- Email: [email protected]
Heteronuclear isotropic mixing in liquids
Volume 163, number 4,5
CHEMICAL PHYSICS LETTERS
17 November 1989
HETERONUCLXAR ISOTROPIC MIXING IN LIQUIDS Daniel W. BEARDEN
and Larry R. BROWN
Research School of Chemistry,AustralianNational University,Canberra, ACT2601, Australia Received 10 July 1989
We report on a new experimental NMR technique for heteronuclear polarization transfer in the rotating frame which utilizes a WALTZ 16 decoupling sequence applied synchronously to both nuclear species to effect magnetization transfer among J-coupled spins. The technique increases the tolerance to Hartmann-Hahn mismatch between coupled spins and provides easily phased one-
bond correlation spectra. Lang-rangeconnectivities can also be elucidated with longermixingtimes.
1. Introduction Heteronuclear dipolar cross-polarization (CP) is used in many solid-state NMR experiments to tmnsfer magnetization from an abundant nuclear species to a less abundant species in order to enhance the sensitivity of the less abundant species [ 11. The analogous heteronuclear experiment in liquid-state NMR, commonly referred to as J-cross-polarization (JCP) [2-41, has been used with limited success primarily because of the difficulties associated with matching the Hartmann-Hahn [5] condition well enough to ensure efficient polarization transfer. We report a modified version of the JCP experiment which allows for efficient heteronuclear polarization transfer over one-bond and multiple-bond couplings with much simpler experimental constraints. Magnetization in a liquid-state coupled spin system can be transferred coherently through two primary methods which have a common origin in the indirect- (or J- or scalar-) coupling between nuclei [ 3 1. The first method depends on pulse-interrupted free precession. During a free-precession period, antiphase magnetization evolves from single-quantum magnetization due to the scalar coupling between nuclei. With a suitable combination of pulses, this magnetization is transferred from one nucleus to another and, after another free-precession period to refocus antiphase magnetization, single-quantum magnetization is observed. This technique is the basis for many of the commonly used homonuclear and het432
eronuclear experiments, i.e. COSY, HETCOR, etc. Because of the nature of pulse-interrupted freeprecession experiments, several drawbacks compromise application of the technique, especially in experiments designed to detect long-range couplings. For example, some experiments result in mixed-phase peaks, which are difficult to interpret, and/or antiphase peaks, which require good resolution and narrow linewidths to be observable. In addition, the time dependence of the magnetization transfer function can depend quite strongly on the topology of the scalar coupling network and may have multiple zero crossings which would lead to very different transfer efftciencies within the network [ 61. The second method requires a pulse sequence in which the only effective term in the average Hamiltonian is related to the scalar coupling, JI-S, due to the elimination of all chemical shift terms in the effective Hamiltonian. In this case, a very useful feature is that inphase periodic transfer of magnetization between Cartesian spin components can be obtained [ 7-10 1. The magnetization transfer function is much better behaved than in pulse-interrupted free-precession experiments, and usually does not have multiple zero crossings as a function of the mixing time. As a result, the transfer efficiency within a scalar coupled network is more uniform. This is the basis of transfer in homonuclear TOCSY or HOHAHA experiments and in heteronuclear JCP experiments. In conventional JCP experiments on liquid samples, the mixing is accomplished by continuous-wave
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume
163, number
4,5
CHEMICAL PHYSICS LETTERS
17 November 1989
rf irradiation applied simultaneously to the coupled nuclei. Consequently, offset effects are very impor-
tant and the efficient transfer of magnetization is limited to a relatively narrow range of offsets. This is due to the smallness of the J coupling constant compared to the effective field in the rotating frame. In addition, the smallness of J means that the Hartmann-Hahn matching condition is difftcult to achieve experimentally [ 7,11,12], and as a consequence, the t-f power levels must be adjusted very precisely and be very stable. Some techniques have been devised to overcome this difficulty. For example [ Ill, the amplitude of one or both rf fields can be adiabatically swept with the goal of matching the transfer at some point in the mixing sequence. In homonuclear experiments, however, the difficulty of matching the Hartmann-Hahn condition has been overcome by the use of phase-modulated continuous-wave rf fields which give broader spectral coverage and compensate for rf inhomogeneity and pulse imperfections. These pulse sequences give an effective Hamiltonian which is very close to the full J1.S term resulting in what has been called the isotropic mixing Hamiltonian. Although it is difficult to state whether practical pulse sequences can be designed to give pure isotropic mixing [ 131 and it is not clear how to eliminate certain artifacts and imperfections, quite good homonuclear results can be obtained with MLEV-16, MLEV-17, and WALTZ-16 mixing sequences. We have verified experimentally that heteronuclear polarization transfer in liquids can be done efficiently by using phase-modulated continuous-wave mixing sequences. This verifies suggestions made previously by various workers [ 14,151.
