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EPR spectroscopy at very high field
Volume 165,number 1
EPR SPECTROSCOPY
CHEMICALPHYSICS LETTERS
5 January 1990
AT VERY HIGH FIELD
A.L. BARRA, L.C. BRLJNEL and J.B. ROBERT Service Nationaldes Champs Menses. CNRS, B.P. 166X, 38042 Grenoble Cedex, France Received 7 July 1989; in final form 19 October 1989
A very high field EPR spectrometer, which can operate at magnetic fields up to I9 T, has been constructed. The first EPR spectra of a nitroxide free radical in solution are reported. A discussion of the applications of high frequency EPR (corresponding to the far infrared) is presented.
1. Introduction
Since the early days of high resolution NMR spectroscopy, efforts have been made to build spectrometers operating at magnetic fields as high as possible, in order to increase both the sensitivity and the resolution and, because chemical shifts become more widely dispersed, allowing great simplifications in spectral analysis. Conversely, for technical and historical reasons and because EPR spectra are usually easy to analyze, EPR spectrometers still continue to operate at relatively low field, either Bz0.34 T or Bw 1.25 T. The heart of the spectrometer is the resonant cavity which contains the sample and operates in the X band (~~~9.5 GHz) or in the Q band ( V,E 35 GHz). However, EPR studies of free radicals in the 2 mm wavelength range have been reported [ 13, and Mobius [ 21 and Freed [ 31 described spectrometers which use a Fabry-Perot-type microwave resonator at 94 and 250 GHz respectively. For solid state studies, Wagner [4] built up an absorption
spectrometer
ductors [ 5 ] is presented in fig. 1. This high field EPR spectrometer does not use the typical resonant cavity arrangement. The recorded signal is the absorption of the light delivered by an optically (CO2 laser)-
which operates at a field
of 9 T. We present here the very first results obtained in liquid phase on a high field spectrometer which can operate up to 19 T, thus making an overlap between EPR and IR spectroscopy.
2. Experimental A block diagram of the spectrometer, which was initially devised for solid state studies on semicon-
Fig. 1.Block diagram ofthe EPR spectrometer. M indicates brass mirror.
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Volume 165, number 1
Table 1 Wavelengths and frequencies of selected laser lines, and magnetic field needed to reach the free electron resonance (gz 2.0023 )
B CT)
1 (w)
v (cm-‘)
Y (GHz)
x.74 15.30 18.75
1223.67 699.42 570.57
8.17 14.30 17.53
244.99 428.63 525.43
pumped far infrared waveguide laser providing emission lines at several wavelengths between 570 pm and 2 mm. The detection is performed with a germanium composite bolometer working at low temperature ( 1.4 K). The magnetic field is delivered by a resistive Bitter coil system, which may provide a field up to 19 T, with a homogeneity of about 3 G in an 8 mm diameter sphere. The magnetic field is swept from zero at a speed of about a few thousand G s-l, to reach the EPR resonance at the selected laser line frequency. In recording the spectrum the sweep rate is of the order of 20 G min-‘. The laser line wavelengths which have been used up to now are given in table I, along with the corresponding resonance frequencies (cm-‘) and the magnetic fields required to reach the electron resonance, for the case of a free electron (g,=2.0023). An additional coil superimposes an alternating component,
which may be of a few gauss, to the static magnetic field B,. Thus the derivative of the absorption signal is obtained. For liquid-phase studies, the solution is put in an 8 mm diameter and 6 mm high teflon cell. A mirror system allows to have the light propagation either parallel to B0 (Faraday configuration) or perpendicular to E0 (Voigt configuration). The main improvement to the system which is now under study concerns the field homogeneity of the Bitter coil, which may be corrected by using additional shim coils, and a better stabilization of the electric power supply.
