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Synchronous accumulation of polarisation. A new approach to cross polarisation under fast magic angle spinning
22 April 1994
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 221 (1994) 322-326
Synchronous accumulation of pola~sation. A new approach to cross polarisation under fast magic angle spinning R. Pratima a, K.V. Ramanathan b aDepartmentof Physics,Indian Instituteof Science, Bangalore560 012, India b SophisticatedInstrumentsFacility,Indian Instituteof Science, Bangalore560 012, India Received 17 December 1993; in final form 8 February 1994
Abstract
A new experiment to circumvent the problem of poor cross polarisation (CP) efficiency at the Hartmann-Hahn match under high-speed magic angle spinning is proposed and tested in the case of adamantane. It is based on the fact that the election transferred from the I spin to the S spin, though zero for a full rotor cycle, is finite for a part of a cycle. The polarisation acquired during a part of every rotor cycle is accumulated for several cycles to be finally observed as the signal. This is achieved by a simple modification to the regular CP experiment and has been shown to give rise to enhanced efficiency at the standard CP condition.
1. Introduction Cross polarisation (CP) combined with magic angle spinning (MAS) is a well established technique for the study of organic solids in the solid state. Use of high spinning speeds in these experiments is advantageous because there will be few spinning side bands in the spectrum due to chemical shift anisotropy and thus complications arising from the overlap of different sets of centres and sidebands are minimised. However, this can also lead to poor cross polarisation efficiency and poor signal to noise ratio in certain cases. This aspect of MAS has attracted much attention [l-4] and methods to overcome the problem have been proposed. To transfer polarisation from an abundant nucleus such as the proton (I spin) to a rare nucleus such as carbon (S spin) the energy levels of the two systems 1 and Sin the rotating frame are matched. This is the so-called Hartmann-Hahn
match [ 51 where wlr=ols with wL1=yIH1, and wLs= Y~H,,. Under this condition, the transfer of polarisation from I spins to S spins occurs due to the heteronuclear dipolar interaction, This interaction classified as the inhomogeneous interaction [ 61, averages to zero under MAS over one rotation period. However, in the presence of the much stronger proton-proton homonuclear interaction, the total I-Iamiltonian is considered to be homogeneous. Because of this, polarisation transfer under MAS takes place at low spinning speeds. However, for spinning speeds exceeding the proton linewidth, even the homonuclear interaction is averaged out and thus one does not expect polarisation transfer to occur under MAS, and the peak intensity of the rare nucleus tends to zero. This interference between MAS and CP has been studied by several workers in the field f 7- IO], They have also observed that when the Zeeman energies of the two nuclei in the rotating frame (in frequency
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0009-2614(94)00244-K
R. Pratima. K. V. Ramanathan /Chemical Physics Letters 221(1994)
units) differ by an integer multiple of the rotation frequency, cross polarisation occurs. Thus, for the usual CP contact times, the so-called CP spectrum with intensities plotted against wIs-wlz has several peaks separated by integer multiples of rotation frequency with little intensity for the centre peak corresponding to o,~-w,~=O. Attempts have been initiated so that intensity of the central peak could be increased [ l-41. This is important for carrying out routine experiments at the Hartmann-Hahn match irrespective of spinning speed. Phase and amplitude modulation of the rf have been suggested for making the CP spectrum broad-band. In particular, the rf amplitude modulation methods proposed by Hediger et al. have shown that it is possible to cross polarise with equal efficiency over a large frequency range around oiz= wIs. Many of these techniques require additional hardware which is not standard in commercial spectrometers at the present time. We propose a simple alternate approach which is based on the fact that the cross polarisation process gives rise to zero S spin magnetisation only if it is allowed to proceed through a complete rotor cycle but gives rise to a finite S spin magnetisation if it is restricted to a portion of a rotor cycle which can be accumulated over several rotor cycles.
