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Spectral Simplification and “Pseudo Decoupling” in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients
JOURNAL OF MAGNETIC RESONANCE,
Series A 118, 113–116 (1996)
Article No. 0016
Spectral Simplification and ‘‘Pseudo Decoupling’’ in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients G. A. NAGANAGOWDA Sophisticated Instruments Facility, Indian Institute of Science, Bangalore 560 012, India Received April 3, 1995; revised July 17, 1995
High-resolution proton NMR spectra of complex molecules are generally crowded due to the homonuclear spin– spin couplings. Knowledge of chemical-shift frequencies from such crowded spectra are of utmost importance in identifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decoupling. Since the pioneering work of Aue et al. (1), twodimensional J-resolved spectroscopy seems to have offered the best route to ‘‘broadband-decoupled’’ proton spectra. Subsequently, various attempts have been made to improve upon the technique (2–9), of which the most promising solution seems to be ‘‘purged J spectroscopy’’ (8, 9). Recently, a new proton ‘‘decoupling’’ method has been proposed (10) which uses the classic spin-echo sequence without any purging pulse. A new era has started in the field of NMR with the use of pulsed-field gradients, which in turn, is a result of technological progress in the design of highquality shielded gradients. We have reported the use of these gradients for editing one-dimensional proton NMR spectra (11), where the gradients are used for suppressing artifacts arising from 1807 pulse imperfections and for z filtering. It was shown that the experiment can be used to identify unambiguously singlets and triplets overlapping with other multiplets. This was done by suppressing the signals arising from doublets and quartets by matching the interval between the two 90 7x pulses with t Å 1/2J and with the use of gradient pulses (Fig. 1a). This process leaves the coherences of the singlets unaffected. For triplets, the central line is unaffected and the outer lines are inverted (Fig. 2a). We report here the extensions of this pulse sequence to filter singlets and triplets without any distortions (Fig. 1b) and to achieve ‘‘pseudo-decoupling’’ of the triplets in the proton NMR spectra (Figs. 1c and 1d). The evolution of the coherences under the influence of radiofrequency pulses and t delays can be followed by the product-operator formalism (12) and by a simple vector picture. The spins under consideration are all homonuclear spin-12 nuclei. The 1807 pulses in the middle of the first and second t or t /2 periods in all the pulse sequences (Figs. 1b to 1d) serve to refocus the chemical-shift evolution of
the spins, and the gradient pulse pairs G1 , G2 and G4 , G5 , sandwiching 1807 pulses, eliminate the artifacts that might be created due to 1807 pulse imperfections. In the pulse sequence of Fig. 1b, for an uncoupled spin I (singlet), the equilibrium magnetization Iz is transformed into 0 Iy after the first 90 7x pulse, the second 90 7x pulse converts it back into Iz which is unaffected during the gradient pulse G3 . The last 90 7x pulse transforms it into 0 Iy , which becomes Iy by the end of the subsequent refocusing t period. This echoed signal is detected during acquisition. For a coupled spin, 0 Iy magnetization after the first 90 7x pulse evolves under the J couplings to other spins. For a two-spin system I and S, the spin I evolves under the coupling with spin S giving rise to the terms 0 Iy cos( p Jt ) and 2Ix Sz sin( p Jt ). The first term goes to zero for t Å 1/2J. The second term after the second 90 7x pulse is converted into 02Ix Sy , a mixture of zero, and double-quantum coherences. The doublequantum coherences are dephased by the gradient pulse, G3 , while the zero-quantum coherences, which are insensitive to the magnetic field gradients, are suppressed by the random variation of the delay D, which is comparable with the reciprocal of the zero-quantum frequencies (13). Thus, at this point, all the signals arising from the two-spin system (doublet) are completely suppressed. Similarly, it can be shown that the signals arising from a spin coupled to three other equivalent spins (quartet) are also suppressed by the end of the delay D. For the spin coupled to two other equivalent spins (triplet), it may be noted that after the third 90 7x pulse, the central line is