3. Discussion The spectra recorded at 8.7 and 15.3 T of ICY3and low2 M deoxygenated solutions of the stable free radical tempo (2,2,6,6-tetramethylpiperidine-1-oxyl) in perdeuterated toluene are shown in fig. 2. Each spectrum exhibits the expected 1.1.1 triplet due to the hyperfine coupling ( uNz 15 G) of the free electron with the spin 1 nitrogen nucleus. The peak-topeak distances within the components of the triplet are 4.5 and 5.5 G respectively. Part of the line broadening is due to coupling of the free electron with the
N-0’ Bz8.7T
5 January 1990
CHEMICAL PHYSICS LETTERS
B-15.3 T
,15G,
Fig. 2. EPR spectra of 2,2,6,6-tetramethylpiperidine-1-oxyl in perdeuterated toluene: (A) at B,,= 8.75 T and with a concentration of lo-’ M; (B) at Bo= 15.31 T and with a concentration of IO-‘M.
108
Volume 165,number 1
CHEMICALPHYSICS LETTERS
methyl and methylene ring protons, i.e. nine protons with a,=0.23 G and four protons with a,=0.39 G [ 61. An intrinsic line broadening due to the magnetic field increase can also be considered, as it clearly appears when comparing the PD-Tempone spectra recorded at 9.5 and 250 GHz at the same concentration and in the same solvent [ 3 1. The switch to stronger magnetic fields and the corresponding increase in the resonance frequency of the studied paramagnetic species offer the possibility of several kinds of new or more accurate studies. It must be stressed that the increase in the frequency which is considered here is more than one order of magnitude as compared to conventional EPR. In the solid state one may consider the obvious use of high frequency to resolve features of the g tensor and of the electron-nucleus hyperfine tensor a. The highfrequency detector provides the possibility to record the spectra of systems in which a large zero-field splitting prevents the detection of transitions with conventional EPR spectrometers. In such spectra the line splitting provides aration
information
of two unpaired
pole-dipole
electrons
on the mean septhrough
the di-
interaction.
In the liquid state, the order of magnitude increase in field, even if the resolution remains lower, offers great advantages to resolve the spectra of individual radicals which are often unresolved. A particularly interesting case in recording EPR spectra at high field concerns biradicals, where the two coupled electrons have only a slightly different g factor [7]. Using
5 January 1990
NMR nomenclature the two electrons give an AB spectrum, where the inner line splitting increases as the square of the magnetic field. Another domain of interest concerns the study of the field dependence of the relaxation times T, and T2, which both depend upon the product of the measured frequency and of the reorientational correlation time. By recording the spectra of labelled species at different frequencies between 35 (B,,=0.3 T) and 500 GHz (B,- 17.8 T) one explores the spectral density function J(w) over a wide frequency range and this may provide information concerning molecular motions at a slow rate [ 81.
References
[ I ] O.Ya. Grynberg, A.A. Dubinskii and Ya.S. Lebedev, Uspek. Khim. 52 (1983) 1490;Russian Chem. Rev. 52 (1983) 850. [2] E. Haindl, K. Mijbius and H. Oloff, Z. Naturfotsch. 40a (1985) 169. 13) B. Lynch, K.A. Earle and J.H. Freed, Rev. Sci. Instr. 59 (1988) 1345. [ 41 R.J. Vagnerand A.M.W&e, Solid StateCommun. 32 ( 1979) 399. [5] F. Muller, L.C. Brunei, M. Grynberg, J. Blinowski and G. Martinez, Europhys. Letters 8 (1989) 291; F. Muller,M.A. Hopkins, L.C. Brunei,N. Coron, M. Grynbcrg and G. Martinez, Rev. Sci. Instr., in press. [6] R. Briire, H. Lemaire, A. Rassat and A. Rousseau, Bull.Sot. Chim. France (1967) 4479. [ 7 ] G.R. Eaton and S.S. Eaton, Accounts Chem. Res. 2 I ( 1988) 107. [S] L.J. Berliner, ed., Spin labelling theory and applications (Academic Press, New York, 1976).