2. Experimental details and results
DECOUPLING
,aC
1
(
1
WALTZ-16
L
h r\ r\ n *,
UVV” Fig. 1. The WHIM pulse sequence. The phase cycle consists of incrementing the phase of the first ‘H pulse and the receiver by 180” on alternate scans. Ta obtain quadrature in w,, a separate experiment was run in which the phase of the first ‘H pulse was incremented by 90”. For one-dimensional experiments, t, =O and the composite 180” pulse on “C in the middle of 1, was omitted to allow easier phasing.
-10 -5 ’ H Power Level in db
0
Fig. 2. The effect of reducing the ‘H channel power level below the optimal matching condition as determined with a one-dimensionalWHIM experiment. The data were collected with both “C and ‘H carriers located on the respective “C” resonance. A mixing time of 7 ms was used and four transients were recorded for each data point. The normalized intensities are taken from absolute value spectra.
as critical in this experiment as in a conventional JCP experiment [7,11,12]. In fig.2 we show the effect of varying the ‘H channel power level over a 12 dB range below the optimal matching level. The depen-
The pulse sequence used in this experiment
is shown in fig. 1. We found that the best results in terms of spectral coverage and signal phase were obtained with a WALTA- 16 mixing sequence. MLEV-16 gives slightly reduced spectral coverage and MLEV- 17 gave unacceptably poor results. The modulation of the rf is synchronous on both of the channels in this experiment, and the power level on one of the channels must be adjusted to give the matching required for polarization transfer. However, this adjustment is not
dence on power level is not sharply peaked near the optimum matching level and it is still possible to transfer magnetization over one-bond Jeu couplings when the mismatch in spin-locking levels is greater than a factor of 2. This is in sharp contrast to a regular JCP experiment where the level matching is notoriously difficult and is usually obtained by adjusting an attenuator in 0.1 dB steps on one of the channels [ 111. Additionally, it is not necessary to use a double-tuned, single-coil probe to match rflev433
Volume163,number 4,5
17November 1989
CHEMICAL PHYSICSLETTERS
for resonance offsets from the carrier of 40 to 50 ppm. This demonstrates again the good tolerance to offset effects which is possible with this sequence. In all cases the phase of the WHIM spectrum was readily adjusted to pure absorption mode. We have obtained full 2D spectra of dihydroresorcinol for both short and long mixing times to verify the efficacy of this experiment. Selected traces parallel to the w, axis are shown in fig. 3. The traces taken at the positions of the A, B, C and D (figs. 3a3d) carbons show unambiguous connectivities to their respective hydrogens even though the C hydrogen involves a ‘5 coupling over oxygen. Although traces are not shown, the carbonyl line showed no cross peaks at this mixing time and residual solvent lines were completely suppressed. Our experience with WHIM on a larger organic molecule and on a small polypeptide show that spectral coverage is sufficient to determine one-bond connectivities over the entire carbon and hydrogen range using the strength of spin-lock field we were able to achieve (yB,/ 2n= 13 kHz). The short mixing-time traces at the position of the A and I3 carbons (figs. 3a,3b) show additional small cross peaks indicating that strong homonuclear coupling has caused a simultaneous transfer from A to B and B to A. Cross peaks of this nature provide additional information which may be useful for obtaining total spectral correlation of molecules. The long mixing-time trace through the po-
