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Spin energy transfer in nuclear magnetic resonance spectroscopy using a zero magnetic field and spin echoes resulting from pulsing the magnetic field to zero
JOURNAL
OF
MAGNETIC
RESONANCE
33,
205-208
(1979)
COMMUNICATIONS Spin Energy Transfer in Nuclear Magnetic Resonance Spectroscopy Using a Zero Magnetic Field and Spin Echoes Resulting from Pulsing the Magnetic Field to Zero An early nuclear magnetic resonance experiment demonstrated a general procedure for affecting the spin population distribution of one set of nuclei by allowing the spin system to mix with another spin system. The experiment mixed the spin populations of ‘Li and r9F in a crystal of LiF by removing the sample momentarily from the magnet gap (I). The dipole-dipole coupling mechanism was used to explain the mixing of the unlike spin systems at zero field (2,3). Under ordinary highresolution conditions, large changes in intensity are not observed in proton spectra when other peaks in the spectrum are irradiated, except in systems where there is chemical exchange, or in experiments involving spin decoupling. There are exceptions to this generalization. Changes in proton intensity have been reported (4-7); however, these methods are not generally applicable to common liquid samples. Reported here are the preliminary results of an experiment which mixed unlike proton spin systems in a liquid sample by pulsing the magnetic field to zero. The experiment was conducted on a Varian EM300 spectrometer, which operates at 7046 gauss. The instrument was converted from a field sweep to a sawtooth frequency sweep by generating externally the audio sideband. During an interval in one sweep, the amplitude of the sideband could be increased from approximately 0.05 milligauss to well over 0.5 milligauss in order to perform an adiabatic rapid passage on one peak, without affecting the other peaks. At a later time, the sample was removed from the probe to a location of zero field, and then returned to the probe. Approximately 0.1 set lapsed for the removal, and 0.1 set for the return. Zero field means a sufficiently small field to observe the effect. The effect was observed both in the ambient magnetic field just outside the magnet enclosure, and in the zero field produced by a shield consisting of an annealed iron cylinder. The experiment first allowed the spins to come to equilibrium in the magnetic field. An adiabatic rapid passage inverted one set of protons without disturbing the other sets. The sample was put in zero field for a few tenths of a second and returned to the probe. The intensities of all the peaks were observed by sweeping repetitively through the resonances. The repetitive sweeps through the proton resonances of toluene are shown in Fig. 1. In Trace A, the effect of the zero field pulse alone was shown to be minimal, since the time the sample spent at zero field was short compared to the spin-lattice relaxation time. Trace B shows that an adiabatic rapid passage inverting one set of spins did not significantly affect the intensities of the other spins. In Trace C, the 205
0022~2364179/010205-1)4$02.00/O Copyright @ 1979 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
206
COMMUNICATIONS
FIG. 1. Repetitive sweeps through the proton resonances of toluene. Trace A shows the effect of momentarily setting the magnetic field to zero. Trace B shows the effect of a selective adiabatic rapid passage on the ring protons. Trace C shows the effect of inverting the ring proton spins followed immediately by a zero field pulse.
sample was pulsed to zero magnetic field immediately after the ring proton spins were inverted. The large changes in intensity of both peaks demonstrate the mixing of the unlike spin systems. Plots of In&-I,) versus f of all the traces in Fig. 1 gave straight lines. The slopes gave a value of (l/ TI + 2P) equal to 0.067 set-’ for the ring protons, and 0.12 set-’ for the methyl protons. The linewidths were about 1 ppm. The same qualitative behavior was observed between the protons in methyl o-toluate, methyl m-toluate, and methyl p-toluate. The inverted proton resonance of FIG. 2. Repetitive sweeps through the proton resonance of benzene. inversion-recovery sequence, and the lower trace is the inversion-recovery resulting from a zero field pulse.