2. Theory and experiment We shall consider MAS at high spinning speeds at the Hartmann-Hahn match and neglect homonuclear dipolar coupling. Thus, the time-dependent Hamiltonian in the doubly rotating tilted frame for a two-spin I-S system is given by [ 10,111 2fo=.7&+~~+~~~.
(1)
With the assumption of strong rf fields, the Zeeman terms in this frame are given by %z=c&zIr,
%;s=w1&
The truncated given by =%??=b(G2&S,,
*
heteronuclear
(2) dipolar
interaction
is
(3)
where the heteronuclear dipolar coupling constant b(t) is time dependent due to magic angle spinning andisgivenby [12]
b(t)
=wzs[G,
323
322-326
cos(w,t+cb)
+G, cos(2w,t+2@p)]
,
(4)
where PO YZYS
wzs= Gyg
2
G, = 4 sin%,
sin*&
G, = 4 sin(20,) ,
sin(28,)
, (5)
with rzs being the internuclear distance, 0, and @,,the polar angles defining the direction of the dipolar vector in the rotor frame, t9, the magic angle and w, the sample rotation frequency. In the thermodynamic treatment of the cross polarisation process [ lo], Xz and & are considered as two reservoirs and V= Xzs is considered as the perturbation that causes energy exchange between these two reservoirs. From Eq. (4) it is seen that Xzs averages to zero over one rotation period r, ( =2x/w,). Hence, the net transfer of polarisation over long contact times will be only due to the higher-order terms that arise from the non-negligible homonuclear dipolar interaction. This transfer process has been observed to be an order of magnitude slower for spinning speeds comparable to homonuclear dipolar linewidths [ 10 1. In this Letter, we show that with a simple moditication to the regular cross polarisation pulse sequence, substantial S spin intensity can be obtained at the Hartmann-Hahn match itself. This is based on the idea that to average Xzs to zero one full rotor cycle is required. Hence, the net polarization transferred from Z spins to S spins is zero only for a full rotor period but for a part of the rotor period one can expect a finite S spin polarisation. In the experiment that we propose, the cross polarisation process takes place only during a part of every rotor period. The schematics of the pulse scheme is shown in Fig. 1. The S spin polarisation acquired along the Y axis during part of the first rotor period is preserved in the Z direction. At the beginning of the second rotor period this is brought into spin-lock again along the Y direction and further polarisation is achieved during part of the second rotor period. This summed up polarisation is again preserved along the 2 axis. Thus multiple contacts between the Z and S spins are made till maximum magnetization is obtained which will be followed by the normal acquisition period. In the pulse scheme shown both Z and S spin magnetiza-
324
R.Pratima, K. K ~~~t~n
I Chemical Physic Letters 22lfl994) 322-326
DECOUPLE
I
(4
S
Y
ACQUISITION
Fig. 1. (a) Standard CP pulse sequence. (b) SAP pulse scheme for cross polarisation under fast magic angle spinning. tions are brought to the 2 axis after each contact. The method can be expected to work if only the S spin is
brought to the 2 axis leaving the Z spin spin-locked during the whole of the experiment since the only difference between the two experiments is the way the Z spin ma~etisation is preserved - along the static magnetic field in one case and along the spin-lock rf field in the other. We call this technique synchronous a~umulation of pola~~tion (SAP).