cls throughout the sample, although that might improve the sensitivity of the experiment. All of our measurements were done on a standard Varian 5 mm indirect detection probe which has separate coils for ‘H and 13C. No modifications were required in the VXR-500 spectrometer hardware. In order to determine the efficiency of the polarization transfer. we performed some 1D experiments on a concentrated sample of dihydroresorcinol (Aldrich C 10, 160-5 ) in CDC13.The results are shown in table 1. For a short mixing time (7 ms) corresponding approximately to the maximum transfer over onebond couplings, the WALTZ-l 6 heteronuclear isotropic mixing (WHIM) sequence gives intensities comparable to those obtained in a continuously decoupled 13Cspectrum or in a standard ID HETCOR [ 161 experiment. For the longer mixing times (70 ms) required to transfer magnetization through long-
range couplings and with the 13Ccarrier frequency set on the carbonyl resonance, spectra recorded with the WHIM sequence showed about SO-90% of the carbonyl intensity displayed in the one-pulse spectrum (table 1). Because of the complicated nature of the long-range transfer as a function of the mixing time, this may not correspond to the maximum possible intensity in the experiment. When the “C transmitter was moved 500 Hz up-field (4 ppm), essentially the same transfer efficiency was obtained, and we have observed substantial long-range transfer Table I Comparison of intensities obtained from ID experiments Experiment *J
‘3C transmitter position
“C spectrum
“c”
HETCOR WHIM
“c” “C”
“C Spectrum
carbonyl carbonyl
WHIM WHIM
carbonyl+ 500 Hz
Mixing time (ms)
Carbon line intensities b, (shift in ppm) carbonyl
A
B
C
D
(193)
(21)
(32)
(104)
(128)
16
28
62
34
61
8 13
36 49
58 66
65 74
7.1
71.4 71.4
29 24 27
a) The “C spectrumwasa one-pulse spectrum collected with continuous WALTZ decoupling. The HETCOR experiment was collecred with a nominal JCH of 140 Hz. HETCOR and WHIM experiments were collected as difference experiments to suppress natural ‘C magnetization. All experiments were collected with 32 transients and a 90” pulse width on both channels of 18.3 ps. The ‘H transmitter (decoupler) was placed on the C proton line at 5.4 ppm in all experiments. b, The arbitrary-unit intensities are peak intensities from absolute value mode spectra. No difference was observed in the linewidths. Assignments for the 13Cresonances are shown in fig. 3.
434
Volume 163.number4,5
CHEMICAL PHYSICSLETTERS A B
b)
Several variations on the WHIM sequence are easy to visualize, although we have only experimented with the pulse sequence shown in fig. 1. For example, the pulse sequence can be “turned around” to transfer magnetization from a labeled 13Csite to the associated ‘H coupling network. The WHIM sequence could also be incorporated into an indirectdetection scheme to take advantage of the higher sensitivity of hydrogen spectroscopy [ 17 1.
c)
3. Conclusions
I3 fi
,,AJ% D
I
’A_
A carbon
al
I ,J_
B c&ml w
ccarbon
7
6
.