aligned along the y axis, while the outer lines have opposite phase (11) (Figs. 1a and 2a). These outer lines are brought in phase with the central line by allowing them to precess by an additional period of t Å 1/2J. This echoed triplet signal is detected during the acquisition. Thus, this pulse sequence suppresses doublets and quartets, passing only singlets and undistorted triplets. It is clear from the Fig. 1c that, until the third 90 7x pulse, the pulse sequence is the same as that given in Fig. 1b and hence as described in the previous paragraph; by the end of the delay period D, the signals arising from the two-spin system (doublet) and from spins coupled to three other
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1064-1858/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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FIG. 1. The phases of all the radiofrequency pulses are kept constant unless otherwise mentioned. Pulses of 1807 in all the cases serve to refocus the chemical-shift evolution. The gradient pulse pairs G1 , G2 and G4 , G5 sandwiching the 1807 pulses remove pulse imperfections arising from 1807 pulses. Gradient pulses G3 and G6 are used as z filters. The delay t is set to 1/2J in all the cases and D is a random delay to cancel zero-quantum artifacts. The delay d is a fixed z filtering period. (a) Pulse sequence used to identify singlets and triplets by suppressing doublets and quartets ( 11). The outer lines of the triplets are inverted, central lines of the triplets and singlets pass through unaltered. (b) Pulse sequence to filter singlets and undistorted triplets suppressing doublets and quartets. (c) Pulse sequence to achieve both spectral simplification and ‘‘pseudo decoupling’’ by suppressing doublets, quartets, and outer lines of the triplets. Except for the last 907 pulse, which is along the y axis, the phases of all the pulses are kept constant (along the x axis). (d) The results of this pulse sequence are same as that obtained from (c), but the suppression of the quartets is found to be more efficient by this sequence.
equivalent spins (quartet) are suppressed. Also for an uncoupled spin system (singlet), we obtain 0 Iy magnetization after the third 90 7x pulse. This gets transformed into Iy by the subsequent t /2 delay and 180 7x pulse and is detected during the acquisition as it is not affected by the last 90 7y pulse. For the triplet, it is seen earlier that at the end of the third 90 7x pulse, the central signal of the triplet is in phase with the signal of uncoupled spin and the outer lines are 1807 out of phase with respect to the central signal. Evolution of these signals, for a further period of t /2, results in the outer lines being 907 out of phase with respect to the central line. That is, we have, at the end of this t /2 period, the central line along the y axis, with one of the outer lines along the / x and the other along the 0 x axis. Application of the last 90 7y pulse results in the transformation of 0 x and / x components into 0z and /z components, leaving the central line (y magnetization) unaffected, and hence only
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the central line is detected during the acquisition. Thus, by this pulse sequence, doublets, quartets, and the outer lines of the triplets are suppressed completely, allowing only singlets and the central lines of the triplets, which are at their chemical-shift frequencies, to pass through. Since the triplet signals appear as though they are decoupled, we refer to this method of obtaining single lines for triplets at their chemicalshift frequencies as pseudo decoupling of the proton NMR spectra. The pulse sequence of Fig. 1d is same as that given in Fig. 1c until the beginning of the fourth 907 pulse. As described earlier, doublets and quartets are suppressed by the end of the delay D. At the end of the second t /2 delay period (beginning of the fourth 907 pulse), we have the magnetization along the y axis for an uncoupled spin (singlet) and also for the central signal of the triplet, while the outer lines of the triplet are aligned antiphase with respect
