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A single-scan method for NMR 2D J-resolved spectroscopy† Liangjie Lin, Zhiliang Wei, Yanqin Lin* and Zhong Chen
Received 1st October 2014, Accepted 18th November 2014 DOI: 10.1039/c4cc07751b www.rsc.org/chemcomm
A single-scan method is proposed to shorten the acquisition time for an NMR 2D J-resolved spectrum to 1 second. With high SNRs and acceptable resolutions, it constitutes an effective tool for obtaining decoupled proton spectra and fine scalar-coupling splitting patterns.
Nuclear magnetic resonance (NMR) awards us with molecular structure information because NMR parameters, such as chemical shifts and spin–spin coupling constants, possess high sensitivity to structural and conformational features of the molecules observed.1 NMR 2D J-resolved spectroscopy2 was developed to distinctly obtain the aforementioned details by separating the effects of chemical shifts and scalar couplings into two independent dimensions. It has been widely applied to chemical studies,3 metabolomics,4 and in vivo detections.5 Conventional 2D J-resolved spectroscopy (Con-J) suffers from long acquisition time due to the necessity of numerous t1 increments to construct the indirect dimension with a satisfactory resolution. To date numerous methods have been proposed to accelerate the acquisition of multidimensional NMR spectra.6 However, most of them cannot be efficiently applied to 2D J-resolved experiment. The spatially-encoded method7 replaces the conventional temporal encoding with spatial encoding and compresses the acquisition of a multidimensional spectrum into a single scan. It has been successfully applied to 2D J-resolved spectroscopy.8 A typical sequence of spatially encoded J-resolved spectroscopy (SEJ) is shown in Fig. 1(b). Low signal-to-noise ratio (SNR) is arguably the main drawback of the spatially-encoded method for broader applications. In this study, a scheme is proposed for recording 2D J-resolved spectra with high SNRs and decent spectral resolutions in a single scan (termed as SSJ, as shown in Fig. 1(c)). Every single acquisition module records evolutions of both chemical shifts and scalar couplings in the direct (F2) dimension, and it is repeated to
Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc07751b
1234 | Chem. Commun., 2015, 51, 1234--1236
Fig. 1 (a) Conventional 2D J-resolved spectroscopic sequence, (b) SEJ sequence, and (c) SSJ sequence. The filled and open bars represent the p/2 and p pulses, respectively. The rectangles with sloping arrows indicate adiabatic frequency-swept p (chirp-p) pulses, GE the encoding gradient, GP the purge gradient, and GD the decoding gradient.
trace scalar-coupling evolutions in the indirect (F1) dimension. Phases of the repeated 1801 pulses are altered by referencing to (x, x, x, x) for alleviating pulse imperfections. Signals during even (or odd) acquisition modules are arranged in sequential order to yield a 2D array. For providing sufficient spectral widths in F1 dimensions (SW1 = 1/(2 Ta)), the acquisition time within one module (Ta) will be limited. As a result, the raw data of SSJ suffer from serious truncations in the F2 dimension. As it is known, liquid-state NMR signals can be efficiently modeled by the summation of a finite number of damped sinusoids. This character is fully used by several reported algorithms to achieve spectral resolutions beyond the restriction of the time-frequency uncertainty principle such as linear prediction9a (LP), matrix pencil method,9b and filter diagonalization method.9c The LP extrapolation, capable of alleviating truncation errors and enhancing resolutions of discrete Fourier transform (DFT) spectra, is applied to further predict the raw data of SSJ to fully decay in the F2 dimension. A sample of 0.5 M ethyl 3-bromopropionate in CDCl3 was tested with Con-J, SEJ, and SSJ sequences, and the results are
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Fig. 2 2D J-resolved spectra of ethyl 3-bromopropionate (0.5 M in CDCl3) by SSJ (a), SSJ after LP extrapolation (b), Con-J (c) and SEJ (d). The F2 projections are given on the top of the boxed diagrams, and F1 projections for resonances located at 1.3 and 4.2 ppm are also given inside the boxed diagrams. Experimental parameters for SSJ: SW2 = 2.5 kHz, Ta = 12 ms (N2 = 30, SW1 = 42 Hz) and N = 30; for SEJ: BW = 25 kHz, encoding gradients GE = 1 G cm1, decoding gradients GD = 1.6 G cm1, encoding time TE = 60 ms, decoding time TD = 12 ms, frequency range of chirp pulses Pbw = 15 kHz and N = 30.