J\
5
4
3
2
17November 1989
PPM
Fig. 3. Selectedtraces parallelto w, from two-dimensionalWHIM experiments.Traces (a) through (d) are fmm an experimentwith a 7.1 ms mixing time and with the “C carrier in the middle of the 13Cspectrum (= 108ppm). The traces are taken at (a) 21, (b) 32, (c) 104, and (d) 128 ppm in the 13Cspectrum. Trace (e) is taken at 193ppm from an experiment with a 7 1.4ms mixing time and withthe “C carrier on the carbonylresonance ( z 193 ppm). In both experiments, the ‘H carrier was located in the middle of the ‘H spectrum (~4.5 ppm). The data were collected to produce a phase sensitive data set in both dimensions as described in the caption to fig. 1. A total of I6 transients were recorded for each f, increment and a recycledelayof 2.5 s was used. In each experiment the data sets were collected with 1024data points in the t2dimension and 256 data points in the t, dimension and were subsequentlyzero-filledonce to 2048by 512.The arrows in (a) and (b) indicate small cross peaks which are a result of homonuclear coupling.The structure and assignmentsfor dihydroresorcinol are also shown.
sition corresponding to the carbonyl line (fig. 3e) shows long-range connectivities to the A, B, and C hydrogens and demonstrates that WHIM holds promise for determining long-range ‘H-13C connectivities. This has not been shown in standard JCP spectroscopy. Unfortunately, the spectral coverage in the present form of this experiment is not sufficient to cover the entire carbon spectrum for longrange couplings.
In summary, we have shown that heteronuclear polarization transfer can be easily accomplished by heteronuclear isotropic mixing achieved through WALTZ- 16 modulation applied simultaneously to both nuclear species. The previously outlined difficulties associated with pulse-interrupted free-precession experiments, which are avoided in isotropic mixing experiments, provide motivation to seek still better mixing sequences. The greatly reduced sensitivity to Hartmann-Hahn mismatch, the ability to obtain in-phase magnetization transfer, and the possibility of elucidating long-range couplings may make heteronuclear isotropic mixing an attractive alternative in many pulse sequences.
References
[l] R.R. Ernst, G. Bodenhausen, A. Wokaun, Principles of nuclear magnetic resonance m one and two dimensions (Clarendon Press, Oxford, 1987). [2]R.D. Bertrand, W.B. Moniz, A.N. Garroway and G.C. Chingas,J. Am. Chem. Sot. 100 (1978) 5227. [3] L. Muller and R.R. Ernst, Mol. Phys. 38 (1979) 963. [4] G.C. Chingas, A.N. Garroway, R.D. Bertrand and W.B. Moniz, J. Chem. Phys. ?4 ( 1981) 127. [ 51S.R.Harttnann and EL. Hahn, Phys. Rev. 128(1962) 2042. [ 61 L.R. Brownand J. Bremer,J. Magn.Reson. 68 (1986) 217. [ 71A.A. Maudsley,L. Muller and R.R. Ernst, J. Magn. Reson. 28 (1977) 463. [8] L. Braunschweiler and R.R. Ernst, J. Magn. Reson. 53 (1983) 521. [9] N. Chandrakumar,J. Magn. Reson. 67 (1986) 457. [ 101R. Bavo and J. Boyd,J. Magn. Reson. 75 (1987) 452. [ 111G.C. Chingas,AX. Garroway and W.B. Moniz, in: Topics in carbon-13 NMR spectroscopy, Vol. 4, ed. G.C. Levy (Wiley, NewYork, 1984) p. 159.
435
Volume 163,number 4,5
CHEMICALPHYSICSLETTERS
17November I989
[ 121G.C. Chingas, A.N. Garroway, R.D. Bertrand and W.B.
[ 15] P.B. Barker, A.J. Shaka and R. Freeman, J. Magi. Reson.
Moniz, J. Magn Reson. 35 (1979) 283. [13) J.S. Waugh,J. Magn. Reson. 68 (1986) 189. [ 141D.P. Weitekamp,J.R. Garbow and A. Pines, J. Chem. Phys. 77 (1982) 2870.
65 (1985) 535. [16]A.A. Maudsley and R.R. Ernst, Chem. Phys. Ietters 50 (1977) 368. [ 171N. Chandrakumarand K. Nagayama,Chem. Phys. Letters 133 (1987) 288.