uration rfburst to ensure that the signals are only from cross polarised ~~etization. In Fig. 2 the behaviour of the carbon magnetization as a function of a contact time of the order of two rotor periods for three different spinning speeds in the normal CP experiment is shown. It is observed that at the lower spinning speed, the signal intensity steadily increases as a function of the contact time whereas at the higher spinning speeds, the intensity does not build up and remains relatively small. This indicates that at the lower spinning speed, the homonuclear proton-proton dipolar interaction plays an impo~nt role in the cross polarisation process whereas at the higher spinning speeds its effect is much less and at these speeds one can use the experimental approach outlined earlier. In Figs. 3 and 4 the intensity of the CH2 resonance of adamantane obtained by the normal CP and the SAP experiments are shown for two different spinning speeds, 3050 and 4050 Hz. In each case, the SAP experiment has been performed for different ratios r,, ( = r_5,1rE)of the time of contact z, in a rotor period to the total rotor period z,. For the SAP experiment, the X axis corresponds to the total time T4, used for the cross polarisation process ( nrr). Thus in this case the actual time for which the Z and the S spins are in contact with each other (nrci.) will be less than the 4.00
-
/
3. Results The experiments have been performed on a Bruker MSL-300 FTNMR spectrometer equipped with a double-bearing magic angle spinning probe. The rotor diameter is 7 mm and the maximum spinning speed achievable is 4.2 kHz. The proton and carbon rfpower levels were 40.7 kHz. The experiments were carried out mainly at two spinning speeds, 3050 and 4050 Hz. The “C spectra of a powder sample of adamantane were recorded. The intensity of the CH2 resonance was monitored. In all the experiments natural abundance 13Cmagnetization was destroyed by a sat-
0.00
0.00
1. )O
Cross
Potarisation
Time
(T,$
in ms
Fig. 2. 13CH2signal intensity of adamantane for contact times of the order of two rotor periods obtained from the standard CP experiment for spinning speeds: (A ) 3050 Hz, (*) 3600 HZ, (0) 4050 Hz.
A. Pratima. K. V. Ramanathan /Chemical Physics Letters 221(1994) 322-326
325
1°.OO /
io
0.00
O.OO 0.00
Cross Polarisation
Time (Ted in ms
Fig. 3. 13CH2signal intensity of adamantane obtained from the SAP experiment at the MAS frequency of 3050 Hz as a function of the total cross polarisation time Tcp= nr, for the following ratios rp of contact time per rotor period ( rcr) to the total rotor period (To): (+ ) 0.34, (*) 0.50 and (A) 0.87. Corresponding intensities obtained by the regular CP experiment are also indicated (0). See Fig. 1 for definition of the time periods.
/
4
0.00
~ 0.00
I
4.00 Cross Polarisation
Time (TcP, in ms’
1
0.00
4.00
1 .OO
Fig. 4. 13CH2signal intensity of adamantane obtained from the SAP experiment at the MAS frequency of 4050 Hz as a function of the total cross polarisation time T,= nz, for the following ratios r, of contact time per rotor period (T_) to total rotor period (To): (0) 0.45, (+) 0.65 and (A) 0.81. Corresponding intensities obtained by the regular CP experiment are also indicated (* ) . See Fig. 1 for definition of time the periods.
3.00 Cross Polarisation Time
6. 0 (T,J in ms
Fig. 5. Comparison of “CH2 signal intensities of adamantane for different cross polarisation times TW obtained at a MAS frequency of 4050 Hz with the SAP pulse scheme at the HartmannHahn match for r,,=O.81 (*) and the CP experiment off the Hartmann-Hahn match with w,,=w,,+w, (0).
