most recently, even at 250 GHz (X = 1.2 mm) (6). Additional advantages are envisaged when extending hmf EPR to hmf ENDQR spectroscopy. Due to the larger Zeeman interaction, selective saturation of EPR lines * To whom correspondence should be addressed.
383
0022-2364188$3.00 Copyright Q 1988 by Academic Pm, Inc. All rights of reproduction in any fom reserved.
384
COMMUNICATIONS
becomes possible even in the case of small g-factor anisotropy. Thus one can select ENDOR spectra of different radical sites in crystals or of different species in radical mixtures. Saturation of the EPR at the turning points,in the powder pattern of polycrystalline or amorphous samples or frozen solutions results in single-crystal-like ENDOR spectra provided the g and hyperfme tensors have a common axes system (IO). In view of analyzing complex ENDOR spectra, working at high magnetic fields might result in a considerable simplification, since even for large hyperfine couplings the hmf ENDOR spectra will be symmetric with respect to the Larmor frequency of the free nucleus. A schematic diagram of the EPR and ENDOR spectrometer is given in Fig. 1. An earlier design of the EPR part was described previously (3, 9). We use two different microwave sources. The first one is a low-noise solid-state Gunn oscillator. It is part of a microwave test apparatus (Millimeter Wave Sweeper Model 706, Micro Now Instrument Company, Inc.) and supplies 6.3 mW between 92 and 97 GHz. The second source is a 97 GHz klystron (Varian VRB 200 1) with a maximum output power of 300 mW. Because of the short lifetime of the klystron and its high cost, a sweepable microwave source turned out to be essential for developing and improving the microwave components, particularly the resonator, and performing EPR test experiments. Both mw sources are stabilized by a homebuilt AFC. The microwave sources are shielded from the spurious magnetic field of the cryomagnet by p-metal boxes. The microwave bridge consists of a Fabry-Perot resonator in reflection, a reference arm, and a low-noise preamplifier. To minimize the attenuation losses in W-band waveguides, oversized waveguides are used and their length is reduced to a minimum. Only for the last part to the cavity is a W-band waveguide used, since this determines the microwave mode configuration in the Fabry-Perot resonator. Still the microwave losses between the klystron’s isolator and the cavity are almost 5 dB (over a waveguide length of approximately 40 cm).
1
\
LFP-resonator
•II
magnet rf
coil
(
pick-up coil /
oversized
d digital attenu.
waveguide amplifier
--
MW bridge , t
1
t
computer with I/O
1 FIG. 1. Block
diagram
of the 3 mm ENDOR
-
spectrometer.
ff-‘ock
385
COMMUNICATIONS
The magnetic field is produced by a cryomagnet (Oxford Instruments 250/89 NMR system) with a warm bore of 88 mm diameter. A homebuilt field regulator (9) allows a linear sweep of the magnetic field from 0 to the maximum of 5.87 T. The sweep rate can be set from 0 to 500 G/min. The cooling system gives stable temperatures in the range of 30-300 K. It is constructed as a heat exchanger and liquid helium or liquid nitrogen can be used as a coolant. The resonator stays in an Nz or He gas atmosphere and is cooled by convection. The B,-, field modulation coils are mechanically separated from the resonator and the sample holder to avoid microphonics. This allows use of modulation rates up to 40 Gpp in a frequency range of 100 Hz to 100 kHz without baseline problems. The resonator presently used is of the Fabry-Perot type with two concave mirrors of 13 mm diameter in almost confocal separation. Resonators of this biconcave type or of plane-concave type have been employed also in other hmf EPR spectrometer designs (3, 6, II, 12). Variable coupling to the waveguide is achieved by a ceramic sheet ( 1.4 X 0.4 X 5 mm) which can be shifted directly behind the iris ofthe removable Fabry-Perot mirror. The special design of the complex Fabry-Perot structure will be described in a forthcoming paper. Figure 2 shows schematically the Fabry-Perot resonator with the two-turn ENDOR coil. All the magnetic fields (&, B,, &) are perpendicular to each other to minimize undesirable cross couplings. In this respect it is also necessary to make the
I /
-
0
‘is
Sample
\I0’
, Endor
position
1
coil
‘1
13mm
u!J E
Capacitor 1 RF power
amplifier
FIG. 2. The Fabry-Perot ENDOR resonator. The ceramic coupling sheet is placed directly behind the iris with a X/4 choke. The diameter of the shown quartz capillary for liquid samples lies between 0.4 and 0.8 mm with a wall thickness of about 0.02 mm. In the measurements described the capillary in sample position 1 is replaced by the single crystal mounted on a quartz thread. Sample position 2 lies in the center of the resonator perpendicular to the drawing plane. The fundamental resonator mode is used with eight nodes of the BI field on the resonator axis. The ENDOR coil is made of the inner wire of a semirigid coaxial cable.