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FIG. 2. Proton NMR spectra of the mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane recorded at 400 MHz. The bottom trace is a normal one-dimensional spectrum recorded on a high-resolution probe for comparison. All the other spectra were recorded on an imaging probe without field frequency locking. Eight scans were accumulated with a relaxation delay of two seconds in all the cases. Widths of 907 and 1807 pulses were 9.5 and 19.0 ms respectively on the imaging probe. The delays t /2 and t /4 were 35.7 and 17.85 ms, respectively, which correspond to the J of 7.0 Hz. Actual proton J values in ethyl acetate and ethyl alcohol are 7.17 and 7.02 Hz respectively. The delay D was varied randomly between 4 and 6 ms to suppress the artifacts arising from zero-quantum frequencies. The delay d was fixed at 4 ms. Durations and strength of the z-gradient pulses were G1 Å G2 Å G4 Å G5 Å (1.5 ms, 5 G/cm), G3 Å (3 ms, 5 G/ cm), and G6 Å (0.8 ms, 5 G/cm). (a) Spectrum recorded using the pulse sequence of Fig. 1a. (b) Spectrum recorded using the pulse sequence of Fig. 1b. The two quartets at 3.69 and 4.12 ppm are completely suppressed, leaving triplets and singlets unaffected. (c) Spectrum recorded using the pulse sequence of Fig. 1c. Here the outer lines of the two partially overlapping triplets at 1.22 and 1.26 ppm are suppressed in addition to the quartets. (d) Spectrum recorded using the pulse sequence of Fig. 1d. The results of this experiment are same as that obtained from sequence 1c. But, as seen in the spectrum, the suppression of quartets using the sequence of Fig. 1d is more efficient than the suppression using the sequence of Fig. 1c.
to the x axis. Application of the 90 7x pulse at this point transforms the y magnetization of the singlet and the central line of the triplet into z magnetization, leaving the outer lines of the triplet unaffected. The latter are subsequently destroyed by the gradient pulse G6 . The last 90 7x pulse brings back the stored z magnetization of the singlet and central line of the triplet into the transverse plane which is detected during the acquisition. Thus, this sequence, like the pulse sequence of Fig. 1c, achieves the suppression of the doublets,
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quartets, and outer lines of the triplets, filtering only singlets and central lines of the triplets, thus achieving pseudo decoupling in the proton NMR spectra. It is noticed from the experimental results that this sequence works more efficiently than the sequence of Fig. 1c. Results of the proposed pulse sequences are demonstrated on a simple system, a mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane. Proton NMR spectra were recorded on a Bruker AMX-400 NMR spectrometer equipped with a microimaging accessory. Since the imaging probe does not have a field–frequency locking facility, shimming was done on the FID. Uncompensated rectangular gradient pulses were used in each of the cases. The durations and strength of the gradient pulses were G1 Å G2 Å G4 Å G5 Å 1.5 ms, 5 G/ cm, G3 Å 3 ms, 5 G/cm, and G6 Å 0.8 ms, 5 G/cm. The delay D was varied randomly between 4 and 6 ms in each of the experiments to destroy the zero-quantum frequencies. The delay d in the pulse sequence of Fig. 1d was 4 ms. The normal one-dimensional proton NMR spectrum recorded on a high-resolution probe is shown for comparison in the bottom trace, Fig. 2e. Figure 2b shows the spectrum recorded using the pulse sequence of Fig. 1b. It is seen from the spectrum that the two quartets centered at 3.69 and 4.12 ppm are completely suppressed and the triplets, whose outer lines get inverted by the pulse sequence of Fig. 1a (as shown in Fig. 2a), are in phase with the central lines, thus removing the distortions of the triplet in addition to simplifying the spectrum. The singlets of the tetramethylsilane (0.0 ppm), acetyl (2.04 ppm), and hydroxyl (2.78 ppm) protons are not affected by the pulse sequence. The spectrum recorded using the pulse sequence of Fig. 1c is shown in Fig. 2c. As seen in the figure, both quartets are suppressed, and, for the triplets, the outer lines are completely suppressed while retaining the central lines which are at their chemical-shift frequencies (1.22 and 1.26 ppm), thus achieving pseudo decoupling of the spectrum. The singlets pass through the sequence unaltered. Figure 2d shows the spectrum obtained using the pulse sequence of Fig. 1d. It is clear from the spectrum that the suppression of the quartets from this pulse sequence is more efficient than suppression with the sequence of Fig. 1c. All the other features are the same as those shown in Fig. 2c. The suppression of the outer lines of the triplets by the proposed sequences (Figs. 1c and 1d) results in the reduction of the overall intensity of each of the triplets by 50%, which must be accounted for while making quantitative measurements, although quantitative measurements are difficult due to the long evolution of the magnetization in the transverse plane. Experiments are also performed with deliberate missetting of some of the important parameters like the t delays and the pulse widths in order to appraise these pulse sequences for routine use for complex proton NMR spectra. Figure 3 shows the set of spectra recorded using the pulse sequence