corresponding contact time for the normal CP experiment. At 3050 Hz (Fig. 3) it is observed that the proposed SAP experiment is at best comparable to the normal CP experiment for cross polarisation times of the order of 2 to 3 ms. For longer T,,the latter experiment gives better results. However, the advantage of the SAP experiment is clearly demonstrated at the higher spinning speed of 4050 Hz (Fig. 4). For comparable contact times, the SAP experiment gives rise to a several-fold increase in intensity as compared to the normal CP experiment for all the values of rp shown. It is observed that the value of r,=O.8 gives maximum enhancement. It is to be noted that the buildup of intensity takes place much faster compared to the regular CP experiment at this MAS frequency. In Fig. 5 the buildup of intensity in the CP experiment at one of the sidebands of the CP spectrum for which wI= W,+ o, is compared with the SAP experiment at the Hartmann-Hahn match. It is observed that the final intensities and the rate at which the signals build up are comparable in both cases. 4. Conclusion The disadvantage of the standard cross polarisation experiment under high speed magic angle spin-
326
R. Pratima, K. V. Ramanathan /Chemical Physics Letters 221(1994) 322-326
ning is that the cross polarisation efficiency is poor at the Hartmann-Hahn match condition. Away from this match there are other sidebands in the CP spectrum which can give intense cross polarised signals. However, for routing use, adjustments of the rfpower levels for the Z and S spins will be tedious especially since the width of the sidebands is small and the rf inhomogeneities can adversely affect the sensitivity [ I]. The SAP experiment described here has been shown to considerably improve the efficiency of transfer of polarisation at the Hartmann-Hahn condition in the case of adamantane. The method proposed is synchronized with sample spinning. The modification to the pulse scheme is the introduction of additional 90” pulses within a rotor cycle to periodically store the acquired magnetisation in the 2 axis, which is easily implemented. We have observed that a period of contact per rotor cycle T_ which is around 80% of a rotor period r, gives rise to a good signal to noise ratio. The rate of buildup as well as the maximum signal obtainable are also comparable to those obtained at the + 1 sideband of the CP spectrum. It has been observed that other samples with strong homonuclear and heteronuclear dipolar couplings such as glycine behave similar to adamantane with low intensities for cross polarised “C signals at spinning speeds of the order 15 kHz [ 4 1. The application of the present SAP technique to such samples at these high spinning speeds appears promising. The short rotor period r, encountered at high spinning speeds may not really be a constraint since for a typical spinning speed of 20 kHz and an rf field strength of 40 kHz, an r, of 0.75 can be used giving rise to considerable enhancement of the signal intensity. It may be noted that there are similarities between the S-AMCP (q, 0) pulse scheme proposed by Hediger et al. [ 1] and the SAP scheme discussed here. It is possible to visualise the former scheme also as one in which the S magnetisation is allowed to build
up for part of a rotor cycle but without preserving the magnetisation along the Z axis as in the case of SAP. Hence, dephasing of the magnetisation during the period for which the S spin is not spin-locked may degrade the performance of the S-AMCP (q, 0) scheme in comparison to the SAP scheme. It may be noted that at slow spinning speeds the SAP experiment may give rise to intensities smaller than the regular CP experiment. Thus, depending on the spinning speed used and the nature of the sample studied, the SAP experiment may be used as complementary to the regular CP experiment.
Acknowledgement The authors would like to thank Professor C.L. Khetrapal and Professor Anil Kumar for scientific advice and constant encouragement.
References [ 1] S. Hediger, B.H. Meier and R.R. Ernst, Chem. Phys. Letters 213 (1993) 627. [2] X. Wu and K-W. Zilm, J. Magn. Reson. A 104 (1993) 154. [ 3 ] T.M. Barbara and E.H. Williams, J. Magn. Reson. 99 ( 1992) 439. [4] O.B. Peirsen, X. Wu, I. Kustanovich and SO. Smith, J. Magn. Reson. A 104 (1993) 334. [5]S.R.HartmannandE.L.Hahn,Phys.Rev. 128 (1962)2042. [6] M.M. Maricq and J.S. Waugh, J. Chem. Phys. 70 (1979) 3300. [7] E.O. Stejskal, J. Schaefer and J.S. Watt&J. Magn. Reson. 28 (1977) 105. [ 81 M. Sardashti and G.E. Maciel, J. Magn. Reson. 72 (1987) 467. [9] R.A. Wind, S.F. Dee, H. Lock and G.E. Mariel, J. Magn. Reson. 79 (1988) 136. [lo] B.H. Meier, Chem. Phys. Letters 188 (1992) 201. [ 111M.H. Levitt, D. Suter and R.R. Ernst, J. Chem. Phys. 84 (1986) 4242. [ 121 A. Schmidt and S. Vega, J. Chem. Phys. 96 (1992) 2655.