436
CHEMICAL PHYSICS LETTERS
17 November 1989
HETERONUCLXAR ISOTROPIC MIXING IN LIQUIDS Daniel W. BEARDEN
and Larry R. BROWN
Research School of Chemistry,AustralianNational University,Canberra, ACT2601, Australia Received 10 July 1989
We report on a new experimental NMR technique for heteronuclear polarization transfer in the rotating frame which utilizes a WALTZ 16 decoupling sequence applied synchronously to both nuclear species to effect magnetization transfer among J-coupled spins. The technique increases the tolerance to Hartmann-Hahn mismatch between coupled spins and provides easily phased one-
bond correlation spectra. Lang-rangeconnectivities can also be elucidated with longermixingtimes.
1. Introduction Heteronuclear dipolar cross-polarization (CP) is used in many solid-state NMR experiments to tmnsfer magnetization from an abundant nuclear species to a less abundant species in order to enhance the sensitivity of the less abundant species [ 11. The analogous heteronuclear experiment in liquid-state NMR, commonly referred to as J-cross-polarization (JCP) [2-41, has been used with limited success primarily because of the difficulties associated with matching the Hartmann-Hahn [5] condition well enough to ensure efficient polarization transfer. We report a modified version of the JCP experiment which allows for efficient heteronuclear polarization transfer over one-bond and multiple-bond couplings with much simpler experimental constraints. Magnetization in a liquid-state coupled spin system can be transferred coherently through two primary methods which have a common origin in the indirect- (or J- or scalar-) coupling between nuclei [ 3 1. The first method depends on pulse-interrupted free precession. During a free-precession period, antiphase magnetization evolves from single-quantum magnetization due to the scalar coupling between nuclei. With a suitable combination of pulses, this magnetization is transferred from one nucleus to another and, after another free-precession period to refocus antiphase magnetization, single-quantum magnetization is observed. This technique is the basis for many of the commonly used homonuclear and het432
eronuclear experiments, i.e. COSY, HETCOR, etc. Because of the nature of pulse-interrupted freeprecession experiments, several drawbacks compromise application of the technique, especially in experiments designed to detect long-range couplings. For example, some experiments result in mixed-phase peaks, which are difficult to interpret, and/or antiphase peaks, which require good resolution and narrow linewidths to be observable. In addition, the time dependence of the magnetization transfer function can depend quite strongly on the topology of the scalar coupling network and may have multiple zero crossings which would lead to very different transfer efftciencies within the network [ 61. The second method requires a pulse sequence in which the only effective term in the average Hamiltonian is related to the scalar coupling, JI-S, due to the elimination of all chemical shift terms in the effective Hamiltonian. In this case, a very useful feature is that inphase periodic transfer of magnetization between Cartesian spin components can be obtained [ 7-10 1. The magnetization transfer function is much better behaved than in pulse-interrupted free-precession experiments, and usually does not have multiple zero crossings as a function of the mixing time. As a result, the transfer efficiency within a scalar coupled network is more uniform. This is the basis of transfer in homonuclear TOCSY or HOHAHA experiments and in heteronuclear JCP experiments. In conventional JCP experiments on liquid samples, the mixing is accomplished by continuous-wave
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
Volume
163, number
4,5
CHEMICAL PHYSICS LETTERS
17 November 1989
rf irradiation applied simultaneously to the coupled nuclei. Consequently, offset effects are very impor-
tant and the efficient transfer of magnetization is limited to a relatively narrow range of offsets. This is due to the smallness of the J coupling constant compared to the effective field in the rotating frame. In addition, the smallness of J means that the Hartmann-Hahn matching condition is difftcult to achieve experimentally [ 7,11,12], and as a consequence, the t-f power levels must be adjusted very precisely and be very stable. Some techniques have been devised to overcome this difficulty. For example [ Ill, the amplitude of one or both rf fields can be adiabatically swept with the goal of matching the transfer at some point in the mixing sequence. In homonuclear experiments, however, the difficulty of matching the Hartmann-Hahn condition has been overcome by the use of phase-modulated continuous-wave rf fields which give broader spectral coverage and compensate for rf inhomogeneity and pulse imperfections. These pulse sequences give an effective Hamiltonian which is very close to the full J1.S term resulting in what has been called the isotropic mixing Hamiltonian. Although it is difficult to state whether practical pulse sequences can be designed to give pure isotropic mixing [ 131 and it is not clear how to eliminate certain artifacts and imperfections, quite good homonuclear results can be obtained with MLEV-16, MLEV-17, and WALTZ-16 mixing sequences. We have verified experimentally that heteronuclear polarization transfer in liquids can be done efficiently by using phase-modulated continuous-wave mixing sequences. This verifies suggestions made previously by various workers [ 14,151.