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pulses were varied simultaneously, keeping the rest of the parameters constant as mentioned in the caption of Fig. 2. It is very interesting to note that the missetting of these pulses, even to a large extent, does not degrade the quality of the spectra. New methods for spectral simplification and pseudo decoupling of proton NMR spectra are presented. This is achieved by suppressing doublets, quartets, and outer lines of triplets based on proton spin–spin couplings. The proposed one-dimensional experiments make use of pulsed field gradients for suppressing artifacts arising from 1807 pulse imperfections and for z filtering. Theoretical and experimental results are discussed. The results are demonstrated on a simple system, a mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane. It is observed that the quality of the spectra does not degrade even when the parameters, like 1/2J, and pulse width are misset to a large extent. This considerable simplification should prove useful in one-dimensional proton NMR spectroscopy for identifying the signals in the overlapping regions of the spectra. ACKNOWLEDGMENTS FIG. 3. Proton NMR spectra of the mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane recorded at 400 MHz on an imaging probe using the pulse sequence of Fig. 1d at different values of t, keeping other parameters constant. The values of t were varied between 63.4 and 83.4 ms and they are marked at the right side of the corresponding spectrum. The values for 907 and 1807 pulses were 9.5 and 19 ms, respectively. The delay D was varied randomly between 4 and 6 ms while the delay d was 4 ms. Durations and strength of the gradients used were G1 Å G2 Å G4 Å G5 Å (1.5 ms, 5 G/cm), G3 Å (3 ms, 5 G/cm), and G6 Å (0.8 ms, 5 G/ cm). The bottom trace is a normal one-dimensional spectrum recorded on a high-resolution probe for comparison.
of Fig. 1d with different values of t, keeping the other parameters constant as described in the figure caption. The values of t were varied between 63.4 and 83.4 ms, which correspond to a variation of J between 7.88 and 5.99 Hz. It is clear from these results that, although the spectral simplification and pseudo decoupling is based on the parameter J, it is not very critical that these values are adjusted accurately for better results. Thus, these sequences are shown to be useful even for samples with spin–spin coupling constants varying over a range of values. The spectra were also recorded using the pulse sequence of Fig. 1d, varying the pulse widths between 7.5 and 10.5 ms for the 907 pulse and between 15 and 21 ms for the 1807 pulse. Both 907 and 1807
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The author is indebted to Professor C. L. Khetrapal, Professor Anilkumar, and Dr. K. V. Ramanathan for encouragement.
REFERENCES 1. W. P. Aue, J. Karhan, and R. R. Ernst, J. Chem. Phys. 64, 4226 (1976). 2. K. Nagayama, P. Bachmann, K. Wu¨thrich, and R. R. Ernst, J. Magn. Reson. 31, 133 (1978). 3. A. Bax, R. Freeman, and G. A. Morris, J. Magn. Reson. 43, 333 (1981). 4. A. Bax, A. F. Mehlkoof, and J. Smidt, J. Magn. Reson. 35, 167 (1979). 5. A. Bax and R. Freeman, J. Magn. Reson. 44, 542 (1981). 6. J. A. Hogbom, Astron. Astrophys. Suppl. 15, 417 (1974). 7. A. J. Shaka, J. Keeler, and R. Freeman, J. Magn. Reson. 56, 294 (1984). 8. P. Xu, X.-L. Wu, and R. Freeman, J. Am. Chem. Soc. 113, 3596 (1991). 9. P. Xu, X.-L. Wu, and R. Freeman, J. Magn. Reson. 95, 132 (1991). 10. M. Woodley and R. Freeman, J. Magn. Reson. A 109, 103 (1994). 11. G. A. Naganagowda, J. Magn. Reson. A 113, 235 (1995). 12. O. W. Sørensen, G. W. Eich, M. H. Levitt, G. Bodenhausen, R. R. Ernst, Prog. NMR Spectrosc. 16, 163 (1983). 13. A. Kumar, G. Wagner, R. R. Ernst, and K. Wu¨thrich, J. Am. Chem. Soc. 103, 3654 (1981).