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 221 (1994) 322-326
Synchronous accumulation of pola~sation. A new approach to cross polarisation under fast magic angle spinning R. Pratima a, K.V. Ramanathan b aDepartmentof Physics,Indian Instituteof Science, Bangalore560 012, India b SophisticatedInstrumentsFacility,Indian Instituteof Science, Bangalore560 012, India Received 17 December 1993; in final form 8 February 1994
Abstract
A new experiment to circumvent the problem of poor cross polarisation (CP) efficiency at the Hartmann-Hahn match under high-speed magic angle spinning is proposed and tested in the case of adamantane. It is based on the fact that the election transferred from the I spin to the S spin, though zero for a full rotor cycle, is finite for a part of a cycle. The polarisation acquired during a part of every rotor cycle is accumulated for several cycles to be finally observed as the signal. This is achieved by a simple modification to the regular CP experiment and has been shown to give rise to enhanced efficiency at the standard CP condition.
1. Introduction Cross polarisation (CP) combined with magic angle spinning (MAS) is a well established technique for the study of organic solids in the solid state. Use of high spinning speeds in these experiments is advantageous because there will be few spinning side bands in the spectrum due to chemical shift anisotropy and thus complications arising from the overlap of different sets of centres and sidebands are minimised. However, this can also lead to poor cross polarisation efficiency and poor signal to noise ratio in certain cases. This aspect of MAS has attracted much attention [l-4] and methods to overcome the problem have been proposed. To transfer polarisation from an abundant nucleus such as the proton (I spin) to a rare nucleus such as carbon (S spin) the energy levels of the two systems 1 and Sin the rotating frame are matched. This is the so-called Hartmann-Hahn
match [ 51 where wlr=ols with wL1=yIH1, and wLs= Y~H,,. Under this condition, the transfer of polarisation from I spins to S spins occurs due to the heteronuclear dipolar interaction, This interaction classified as the inhomogeneous interaction [ 61, averages to zero under MAS over one rotation period. However, in the presence of the much stronger proton-proton homonuclear interaction, the total I-Iamiltonian is considered to be homogeneous. Because of this, polarisation transfer under MAS takes place at low spinning speeds. However, for spinning speeds exceeding the proton linewidth, even the homonuclear interaction is averaged out and thus one does not expect polarisation transfer to occur under MAS, and the peak intensity of the rare nucleus tends to zero. This interference between MAS and CP has been studied by several workers in the field f 7- IO], They have also observed that when the Zeeman energies of the two nuclei in the rotating frame (in frequency
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0009-2614(94)00244-K
R. Pratima. K. V. Ramanathan /Chemical Physics Letters 221(1994)
units) differ by an integer multiple of the rotation frequency, cross polarisation occurs. Thus, for the usual CP contact times, the so-called CP spectrum with intensities plotted against wIs-wlz has several peaks separated by integer multiples of rotation frequency with little intensity for the centre peak corresponding to o,~-w,~=O. Attempts have been initiated so that intensity of the central peak could be increased [ l-41. This is important for carrying out routine experiments at the Hartmann-Hahn match irrespective of spinning speed. Phase and amplitude modulation of the rf have been suggested for making the CP spectrum broad-band. In particular, the rf amplitude modulation methods proposed by Hediger et al. have shown that it is possible to cross polarise with equal efficiency over a large frequency range around oiz= wIs. Many of these techniques require additional hardware which is not standard in commercial spectrometers at the present time. We propose a simple alternate approach which is based on the fact that the cross polarisation process gives rise to zero S spin magnetisation only if it is allowed to proceed through a complete rotor cycle but gives rise to a finite S spin magnetisation if it is restricted to a portion of a rotor cycle which can be accumulated over several rotor cycles.