109
EPR SPECTROSCOPY
CHEMICALPHYSICS LETTERS
5 January 1990
AT VERY HIGH FIELD
A.L. BARRA, L.C. BRLJNEL and J.B. ROBERT Service Nationaldes Champs Menses. CNRS, B.P. 166X, 38042 Grenoble Cedex, France Received 7 July 1989; in final form 19 October 1989
A very high field EPR spectrometer, which can operate at magnetic fields up to I9 T, has been constructed. The first EPR spectra of a nitroxide free radical in solution are reported. A discussion of the applications of high frequency EPR (corresponding to the far infrared) is presented.
1. Introduction
Since the early days of high resolution NMR spectroscopy, efforts have been made to build spectrometers operating at magnetic fields as high as possible, in order to increase both the sensitivity and the resolution and, because chemical shifts become more widely dispersed, allowing great simplifications in spectral analysis. Conversely, for technical and historical reasons and because EPR spectra are usually easy to analyze, EPR spectrometers still continue to operate at relatively low field, either Bz0.34 T or Bw 1.25 T. The heart of the spectrometer is the resonant cavity which contains the sample and operates in the X band (~~~9.5 GHz) or in the Q band ( V,E 35 GHz). However, EPR studies of free radicals in the 2 mm wavelength range have been reported [ 13, and Mobius [ 21 and Freed [ 31 described spectrometers which use a Fabry-Perot-type microwave resonator at 94 and 250 GHz respectively. For solid state studies, Wagner [4] built up an absorption
spectrometer
ductors [ 5 ] is presented in fig. 1. This high field EPR spectrometer does not use the typical resonant cavity arrangement. The recorded signal is the absorption of the light delivered by an optically (CO2 laser)-
which operates at a field
of 9 T. We present here the very first results obtained in liquid phase on a high field spectrometer which can operate up to 19 T, thus making an overlap between EPR and IR spectroscopy.
2. Experimental A block diagram of the spectrometer, which was initially devised for solid state studies on semicon-
Fig. 1.Block diagram ofthe EPR spectrometer. M indicates brass mirror.
107
Volume 165, number 1
Table 1 Wavelengths and frequencies of selected laser lines, and magnetic field needed to reach the free electron resonance (gz 2.0023 )
B CT)
1 (w)
v (cm-‘)
Y (GHz)
x.74 15.30 18.75
1223.67 699.42 570.57
8.17 14.30 17.53
244.99 428.63 525.43
pumped far infrared waveguide laser providing emission lines at several wavelengths between 570 pm and 2 mm. The detection is performed with a germanium composite bolometer working at low temperature ( 1.4 K). The magnetic field is delivered by a resistive Bitter coil system, which may provide a field up to 19 T, with a homogeneity of about 3 G in an 8 mm diameter sphere. The magnetic field is swept from zero at a speed of about a few thousand G s-l, to reach the EPR resonance at the selected laser line frequency. In recording the spectrum the sweep rate is of the order of 20 G min-‘. The laser line wavelengths which have been used up to now are given in table I, along with the corresponding resonance frequencies (cm-‘) and the magnetic fields required to reach the electron resonance, for the case of a free electron (g,=2.0023). An additional coil superimposes an alternating component,
which may be of a few gauss, to the static magnetic field B,. Thus the derivative of the absorption signal is obtained. For liquid-phase studies, the solution is put in an 8 mm diameter and 6 mm high teflon cell. A mirror system allows to have the light propagation either parallel to B0 (Faraday configuration) or perpendicular to E0 (Voigt configuration). The main improvement to the system which is now under study concerns the field homogeneity of the Bitter coil, which may be corrected by using additional shim coils, and a better stabilization of the electric power supply.