AP: Mag Res
Series A 118, 113–116 (1996)
Article No. 0016
Spectral Simplification and ‘‘Pseudo Decoupling’’ in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients G. A. NAGANAGOWDA Sophisticated Instruments Facility, Indian Institute of Science, Bangalore 560 012, India Received April 3, 1995; revised July 17, 1995
High-resolution proton NMR spectra of complex molecules are generally crowded due to the homonuclear spin– spin couplings. Knowledge of chemical-shift frequencies from such crowded spectra are of utmost importance in identifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decoupling. Since the pioneering work of Aue et al. (1), twodimensional J-resolved spectroscopy seems to have offered the best route to ‘‘broadband-decoupled’’ proton spectra. Subsequently, various attempts have been made to improve upon the technique (2–9), of which the most promising solution seems to be ‘‘purged J spectroscopy’’ (8, 9). Recently, a new proton ‘‘decoupling’’ method has been proposed (10) which uses the classic spin-echo sequence without any purging pulse. A new era has started in the field of NMR with the use of pulsed-field gradients, which in turn, is a result of technological progress in the design of highquality shielded gradients. We have reported the use of these gradients for editing one-dimensional proton NMR spectra (11), where the gradients are used for suppressing artifacts arising from 1807 pulse imperfections and for z filtering. It was shown that the experiment can be used to identify unambiguously singlets and triplets overlapping with other multiplets. This was done by suppressing the signals arising from doublets and quartets by matching the interval between the two 90 7x pulses with t Å 1/2J and with the use of gradient pulses (Fig. 1a). This process leaves the coherences of the singlets unaffected. For triplets, the central line is unaffected and the outer lines are inverted (Fig. 2a). We report here the extensions of this pulse sequence to filter singlets and triplets without any distortions (Fig. 1b) and to achieve ‘‘pseudo-decoupling’’ of the triplets in the proton NMR spectra (Figs. 1c and 1d). The evolution of the coherences under the influence of radiofrequency pulses and t delays can be followed by the product-operator formalism (12) and by a simple vector picture. The spins under consideration are all homonuclear spin-12 nuclei. The 1807 pulses in the middle of the first and second t or t /2 periods in all the pulse sequences (Figs. 1b to 1d) serve to refocus the chemical-shift evolution of
the spins, and the gradient pulse pairs G1 , G2 and G4 , G5 , sandwiching 1807 pulses, eliminate the artifacts that might be created due to 1807 pulse imperfections. In the pulse sequence of Fig. 1b, for an uncoupled spin I (singlet), the equilibrium magnetization Iz is transformed into 0 Iy after the first 90 7x pulse, the second 90 7x pulse converts it back into Iz which is unaffected during the gradient pulse G3 . The last 90 7x pulse transforms it into 0 Iy , which becomes Iy by the end of the subsequent refocusing t period. This echoed signal is detected during acquisition. For a coupled spin, 0 Iy magnetization after the first 90 7x pulse evolves under the J couplings to other spins. For a two-spin system I and S, the spin I evolves under the coupling with spin S giving rise to the terms 0 Iy cos( p Jt ) and 2Ix Sz sin( p Jt ). The first term goes to zero for t Å 1/2J. The second term after the second 90 7x pulse is converted into 02Ix Sy , a mixture of zero, and double-quantum coherences. The doublequantum coherences are dephased by the gradient pulse, G3 , while the zero-quantum coherences, which are insensitive to the magnetic field gradients, are suppressed by the random variation of the delay D, which is comparable with the reciprocal of the zero-quantum frequencies (13). Thus, at this point, all the signals arising from the two-spin system (doublet) are completely suppressed. Similarly, it can be shown that the signals arising from a spin coupled to three other equivalent spins (quartet) are also suppressed by the end of the delay D. For the spin coupled to two other equivalent