The upper trace is the traditional sequence including an echo
COMMUNICATIONS
207
7 I F
_..- __.- -- _. _ - - .. -_. ._____.__ ___ __.._ -.-. __^-__ . - -.3
COMMUNICATIONS
208
chloroform increased after pulsing the field to zero. The chlorine resonance was not observed because of instrument limitations but should have decreased. The direct effect of proton-rich solvents was examined by performing the experiment on nonassociating solvent mixtures. In solutions of benzene and cyclohexane, and of benzene and acetone, no detectable changes in intensity were observed after pulsing the field to zero. In addition to observing the mixing of spin systems at zero field, a second phenomenon was observed upon sampling the spins the first time after pulsing a nonequilibrium spin system to zero magnetic field. It appeared as a large positive peak, called an echo, instead of the negative peak usually associated with the spectrum of an inverted spin system. The time spent at zero field did not significantly affect the echo as long as it was much less than the relaxation time. The time between the return of the sample to the probe and the first sampling sweep was important. An echo appeared only when this time did not exceed several tenths of a second. Figure 2 shows repetitive sweeps through the proton resonance of a degassed benzene sample. The adiabatic rapid passage occurred at the usual place, and the zero field pulse in the lower trace occurred immediately before the echo appeared as a positive peak in the midst of the negative peaks. As long as the thermal equilibrium of the spin system was disturbed, the echo always appeared whenever the sample had been pulsed to zero field. No echo was observed after zero-field pulsing a sample with spins at thermal equilibrium, Plots of ln(L - 1,) versus t for the traces, excluding the echo, gave identical straight lines. The slope gave a value of (l/ T1 + 2P) equal to 0.56 set-‘. The intensity of the echo in the lower trace of Fig. 2 was 1.8 times greater in absolute magnitude than the negative peak which would have appeared in the absence of the zero field pulse. In the past, spin echoes were produced by any of a variety of methods of rf pulsing (B), and by other techniques such as reversing the inhomogeneity of the laboratory magnetic field (9). This experiment was similar, and in this case the pulse occurred in the laboratory magnetic field. REFERENCES 1. R. V. POUND, Whys. Rev. 81, 156 (1951). 2. E.M.PURCELLANDW.G.PROCTOR, Phys.Rev. g&279(1951). 3. A.ABRAHAMANDW.G.PROCTOR, Phys.Rev. 109,144(1958). 4. F. A. L. ANET AND A. J. R. BROWN, J. Am. Chem. Sot. 87,525O (1965). 5. R.A.BELLANDJ.K.SAUNDERS,C~~ J. Chem.46,3421(1968). 6. S.I.CHANANDG.P.KREISHMAN, J. Am. Chem.Soc.92,1102(1970). 7. P.BALARAM,A.A.BOTHNER-BY,ANDJ.DADOK, J. Am. Chem.Soc.94,4015(1972). 8. E. L. HAHN, Phys. Rev. 80, 580 (1950). 9. A. ABRAGAM, “The Principles of Nuclear Magnetism," Oxford University Press,
1961.
DONPAULMILLER Department of Chemistry Washburn University Topeka, Kansas 66621 Received
February
22, 1978;
revised
October
10, 1978
OF
MAGNETIC
RESONANCE
33,
205-208
(1979)
COMMUNICATIONS Spin Energy Transfer in Nuclear Magnetic Resonance Spectroscopy Using a Zero Magnetic Field and Spin Echoes Resulting from Pulsing the Magnetic Field to Zero An early nuclear magnetic resonance experiment demonstrated a general procedure for affecting the spin population distribution of one set of nuclei by allowing the spin system to mix with another spin system. The experiment mixed the spin populations of ‘Li and r9F in a crystal of LiF by removing the sample momentarily from the magnet gap (I). The dipole-dipole coupling mechanism was used to explain the mixing of the unlike spin systems at zero field (2,3). Under ordinary highresolution conditions, large changes in intensity are not observed in proton spectra when other peaks in the spectrum are irradiated, except in systems where there is chemical exchange, or in experiments involving spin decoupling. There are exceptions to this generalization. Changes in proton intensity have been reported (4-7); however, these methods are not generally applicable to common liquid samples. Reported here are the preliminary results of an experiment which mixed unlike proton spin systems in a liquid sample by pulsing the magnetic field to zero. The experiment was conducted on a Varian EM300 spectrometer, which operates at 7046 gauss. The instrument was converted from a field sweep to a sawtooth frequency sweep by generating externally the audio sideband. During an interval in one sweep, the amplitude of the sideband could be increased from approximately 0.05 milligauss to well over 0.5 milligauss in order to perform an adiabatic rapid passage on one peak, without affecting the other peaks. At a later time, the sample was removed from the probe to a location of zero field, and then returned to the probe. Approximately 0.1 set lapsed for the removal, and 0.1 set for the return. Zero field means a sufficiently small field to observe the effect. The effect was observed both in the ambient magnetic field just outside the magnet enclosure, and in the zero field produced by a shield consisting of an annealed iron cylinder. The experiment first allowed the spins to come to equilibrium in the magnetic field. An adiabatic rapid passage inverted one set of protons without disturbing the other sets. The sample was put in zero field for a few tenths of a second and returned to the probe. The intensities of all the peaks were observed by sweeping repetitively through the resonances. The repetitive sweeps through the proton resonances of toluene are shown in Fig. 1. In Trace A, the effect of the zero field pulse alone was shown to be minimal, since the time the sample spent at zero field was short compared to the spin-lattice relaxation time. Trace B shows that an adiabatic rapid passage inverting one set of spins did not significantly affect the intensities of the other spins. In Trace C, the 205
0022~2364179/010205-1)4$02.00/O Copyright @ 1979 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain
206
COMMUNICATIONS
FIG. 1. Repetitive sweeps through the proton resonances of toluene. Trace A shows the effect of momentarily setting the magnetic field to zero. Trace B shows the effect of a selective adiabatic rapid passage on the ring protons. Trace C shows the effect of inverting the ring proton spins followed immediately by a zero field pulse.