386
COMMUNICATIONS
ENDOR coil diameter larger than the mirror diameter. This large diameter of the coil ( 14 mm) and its high impedance at frequencies around 150 MHz require a resonance circuit to produce high enough B2 fields with the available power amplifier of only 50 W (EN1 550L). But unlike the situation in a similar X-band design there is no need to tune the resonance frequency of the LC circuit while sweeping the frequency of the B2field. A Q of 15 for the resonance circuit resulted in a B2field strength of at least 4 Gmt (rotating frame) in a frequency range between 140 and 155 MHz without retuning the RF circuit. This configuration allowed ENDOR measurements to be made of hyperfine interactions up to 10 MHz at a proton Larmor frequency of 147 MHz. By using a tuned circuit with higher Q it is, of course, possible to produce higher B2 fields, but in this case the full frequency range can only be swept in segments. A pickup coil near the ENDOR coil allows continuous detection of the B2field strength. A digital attenuator is set by a computer-controlled regulation circuit reacting on the B2 field strength changes which are detected by the pickup coil, thus keeping the B2 field at an almost constant level. Sample access to the Fabry-Perot resonator is either along the axis of the cryomagnet or perpendicular to it. In the first option exchange of samples is simple and can be done at any temperature without removing the resonator unit from the bore of the cryomagnet. However, no angular variation of the sample with respect to the B. field can be performed in this option. In the second option the sample can be rotated perpendicular to the B. field, but in order to change the sample the resonator unit must be removed completely. For ENDOR experiments we use a double-modulation scheme, decoding first the 20 kHz RF frequency modulation and then the 950 Hz field modulation. The magnetic field is stabilized with a field-frequency (ff) lock. Because of the double decoding the ff lock works on the second derivative of the EPR line. Figure 3 shows a 3 mm EPR spectrum of a single crystal of y-irradiated cu-aminoisobutyric acid (13) at 100 K together with the structural formula of the radical. The dimensions of the crystal are 0.3 X 2.1 X 2.3 mm. The arrow indicates the EPR transition on which ENDOR was performed at the same temperature. The 3 mm ENDOR spectrum is depicted in Figure 4. The spectrum consists of two frequency regions, the ENDOR lines appearing at frequencies (14)
(vu = free proton Larmor frequency, Aj = hypertine coupling constant). The resonance frequency of the RF circuit was not changed during accumulation of the ENDOR spectra. The first region from 114 to 118 MHz with the RF resonance fixed at 115 MHz with a Q of 10 shows the low-frequency ENDOR transitions of the two groups of methyl protons. The spectrum was scanned four times with a time constant of 1.25 s of the field modulation lock-in decoding. Accumulation took 15 min. The B2 field strength was about 3.5 Grot. The second frequency region between 145 and 149.5 MHz shows the small hyperhne couplings around the proton Larmor frequency Vn = 147.25 MHz. The B2field strength reached values of about 5 Gmt when the resonance frequency of the RF circuit with a Q of 15 was set at 147 MHz. Accumulation of this spectrum took 60 min for 16 scans. The 20 kHz fm modulation was set at +-40 kHz deviation in both plots. Because of the limited variation range of the
COMMUNICATIONS
387
OH
FIG. 3. EPR spectrum of y-irradiated a-aminoisobutyric acid with crystal axis a I&, at 100 K and structural formula of the radical. The arrow indicates the EPR transition on which ENDOR is performed.
capacitor in the RF resonance circuit, up to now we are not able to measure the highfrequency lines of the large hyperfine couplings. In this communication no analysis of the measured hyperfme couplings in terms of molecular conformations of the free radical in the irradiated amino acid will be given (see, for instance, Ref. (13)).
mm
114 115 116 117 MHz
145
146
147
148
149 MHz
FIG. 4. ENDOR spectrum of +-radiated cu-aminoisobutyric acid at 100 K. From the line positions the hypetine coupling constants, Ai, can be deduced (e.g., A, = 1.55 MHz, A2 = 2.32 MHz, A, = 3.08 MHz, Ad = 63.46 MHz).