2. Theory and experiment We shall consider MAS at high spinning speeds at the Hartmann-Hahn match and neglect homonuclear dipolar coupling. Thus, the time-dependent Hamiltonian in the doubly rotating tilted frame for a two-spin I-S system is given by [ 10,111 2fo=.7&+~~+~~~.
(1)
With the assumption of strong rf fields, the Zeeman terms in this frame are given by %z=c&zIr,
%;s=w1&
The truncated given by =%??=b(G2&S,,
*
heteronuclear
(2) dipolar
interaction
is
(3)
where the heteronuclear dipolar coupling constant b(t) is time dependent due to magic angle spinning andisgivenby [12]
b(t)
=wzs[G,
323
322-326
cos(w,t+cb)
+G, cos(2w,t+2@p)]
,
(4)
where PO YZYS
wzs= Gyg
2
G, = 4 sin%,
sin*&
G, = 4 sin(20,) ,
sin(28,)
, (5)
with rzs being the internuclear distance, 0, and @,,the polar angles defining the direction of the dipolar vector in the rotor frame, t9, the magic angle and w, the sample rotation frequency. In the thermodynamic treatment of the cross polarisation process [ lo], Xz and & are considered as two reservoirs and V= Xzs is considered as the perturbation that causes energy exchange between these two reservoirs. From Eq. (4) it is seen that Xzs averages to zero over one rotation period r, ( =2x/w,). Hence, the net transfer of polarisation over long contact times will be only due to the higher-order terms that arise from the non-negligible homonuclear dipolar interaction. This transfer process has been observed to be an order of magnitude slower for spinning speeds comparable to homonuclear dipolar linewidths [ 10 1. In this Letter, we show that with a simple moditication to the regular cross polarisation pulse sequence, substantial S spin intensity can be obtained at the Hartmann-Hahn match itself. This is based on the idea that to average Xzs to zero one full rotor cycle is required. Hence, the net polarization transferred from Z spins to S spins is zero only for a full rotor period but for a part of the rotor period one can expect a finite S spin polarisation. In the experiment that we propose, the cross polarisation process takes place only during a part of every rotor period. The schematics of the pulse scheme is shown in Fig. 1. The S spin polarisation acquired along the Y axis during part of the first rotor period is preserved in the Z direction. At the beginning of the second rotor period this is brought into spin-lock again along the Y direction and further polarisation is achieved during part of the second rotor period. This summed up polarisation is again preserved along the 2 axis. Thus multiple contacts between the Z and S spins are made till maximum magnetization is obtained which will be followed by the normal acquisition period. In the pulse scheme shown both Z and S spin magnetiza-
324
R.Pratima, K. K ~~~t~n
I Chemical Physic Letters 22lfl994) 322-326
DECOUPLE
I
(4
S
Y
ACQUISITION
Fig. 1. (a) Standard CP pulse sequence. (b) SAP pulse scheme for cross polarisation under fast magic angle spinning. tions are brought to the 2 axis after each contact. The method can be expected to work if only the S spin is
brought to the 2 axis leaving the Z spin spin-locked during the whole of the experiment since the only difference between the two experiments is the way the Z spin ma~etisation is preserved - along the static magnetic field in one case and along the spin-lock rf field in the other. We call this technique synchronous a~umulation of pola~~tion (SAP).