3. Discussion The spectra recorded at 8.7 and 15.3 T of ICY3and low2 M deoxygenated solutions of the stable free radical tempo (2,2,6,6-tetramethylpiperidine-1-oxyl) in perdeuterated toluene are shown in fig. 2. Each spectrum exhibits the expected 1.1.1 triplet due to the hyperfine coupling ( uNz 15 G) of the free electron with the spin 1 nitrogen nucleus. The peak-topeak distances within the components of the triplet are 4.5 and 5.5 G respectively. Part of the line broadening is due to coupling of the free electron with the
N-0’ Bz8.7T
5 January 1990
CHEMICAL PHYSICS LETTERS
B-15.3 T
,15G,
Fig. 2. EPR spectra of 2,2,6,6-tetramethylpiperidine-1-oxyl in perdeuterated toluene: (A) at B,,= 8.75 T and with a concentration of lo-’ M; (B) at Bo= 15.31 T and with a concentration of IO-‘M.
108
Volume 165,number 1
CHEMICALPHYSICS LETTERS
methyl and methylene ring protons, i.e. nine protons with a,=0.23 G and four protons with a,=0.39 G [ 61. An intrinsic line broadening due to the magnetic field increase can also be considered, as it clearly appears when comparing the PD-Tempone spectra recorded at 9.5 and 250 GHz at the same concentration and in the same solvent [ 3 1. The switch to stronger magnetic fields and the corresponding increase in the resonance frequency of the studied paramagnetic species offer the possibility of several kinds of new or more accurate studies. It must be stressed that the increase in the frequency which is considered here is more than one order of magnitude as compared to conventional EPR. In the solid state one may consider the obvious use of high frequency to resolve features of the g tensor and of the electron-nucleus hyperfine tensor a. The highfrequency detector provides the possibility to record the spectra of systems in which a large zero-field splitting prevents the detection of transitions with conventional EPR spectrometers. In such spectra the line splitting provides aration
information
of two unpaired
pole-dipole
electrons
on the mean septhrough
the di-
interaction.
In the liquid state, the order of magnitude increase in field, even if the resolution remains lower, offers great advantages to resolve the spectra of individual radicals which are often unresolved. A particularly interesting case in recording EPR spectra at high field concerns biradicals, where the two coupled electrons have only a slightly different g factor [7]. Using
5 January 1990
NMR nomenclature the two electrons give an AB spectrum, where the inner line splitting increases as the square of the magnetic field. Another domain of interest concerns the study of the field dependence of the relaxation times T, and T2, which both depend upon the product of the measured frequency and of the reorientational correlation time. By recording the spectra of labelled species at different frequencies between 35 (B,,=0.3 T) and 500 GHz (B,- 17.8 T) one explores the spectral density function J(w) over a wide frequency range and this may provide information concerning molecular motions at a slow rate [ 81.
References
[ I ] O.Ya. Grynberg, A.A. Dubinskii and Ya.S. Lebedev, Uspek. Khim. 52 (1983) 1490;Russian Chem. Rev. 52 (1983) 850. [2] E. Haindl, K. Mijbius and H. Oloff, Z. Naturfotsch. 40a (1985) 169. 13) B. Lynch, K.A. Earle and J.H. Freed, Rev. Sci. Instr. 59 (1988) 1345. [ 41 R.J. Vagnerand A.M.W&e, Solid StateCommun. 32 ( 1979) 399. [5] F. Muller, L.C. Brunei, M. Grynberg, J. Blinowski and G. Martinez, Europhys. Letters 8 (1989) 291; F. Muller,M.A. Hopkins, L.C. Brunei,N. Coron, M. Grynbcrg and G. Martinez, Rev. Sci. Instr., in press. [6] R. Briire, H. Lemaire, A. Rassat and A. Rousseau, Bull.Sot. Chim. France (1967) 4479. [ 7 ] G.R. Eaton and S.S. Eaton, Accounts Chem. Res. 2 I ( 1988) 107. [S] L.J. Berliner, ed., Spin labelling theory and applications (Academic Press, New York, 1976).
109