sample was pulsed to zero magnetic field immediately after the ring proton spins were inverted. The large changes in intensity of both peaks demonstrate the mixing of the unlike spin systems. Plots of In&-I,) versus f of all the traces in Fig. 1 gave straight lines. The slopes gave a value of (l/ TI + 2P) equal to 0.067 set-’ for the ring protons, and 0.12 set-’ for the methyl protons. The linewidths were about 1 ppm. The same qualitative behavior was observed between the protons in methyl o-toluate, methyl m-toluate, and methyl p-toluate. The inverted proton resonance of FIG. 2. Repetitive sweeps through the proton resonance of benzene. inversion-recovery sequence, and the lower trace is the inversion-recovery resulting from a zero field pulse.
The upper trace is the traditional sequence including an echo
COMMUNICATIONS
207
7 I F
_..- __.- -- _. _ - - .. -_. ._____.__ ___ __.._ -.-. __^-__ . - -.3
COMMUNICATIONS
208
chloroform increased after pulsing the field to zero. The chlorine resonance was not observed because of instrument limitations but should have decreased. The direct effect of proton-rich solvents was examined by performing the experiment on nonassociating solvent mixtures. In solutions of benzene and cyclohexane, and of benzene and acetone, no detectable changes in intensity were observed after pulsing the field to zero. In addition to observing the mixing of spin systems at zero field, a second phenomenon was observed upon sampling the spins the first time after pulsing a nonequilibrium spin system to zero magnetic field. It appeared as a large positive peak, called an echo, instead of the negative peak usually associated with the spectrum of an inverted spin system. The time spent at zero field did not significantly affect the echo as long as it was much less than the relaxation time. The time between the return of the sample to the probe and the first sampling sweep was important. An echo appeared only when this time did not exceed several tenths of a second. Figure 2 shows repetitive sweeps through the proton resonance of a degassed benzene sample. The adiabatic rapid passage occurred at the usual place, and the zero field pulse in the lower trace occurred immediately before the echo appeared as a positive peak in the midst of the negative peaks. As long as the thermal equilibrium of the spin system was disturbed, the echo always appeared whenever the sample had been pulsed to zero field. No echo was observed after zero-field pulsing a sample with spins at thermal equilibrium, Plots of ln(L - 1,) versus t for the traces, excluding the echo, gave identical straight lines. The slope gave a value of (l/ T1 + 2P) equal to 0.56 set-‘. The intensity of the echo in the lower trace of Fig. 2 was 1.8 times greater in absolute magnitude than the negative peak which would have appeared in the absence of the zero field pulse. In the past, spin echoes were produced by any of a variety of methods of rf pulsing (B), and by other techniques such as reversing the inhomogeneity of the laboratory magnetic field (9). This experiment was similar, and in this case the pulse occurred in the laboratory magnetic field. REFERENCES 1. R. V. POUND, Whys. Rev. 81, 156 (1951). 2. E.M.PURCELLANDW.G.PROCTOR, Phys.Rev. g&279(1951). 3. A.ABRAHAMANDW.G.PROCTOR, Phys.Rev. 109,144(1958). 4. F. A. L. ANET AND A. J. R. BROWN, J. Am. Chem. Sot. 87,525O (1965). 5. R.A.BELLANDJ.K.SAUNDERS,C~~ J. Chem.46,3421(1968). 6. S.I.CHANANDG.P.KREISHMAN, J. Am. Chem.Soc.92,1102(1970). 7. P.BALARAM,A.A.BOTHNER-BY,ANDJ.DADOK, J. Am. Chem.Soc.94,4015(1972). 8. E. L. HAHN, Phys. Rev. 80, 580 (1950). 9. A. ABRAGAM, “The Principles of Nuclear Magnetism," Oxford University Press,
1961.
DONPAULMILLER Department of Chemistry Washburn University Topeka, Kansas 66621 Received
February
22, 1978;
revised
October
10, 1978