2. Experimental details and results
DECOUPLING
,aC
1
(
1
WALTZ-16
L
h r\ r\ n *,
UVV” Fig. 1. The WHIM pulse sequence. The phase cycle consists of incrementing the phase of the first ‘H pulse and the receiver by 180” on alternate scans. Ta obtain quadrature in w,, a separate experiment was run in which the phase of the first ‘H pulse was incremented by 90”. For one-dimensional experiments, t, =O and the composite 180” pulse on “C in the middle of 1, was omitted to allow easier phasing.
-10 -5 ’ H Power Level in db
0
Fig. 2. The effect of reducing the ‘H channel power level below the optimal matching condition as determined with a one-dimensionalWHIM experiment. The data were collected with both “C and ‘H carriers located on the respective “C” resonance. A mixing time of 7 ms was used and four transients were recorded for each data point. The normalized intensities are taken from absolute value spectra.
as critical in this experiment as in a conventional JCP experiment [7,11,12]. In fig.2 we show the effect of varying the ‘H channel power level over a 12 dB range below the optimal matching level. The depen-
The pulse sequence used in this experiment
is shown in fig. 1. We found that the best results in terms of spectral coverage and signal phase were obtained with a WALTA- 16 mixing sequence. MLEV-16 gives slightly reduced spectral coverage and MLEV- 17 gave unacceptably poor results. The modulation of the rf is synchronous on both of the channels in this experiment, and the power level on one of the channels must be adjusted to give the matching required for polarization transfer. However, this adjustment is not
dence on power level is not sharply peaked near the optimum matching level and it is still possible to transfer magnetization over one-bond Jeu couplings when the mismatch in spin-locking levels is greater than a factor of 2. This is in sharp contrast to a regular JCP experiment where the level matching is notoriously difficult and is usually obtained by adjusting an attenuator in 0.1 dB steps on one of the channels [ 111. Additionally, it is not necessary to use a double-tuned, single-coil probe to match rflev433
Volume163,number 4,5
17November 1989
CHEMICAL PHYSICSLETTERS
for resonance offsets from the carrier of 40 to 50 ppm. This demonstrates again the good tolerance to offset effects which is possible with this sequence. In all cases the phase of the WHIM spectrum was readily adjusted to pure absorption mode. We have obtained full 2D spectra of dihydroresorcinol for both short and long mixing times to verify the efficacy of this experiment. Selected traces parallel to the w, axis are shown in fig. 3. The traces taken at the positions of the A, B, C and D (figs. 3a3d) carbons show unambiguous connectivities to their respective hydrogens even though the C hydrogen involves a ‘5 coupling over oxygen. Although traces are not shown, the carbonyl line showed no cross peaks at this mixing time and residual solvent lines were completely suppressed. Our experience with WHIM on a larger organic molecule and on a small polypeptide show that spectral coverage is sufficient to determine one-bond connectivities over the entire carbon and hydrogen range using the strength of spin-lock field we were able to achieve (yB,/ 2n= 13 kHz). The short mixing-time traces at the position of the A and I3 carbons (figs. 3a,3b) show additional small cross peaks indicating that strong homonuclear coupling has caused a simultaneous transfer from A to B and B to A. Cross peaks of this nature provide additional information which may be useful for obtaining total spectral correlation of molecules. The long mixing-time trace through the po-