388
COMMUNICATIONS
In summary, the 3 mm EPR spectrometer with a Fabry-Perot resonator was successfully extended to an ENDOR spectrometer with the free proton Larmor frequency as high as 147 MHz. The open structure and the large dimensions of the Fabry-Perot resonator as compared with a cylindrical cavity offer a wide field of new applications. As a first test example, a single crystal of y-irradiated a-aminoisobutyric acid has been investigated. For the Fabry-Perot resonator light irradiation with light pipes or directly focused laser sources poses no problem. The first 3 mm EPR measurements on the triplet state of pyrene-&, in single crystals of fluorene at room temperature were already successful. ACKNOWLEDGMENTS It is a pleasure to acknowledge the important contributions by Dr. E. Haindl (now at Thyssen, Munich) who was involved in earlier stages of our 3 mm EPR/ENDOR project. The expert machine shop work by Mr. J. Claus was essential for the final successof this endeavor, which is highly appreciated. We thank Drs. F. Lendzian, M. Plato, and C. J. Winscom for many helpful discussions. We also thank Dr. A. L. Maniero (University of Padova) for the single crystals of r-irradiated cY-aminoisobutyric acid and Dr. C. v. Borczyskowski (FUB) for the pyrene-&,/tluorene-mixed crystals. Support of this work by the Deutsche Forschungsgemeinschah is gratefully acknowledged. REFERENCES 1. 0. YA. GRINBERG, A. A. DUBINSKII, ANDYA. S. LEBEDEV, Russ. Chem. Rev. (Engl Transl.) 52,850 (1983). 2. R. L. BELFORD, R. B. CLARKSON, J. B. CORNELIUS, K. S. ROTHENBERGER, M. J. NILGES, AND M. D. TIMKEN, in “Electronic Magnetic Resonance of the Solid State” (J. A. Weil, Ed.), p. 2 1, The Canadian Society for Chemistry, Ottawa, 1987. 3. Yu. D. TSVETKOV, V. I. GULIN, S. A. DIKANOV, AND I. A. GRIGOR’EV, in “Electronic Magnetic Resonance of the Solid State” (J. A. We& Ed.), p. 45, The Canadian Society for Chemistry, Ottawa, 1987. 4. YA. S. LEBEDEV, J. Chem. Sot. D. I. Mendelejev 30,187 (1985). 5. E. HAINDL, K. MC~BIUS,AND H. OLOFF, Z. Naturforsch. A 40, 169 (1985). 6. B. LYNCH, K. A. EARLE, AND J. H. FREED, Rev. Sci. Instrum., in press. 7. C. P. POOLE, JR., “Electron Spin Resonance,” 2nd ed., p. 404, Wiley, New York, 1983. 8. E. E. BUDZINSKI, W. R. POTTER, AND H. C. Box, J. Chem. Phys. 70,132O (1979). 9. 0. BURGHAUS, E. HAINDL, M. PLATO, AND K. M~~BIUS,J. Phys. E 18,294 (1985). 10. G. H. RIST AND J. S. HYDE, J. Chem. Phys. 49,2449 (1968); SO, 4532 (1969); 52,4633 (1970). Il. I. AMITY, Rev. Sci. Instrum. 41,1492 (1970). 12. U. HARBARTH, J. KOWALSKI, R. NEUMANN, S. NOEHTE, K. SCHEFFZEK, AND G. zu PUTLITZ, J. Phys. E 20,409 (I 987). 13. J. W. WELLS AND H. C. Box, J. Chem. Phys. 46,2935 (1967). 14. G. FEHER, Phys. Rev. 103,834 (1956).