spins (triplet), it may be noted that after the third 90 7x pulse, the central line is aligned along the y axis, while the outer lines have opposite phase (11) (Figs. 1a and 2a). These outer lines are brought in phase with the central line by allowing them to precess by an additional period of t Å 1/2J. This echoed triplet signal is detected during the acquisition. Thus, this pulse sequence suppresses doublets and quartets, passing only singlets and undistorted triplets. It is clear from the Fig. 1c that, until the third 90 7x pulse, the pulse sequence is the same as that given in Fig. 1b and hence as described in the previous paragraph; by the end of the delay period D, the signals arising from the two-spin system (doublet) and from spins coupled to three other
113
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1064-1858/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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NOTES
FIG. 1. The phases of all the radiofrequency pulses are kept constant unless otherwise mentioned. Pulses of 1807 in all the cases serve to refocus the chemical-shift evolution. The gradient pulse pairs G1 , G2 and G4 , G5 sandwiching the 1807 pulses remove pulse imperfections arising from 1807 pulses. Gradient pulses G3 and G6 are used as z filters. The delay t is set to 1/2J in all the cases and D is a random delay to cancel zero-quantum artifacts. The delay d is a fixed z filtering period. (a) Pulse sequence used to identify singlets and triplets by suppressing doublets and quartets ( 11). The outer lines of the triplets are inverted, central lines of the triplets and singlets pass through unaltered. (b) Pulse sequence to filter singlets and undistorted triplets suppressing doublets and quartets. (c) Pulse sequence to achieve both spectral simplification and ‘‘pseudo decoupling’’ by suppressing doublets, quartets, and outer lines of the triplets. Except for the last 907 pulse, which is along the y axis, the phases of all the pulses are kept constant (along the x axis). (d) The results of this pulse sequence are same as that obtained from (c), but the suppression of the quartets is found to be more efficient by this sequence.
equivalent spins (quartet) are suppressed. Also for an uncoupled spin system (singlet), we obtain 0 Iy magnetization after the third 90 7x pulse. This gets transformed into Iy by the subsequent t /2 delay and 180 7x pulse and is detected during the acquisition as it is not affected by the last 90 7y pulse. For the triplet, it is seen earlier that at the end of the third 90 7x pulse, the central signal of the triplet is in phase with the signal of uncoupled spin and the outer lines are 1807 out of phase with respect to the central signal. Evolution of these signals, for a further period of t /2, results in the outer lines being 907 out of phase with respect to the central line. That is, we have, at the end of this t /2 period, the central line along the y axis, with one of the outer lines along the / x and the other along the 0 x axis. Application of the last 90 7y pulse results in the transformation of 0 x and / x components into 0z and /z components, leaving the central line (y magnetization) unaffected, and hence only
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the central line is detected during the acquisition. Thus, by this pulse sequence, doublets, quartets, and the outer lines of the triplets are suppressed completely, allowing only singlets and the central lines of the triplets, which are at their chemical-shift frequencies, to pass through. Since the triplet signals appear as though they are decoupled, we refer to this method of obtaining single lines for triplets at their chemicalshift frequencies as pseudo decoupling of the proton NMR spectra. The pulse sequence of Fig. 1d is same as that given in Fig. 1c until the beginning of the fourth 907 pulse. As described earlier, doublets and quartets are suppressed by the end of the delay D. At the end of the second t /2 delay period (beginning of the fourth 907 pulse), we have the magnetization along the y axis for an uncoupled spin (singlet) and also for the central signal of the triplet, while the outer lines of the triplet are aligned antiphase with respect