cls throughout the sample, although that might improve the sensitivity of the experiment. All of our measurements were done on a standard Varian 5 mm indirect detection probe which has separate coils for ‘H and 13C. No modifications were required in the VXR-500 spectrometer hardware. In order to determine the efficiency of the polarization transfer. we performed some 1D experiments on a concentrated sample of dihydroresorcinol (Aldrich C 10, 160-5 ) in CDC13.The results are shown in table 1. For a short mixing time (7 ms) corresponding approximately to the maximum transfer over onebond couplings, the WALTZ-l 6 heteronuclear isotropic mixing (WHIM) sequence gives intensities comparable to those obtained in a continuously decoupled 13Cspectrum or in a standard ID HETCOR [ 161 experiment. For the longer mixing times (70 ms) required to transfer magnetization through long-
range couplings and with the 13Ccarrier frequency set on the carbonyl resonance, spectra recorded with the WHIM sequence showed about SO-90% of the carbonyl intensity displayed in the one-pulse spectrum (table 1). Because of the complicated nature of the long-range transfer as a function of the mixing time, this may not correspond to the maximum possible intensity in the experiment. When the “C transmitter was moved 500 Hz up-field (4 ppm), essentially the same transfer efficiency was obtained, and we have observed substantial long-range transfer Table I Comparison of intensities obtained from ID experiments Experiment *J
‘3C transmitter position
“C spectrum
“c”
HETCOR WHIM
“c” “C”
“C Spectrum
carbonyl carbonyl
WHIM WHIM
carbonyl+ 500 Hz
Mixing time (ms)
Carbon line intensities b, (shift in ppm) carbonyl
A
B
C
D
(193)
(21)
(32)
(104)
(128)
16
28
62
34
61
8 13
36 49
58 66
65 74
7.1
71.4 71.4
29 24 27
a) The “C spectrumwasa one-pulse spectrum collected with continuous WALTZ decoupling. The HETCOR experiment was collecred with a nominal JCH of 140 Hz. HETCOR and WHIM experiments were collected as difference experiments to suppress natural ‘C magnetization. All experiments were collected with 32 transients and a 90” pulse width on both channels of 18.3 ps. The ‘H transmitter (decoupler) was placed on the C proton line at 5.4 ppm in all experiments. b, The arbitrary-unit intensities are peak intensities from absolute value mode spectra. No difference was observed in the linewidths. Assignments for the 13Cresonances are shown in fig. 3.
434
Volume 163.number4,5
CHEMICAL PHYSICSLETTERS A B
b)
Several variations on the WHIM sequence are easy to visualize, although we have only experimented with the pulse sequence shown in fig. 1. For example, the pulse sequence can be “turned around” to transfer magnetization from a labeled 13Csite to the associated ‘H coupling network. The WHIM sequence could also be incorporated into an indirectdetection scheme to take advantage of the higher sensitivity of hydrogen spectroscopy [ 17 1.
c)
3. Conclusions
I3 fi
,,AJ% D
I
’A_
A carbon
al
I ,J_
B c&ml w
ccarbon
7
6
.