uration rfburst to ensure that the signals are only from cross polarised ~~etization. In Fig. 2 the behaviour of the carbon magnetization as a function of a contact time of the order of two rotor periods for three different spinning speeds in the normal CP experiment is shown. It is observed that at the lower spinning speed, the signal intensity steadily increases as a function of the contact time whereas at the higher spinning speeds, the intensity does not build up and remains relatively small. This indicates that at the lower spinning speed, the homonuclear proton-proton dipolar interaction plays an impo~nt role in the cross polarisation process whereas at the higher spinning speeds its effect is much less and at these speeds one can use the experimental approach outlined earlier. In Figs. 3 and 4 the intensity of the CH2 resonance of adamantane obtained by the normal CP and the SAP experiments are shown for two different spinning speeds, 3050 and 4050 Hz. In each case, the SAP experiment has been performed for different ratios r,, ( = r_5,1rE)of the time of contact z, in a rotor period to the total rotor period z,. For the SAP experiment, the X axis corresponds to the total time T4, used for the cross polarisation process ( nrr). Thus in this case the actual time for which the Z and the S spins are in contact with each other (nrci.) will be less than the 4.00
-
/
3. Results The experiments have been performed on a Bruker MSL-300 FTNMR spectrometer equipped with a double-bearing magic angle spinning probe. The rotor diameter is 7 mm and the maximum spinning speed achievable is 4.2 kHz. The proton and carbon rfpower levels were 40.7 kHz. The experiments were carried out mainly at two spinning speeds, 3050 and 4050 Hz. The “C spectra of a powder sample of adamantane were recorded. The intensity of the CH2 resonance was monitored. In all the experiments natural abundance 13Cmagnetization was destroyed by a sat-
0.00
0.00
1. )O
Cross
Potarisation
Time
(T,$
in ms
Fig. 2. 13CH2signal intensity of adamantane for contact times of the order of two rotor periods obtained from the standard CP experiment for spinning speeds: (A ) 3050 Hz, (*) 3600 HZ, (0) 4050 Hz.
A. Pratima. K. V. Ramanathan /Chemical Physics Letters 221(1994) 322-326
325
1°.OO /
io
0.00
O.OO 0.00
Cross Polarisation
Time (Ted in ms
Fig. 3. 13CH2signal intensity of adamantane obtained from the SAP experiment at the MAS frequency of 3050 Hz as a function of the total cross polarisation time Tcp= nr, for the following ratios rp of contact time per rotor period ( rcr) to the total rotor period (To): (+ ) 0.34, (*) 0.50 and (A) 0.87. Corresponding intensities obtained by the regular CP experiment are also indicated (0). See Fig. 1 for definition of the time periods.
/
4
0.00
~ 0.00
I
4.00 Cross Polarisation
Time (TcP, in ms’
1
0.00
4.00
1 .OO
Fig. 4. 13CH2signal intensity of adamantane obtained from the SAP experiment at the MAS frequency of 4050 Hz as a function of the total cross polarisation time T,= nz, for the following ratios r, of contact time per rotor period (T_) to total rotor period (To): (0) 0.45, (+) 0.65 and (A) 0.81. Corresponding intensities obtained by the regular CP experiment are also indicated (* ) . See Fig. 1 for definition of time the periods.
3.00 Cross Polarisation Time
6. 0 (T,J in ms
Fig. 5. Comparison of “CH2 signal intensities of adamantane for different cross polarisation times TW obtained at a MAS frequency of 4050 Hz with the SAP pulse scheme at the HartmannHahn match for r,,=O.81 (*) and the CP experiment off the Hartmann-Hahn match with w,,=w,,+w, (0).