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FIG. 2. Proton NMR spectra of the mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane recorded at 400 MHz. The bottom trace is a normal one-dimensional spectrum recorded on a high-resolution probe for comparison. All the other spectra were recorded on an imaging probe without field frequency locking. Eight scans were accumulated with a relaxation delay of two seconds in all the cases. Widths of 907 and 1807 pulses were 9.5 and 19.0 ms respectively on the imaging probe. The delays t /2 and t /4 were 35.7 and 17.85 ms, respectively, which correspond to the J of 7.0 Hz. Actual proton J values in ethyl acetate and ethyl alcohol are 7.17 and 7.02 Hz respectively. The delay D was varied randomly between 4 and 6 ms to suppress the artifacts arising from zero-quantum frequencies. The delay d was fixed at 4 ms. Durations and strength of the z-gradient pulses were G1 Å G2 Å G4 Å G5 Å (1.5 ms, 5 G/cm), G3 Å (3 ms, 5 G/ cm), and G6 Å (0.8 ms, 5 G/cm). (a) Spectrum recorded using the pulse sequence of Fig. 1a. (b) Spectrum recorded using the pulse sequence of Fig. 1b. The two quartets at 3.69 and 4.12 ppm are completely suppressed, leaving triplets and singlets unaffected. (c) Spectrum recorded using the pulse sequence of Fig. 1c. Here the outer lines of the two partially overlapping triplets at 1.22 and 1.26 ppm are suppressed in addition to the quartets. (d) Spectrum recorded using the pulse sequence of Fig. 1d. The results of this experiment are same as that obtained from sequence 1c. But, as seen in the spectrum, the suppression of quartets using the sequence of Fig. 1d is more efficient than the suppression using the sequence of Fig. 1c.
to the x axis. Application of the 90 7x pulse at this point transforms the y magnetization of the singlet and the central line of the triplet into z magnetization, leaving the outer lines of the triplet unaffected. The latter are subsequently destroyed by the gradient pulse G6 . The last 90 7x pulse brings back the stored z magnetization of the singlet and central line of the triplet into the transverse plane which is detected during the acquisition. Thus, this sequence, like the pulse sequence of Fig. 1c, achieves the suppression of the doublets,
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quartets, and outer lines of the triplets, filtering only singlets and central lines of the triplets, thus achieving pseudo decoupling in the proton NMR spectra. It is noticed from the experimental results that this sequence works more efficiently than the sequence of Fig. 1c. Results of the proposed pulse sequences are demonstrated on a simple system, a mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane. Proton NMR spectra were recorded on a Bruker AMX-400 NMR spectrometer equipped with a microimaging accessory. Since the imaging probe does not have a field–frequency locking facility, shimming was done on the FID. Uncompensated rectangular gradient pulses were used in each of the cases. The durations and strength of the gradient pulses were G1 Å G2 Å G4 Å G5 Å 1.5 ms, 5 G/ cm, G3 Å 3 ms, 5 G/cm, and G6 Å 0.8 ms, 5 G/cm. The delay D was varied randomly between 4 and 6 ms in each of the experiments to destroy the zero-quantum frequencies. The delay d in the pulse sequence of Fig. 1d was 4 ms. The normal one-dimensional proton NMR spectrum recorded on a high-resolution probe is shown for comparison in the bottom trace, Fig. 2e. Figure 2b shows the spectrum recorded using the pulse sequence of Fig. 1b. It is seen from the spectrum that the two quartets centered at 3.69 and 4.12 ppm are completely suppressed and the triplets, whose outer lines get inverted by the pulse sequence of Fig. 1a (as shown in Fig. 2a), are in phase with the central lines, thus removing the distortions of the triplet in addition to simplifying the spectrum. The singlets of the tetramethylsilane (0.0 ppm), acetyl (2.04 ppm), and hydroxyl (2.78 ppm) protons are not affected by the pulse sequence. The spectrum recorded using the pulse sequence of Fig. 1c is shown in Fig. 2c. As seen in the