J\
5
4
3
2
17November 1989
PPM
Fig. 3. Selectedtraces parallelto w, from two-dimensionalWHIM experiments.Traces (a) through (d) are fmm an experimentwith a 7.1 ms mixing time and with the “C carrier in the middle of the 13Cspectrum (= 108ppm). The traces are taken at (a) 21, (b) 32, (c) 104, and (d) 128 ppm in the 13Cspectrum. Trace (e) is taken at 193ppm from an experiment with a 7 1.4ms mixing time and withthe “C carrier on the carbonylresonance ( z 193 ppm). In both experiments, the ‘H carrier was located in the middle of the ‘H spectrum (~4.5 ppm). The data were collected to produce a phase sensitive data set in both dimensions as described in the caption to fig. 1. A total of I6 transients were recorded for each f, increment and a recycledelayof 2.5 s was used. In each experiment the data sets were collected with 1024data points in the t2dimension and 256 data points in the t, dimension and were subsequentlyzero-filledonce to 2048by 512.The arrows in (a) and (b) indicate small cross peaks which are a result of homonuclear coupling.The structure and assignmentsfor dihydroresorcinol are also shown.
sition corresponding to the carbonyl line (fig. 3e) shows long-range connectivities to the A, B, and C hydrogens and demonstrates that WHIM holds promise for determining long-range ‘H-13C connectivities. This has not been shown in standard JCP spectroscopy. Unfortunately, the spectral coverage in the present form of this experiment is not sufficient to cover the entire carbon spectrum for longrange couplings.
In summary, we have shown that heteronuclear polarization transfer can be easily accomplished by heteronuclear isotropic mixing achieved through WALTZ- 16 modulation applied simultaneously to both nuclear species. The previously outlined difficulties associated with pulse-interrupted free-precession experiments, which are avoided in isotropic mixing experiments, provide motivation to seek still better mixing sequences. The greatly reduced sensitivity to Hartmann-Hahn mismatch, the ability to obtain in-phase magnetization transfer, and the possibility of elucidating long-range couplings may make heteronuclear isotropic mixing an attractive alternative in many pulse sequences.
References
[l] R.R. Ernst, G. Bodenhausen, A. Wokaun, Principles of nuclear magnetic resonance m one and two dimensions (Clarendon Press, Oxford, 1987). [2]R.D. Bertrand, W.B. Moniz, A.N. Garroway and G.C. Chingas,J. Am. Chem. Sot. 100 (1978) 5227. [3] L. Muller and R.R. Ernst, Mol. Phys. 38 (1979) 963. [4] G.C. Chingas, A.N. Garroway, R.D. Bertrand and W.B. Moniz, J. Chem. Phys. ?4 ( 1981) 127. [ 51S.R.Harttnann and EL. Hahn, Phys. Rev. 128(1962) 2042. [ 61 L.R. Brownand J. Bremer,J. Magn.Reson. 68 (1986) 217. [ 71A.A. Maudsley,L. Muller and R.R. Ernst, J. Magn. Reson. 28 (1977) 463. [8] L. Braunschweiler and R.R. Ernst, J. Magn. Reson. 53 (1983) 521. [9] N. Chandrakumar,J. Magn. Reson. 67 (1986) 457. [ 101R. Bavo and J. Boyd,J. Magn. Reson. 75 (1987) 452. [ 111G.C. Chingas,AX. Garroway and W.B. Moniz, in: Topics in carbon-13 NMR spectroscopy, Vol. 4, ed. G.C. Levy (Wiley, NewYork, 1984) p. 159.
435
Volume 163,number 4,5
CHEMICALPHYSICSLETTERS
17November I989
[ 121G.C. Chingas, A.N. Garroway, R.D. Bertrand and W.B.
[ 15] P.B. Barker, A.J. Shaka and R. Freeman, J. Magi. Reson.
Moniz, J. Magn Reson. 35 (1979) 283. [13) J.S. Waugh,J. Magn. Reson. 68 (1986) 189. [ 141D.P. Weitekamp,J.R. Garbow and A. Pines, J. Chem. Phys. 77 (1982) 2870.
65 (1985) 535. [16]A.A. Maudsley and R.R. Ernst, Chem. Phys. Ietters 50 (1977) 368. [ 171N. Chandrakumarand K. Nagayama,Chem. Phys. Letters 133 (1987) 288.
436