corresponding contact time for the normal CP experiment. At 3050 Hz (Fig. 3) it is observed that the proposed SAP experiment is at best comparable to the normal CP experiment for cross polarisation times of the order of 2 to 3 ms. For longer T,,the latter experiment gives better results. However, the advantage of the SAP experiment is clearly demonstrated at the higher spinning speed of 4050 Hz (Fig. 4). For comparable contact times, the SAP experiment gives rise to a several-fold increase in intensity as compared to the normal CP experiment for all the values of rp shown. It is observed that the value of r,=O.8 gives maximum enhancement. It is to be noted that the buildup of intensity takes place much faster compared to the regular CP experiment at this MAS frequency. In Fig. 5 the buildup of intensity in the CP experiment at one of the sidebands of the CP spectrum for which wI= W,+ o, is compared with the SAP experiment at the Hartmann-Hahn match. It is observed that the final intensities and the rate at which the signals build up are comparable in both cases. 4. Conclusion The disadvantage of the standard cross polarisation experiment under high speed magic angle spin-
326
R. Pratima, K. V. Ramanathan /Chemical Physics Letters 221(1994) 322-326
ning is that the cross polarisation efficiency is poor at the Hartmann-Hahn match condition. Away from this match there are other sidebands in the CP spectrum which can give intense cross polarised signals. However, for routing use, adjustments of the rfpower levels for the Z and S spins will be tedious especially since the width of the sidebands is small and the rf inhomogeneities can adversely affect the sensitivity [ I]. The SAP experiment described here has been shown to considerably improve the efficiency of transfer of polarisation at the Hartmann-Hahn condition in the case of adamantane. The method proposed is synchronized with sample spinning. The modification to the pulse scheme is the introduction of additional 90” pulses within a rotor cycle to periodically store the acquired magnetisation in the 2 axis, which is easily implemented. We have observed that a period of contact per rotor cycle T_ which is around 80% of a rotor period r, gives rise to a good signal to noise ratio. The rate of buildup as well as the maximum signal obtainable are also comparable to those obtained at the + 1 sideband of the CP spectrum. It has been observed that other samples with strong homonuclear and heteronuclear dipolar couplings such as glycine behave similar to adamantane with low intensities for cross polarised “C signals at spinning speeds of the order 15 kHz [ 4 1. The application of the present SAP technique to such samples at these high spinning speeds appears promising. The short rotor period r, encountered at high spinning speeds may not really be a constraint since for a typical spinning speed of 20 kHz and an rf field strength of 40 kHz, an r, of 0.75 can be used giving rise to considerable enhancement of the signal intensity. It may be noted that there are similarities between the S-AMCP (q, 0) pulse scheme proposed by Hediger et al. [ 1] and the SAP scheme discussed here. It is possible to visualise the former scheme also as one in which the S magnetisation is allowed to build
up for part of a rotor cycle but without preserving the magnetisation along the Z axis as in the case of SAP. Hence, dephasing of the magnetisation during the period for which the S spin is not spin-locked may degrade the performance of the S-AMCP (q, 0) scheme in comparison to the SAP scheme. It may be noted that at slow spinning speeds the SAP experiment may give rise to intensities smaller than the regular CP experiment. Thus, depending on the spinning speed used and the nature of the sample studied, the SAP experiment may be used as complementary to the regular CP experiment.
Acknowledgement The authors would like to thank Professor C.L. Khetrapal and Professor Anil Kumar for scientific advice and constant encouragement.
References [ 1] S. Hediger, B.H. Meier and R.R. Ernst, Chem. Phys. Letters 213 (1993) 627. [2] X. Wu and K-W. Zilm, J. Magn. Reson. A 104 (1993) 154. [ 3 ] T.M. Barbara and E.H. Williams, J. Magn. Reson. 99 ( 1992) 439. [4] O.B. Peirsen, X. Wu, I. Kustanovich and SO. Smith, J. Magn. Reson. A 104 (1993) 334. [5]S.R.HartmannandE.L.Hahn,Phys.Rev. 128 (1962)2042. [6] M.M. Maricq and J.S. Waugh, J. Chem. Phys. 70 (1979) 3300. [7] E.O. Stejskal, J. Schaefer and J.S. Watt&J. Magn. Reson. 28 (1977) 105. [ 81 M. Sardashti and G.E. Maciel, J. Magn. Reson. 72 (1987) 467. [9] R.A. Wind, S.F. Dee, H. Lock and G.E. Mariel, J. Magn. Reson. 79 (1988) 136. [lo] B.H. Meier, Chem. Phys. Letters 188 (1992) 201. [ 111M.H. Levitt, D. Suter and R.R. Ernst, J. Chem. Phys. 84 (1986) 4242. [ 121 A. Schmidt and S. Vega, J. Chem. Phys. 96 (1992) 2655.