figure, both quartets are suppressed, and, for the triplets, the outer lines are completely suppressed while retaining the central lines which are at their chemical-shift frequencies (1.22 and 1.26 ppm), thus achieving pseudo decoupling of the spectrum. The singlets pass through the sequence unaltered. Figure 2d shows the spectrum obtained using the pulse sequence of Fig. 1d. It is clear from the spectrum that the suppression of the quartets from this pulse sequence is more efficient than suppression with the sequence of Fig. 1c. All the other features are the same as those shown in Fig. 2c. The suppression of the outer lines of the triplets by the proposed sequences (Figs. 1c and 1d) results in the reduction of the overall intensity of each of the triplets by 50%, which must be accounted for while making quantitative measurements, although quantitative measurements are difficult due to the long evolution of the magnetization in the transverse plane. Experiments are also performed with deliberate missetting of some of the important parameters like the t delays and the pulse widths in order to appraise these pulse sequences for routine use for complex proton NMR spectra. Figure 3 shows the set of spectra recorded using the pulse sequence
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NOTES
pulses were varied simultaneously, keeping the rest of the parameters constant as mentioned in the caption of Fig. 2. It is very interesting to note that the missetting of these pulses, even to a large extent, does not degrade the quality of the spectra. New methods for spectral simplification and pseudo decoupling of proton NMR spectra are presented. This is achieved by suppressing doublets, quartets, and outer lines of triplets based on proton spin–spin couplings. The proposed one-dimensional experiments make use of pulsed field gradients for suppressing artifacts arising from 1807 pulse imperfections and for z filtering. Theoretical and experimental results are discussed. The results are demonstrated on a simple system, a mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane. It is observed that the quality of the spectra does not degrade even when the parameters, like 1/2J, and pulse width are misset to a large extent. This considerable simplification should prove useful in one-dimensional proton NMR spectroscopy for identifying the signals in the overlapping regions of the spectra. ACKNOWLEDGMENTS FIG. 3. Proton NMR spectra of the mixture of ethyl acetate, ethyl alcohol, and tetramethylsilane recorded at 400 MHz on an imaging probe using the pulse sequence of Fig. 1d at different values of t, keeping other parameters constant. The values of t were varied between 63.4 and 83.4 ms and they are marked at the right side of the corresponding spectrum. The values for 907 and 1807 pulses were 9.5 and 19 ms, respectively. The delay D was varied randomly between 4 and 6 ms while the delay d was 4 ms. Durations and strength of the gradients used were G1 Å G2 Å G4 Å G5 Å (1.5 ms, 5 G/cm), G3 Å (3 ms, 5 G/cm), and G6 Å (0.8 ms, 5 G/ cm). The bottom trace is a normal one-dimensional spectrum recorded on a high-resolution probe for comparison.
of Fig. 1d with different values of t, keeping the other parameters constant as described in the figure caption. The values of t were varied between 63.4 and 83.4 ms, which correspond to a variation of J between 7.88 and 5.99 Hz. It is clear from these results that, although the spectral simplification and pseudo decoupling is based on the parameter J, it is not very critical that these values are adjusted accurately for better results. Thus, these sequences are shown to be useful even for samples with spin–spin coupling constants varying over a range of values. The spectra were also recorded using the pulse sequence of Fig. 1d, varying the pulse widths between 7.5 and 10.5 ms for the 907 pulse and between 15 and 21 ms for the 1807 pulse. Both 907 and 1807
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The author is indebted to Professor C. L. Khetrapal, Professor Anilkumar, and Dr. K. V. Ramanathan for encouragement.
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AP: Mag Res