shown in Fig. 2. Fig. 2(a) shows the SSJ spectrum without any post-processing other than 2D DFT. The resolution in the F2 dimension is 102.0 Hz due to the limited acquisition time (Ta = 12 ms). With the introduction of LP extrapolation, it can be improved to 7.8 Hz (Fig. 2(b)). Fig. 2(c) and (d) depict the Con-J and SEJ spectra, respectively. All these methods can provide correct multiplet patterns in F1 dimensions as F1 projections shown in Fig. 2(a)–(d). Detailed comparisons among Fig. 2(b)–(d) are summarized in Table 1. From the table, we can see that the Con-J spectrum requires an experimental duration of 10 minutes that can be shortened by SSJ and SEJ to less than one second. The Con-J spectrum shows the best resolution in the F2 dimensions because of its full sampling, while resolutions of SSJ (7.8 Hz) and SEJ (24.0 Hz) in the F2 dimensions are limited by the performance of LP extrapolation and the spatial-encoding time (TE), respectively. The SNR of the SSJ spectrum (2537) is similar to that of the Con-J spectrum (2606), whereas the SEJ spectrum suffers from a serious SNR decrease (309), which primarily results from the enlarged receiver bandwidth (BW), molecular diffusion, and long spatial-encoding time. The SSJ and
Con-J sequences were also verified to exhibit satisfactory spectra for a low-concentration sample (5 mM ethyl 3-bromopropionate in CDCl3, see Fig. S1, ESI†). The limiting solute concentration for SEJ is approximately 100 mM with a reasonable linewidth in the F2 dimension. These experimental results demonstrate the time-cost advantage of SSJ over the conventional method and the SNR and resolution advantages of SSJ over SEJ. For the spatially-encoded technique, samples detected should be sufficiently homogeneous for applying spatial encoding. Complex pulse sequences also place strict requirements on hardware and experimental skills. The intrinsic shortcomings of SEJ indicate that the SSJ can be more suitable under certain conditions. A mixture solution comprising pyridine, 2,3-dihydrofuran and n-propanol with the same concentration of 100 mM in dimethyl sulfoxide-d6 (DMSO-d6) was detected by the SSJ sequence, and the results are shown in Fig. 3(c). The Con-J spectrum is shown in Fig. 3(b). The F2 projections shown in Fig. 3(b) and (c) indicate that SSJ can provide decoupled 1H spectra as Con-J can. The F1 projections for resonances marked with blue dashed rectangles in Con-J and SSJ spectra are shown in Fig. 3(d). There are complex coupling systems in the molecules of pyridine and dihydrofuran, resulting in complicated scalar-coupling splitting patterns. The SSJ can provide similar F1 projections as Con-J can, such as the F1 projections of resonances J, J0 and F, as shown in Fig. 3(d). Conventional 1D 1H experiments can provide superior spectral resolutions for small molecules in mobile solvents (0.9 Hz for the mixture solution). However, the performances of 1D 1H spectra
Table 1 Detailed comparisons of ethyl 3-bromopropionate spectra acquired with Con-J, SSJ, and SEJ
a
LW1 (Hz) LW2b (Hz) SNRc Duration
Con-J
SSJ
SEJ
1.0 3.6 2606 10 min
1.6 7.8 2537 720 ms
2.2 24.0 310 780 ms
a
Linewidths in F1 dimensions (LW1) were measured from F1 projections of the resonance located at 1.3 ppm. b Linewidths in F2 dimensions (LW2) were measured from F2 projections of the resonance located at 1.3 ppm. c The SNRs of resulting spectra were calculated by dividing the height of the peak located at 1.3 ppm by the root-mean-square of noise ranging from 1.8 to 2.2 ppm.
This journal is © The Royal Society of Chemistry 2015
Fig. 3 (a) Molecular structures for pyridine, 2,3-dihydrofuran and, n-propanol, (b) Con-J spectrum and its F2 projection of the mixture solution, (c) SSJ spectrum and its F2 projection, and (d) F1 projections of the resonances marked with blue dashed rectangles in Con-J (first row) and SSJ (second row) spectra. Experimental parameters for SSJ: SW2 = 5 kHz, Ta = 10 ms (N2 = 50, SW1 = 50 Hz) and N = 50. The total acquisition time was 1 second.
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can be readily influenced by inhomogeneous magnetic fields. In addition, it is difficult for 1D 1H spectroscopy to interpret complicated molecules or mixtures due to its narrow chemicalshift dispersion range and scalar-coupling splitting. The time required for SSJ is identical to that for the conventional singlepulse experiment. The spin echo modules within the SSJ sequence suppress the effects from static magnetic field inhomogeneity, rendering F1 linewidths of the resultant J-resolved spectra approaching the natural limit of 1/(pT2). Moreover, by separating chemical shifts and scalar-coupling splitting into orthogonal dimensions, SSJ can present molecular structural information more clearly. Therefore, SSJ can serve as a complement for 1D spectroscopy when we analyze some complicated samples. Similar to the case of Con-J spectra, SSJ spectra need to be shown in the absolute-value mode due to the presence of dispersive antiphase contributions, which result in broadened linewidths. Therefore, J values measured may be unreliable. There are several ways to obtain phase-sensitive J-resolved spectra for more accurate J values, but they suffer from corresponding drawbacks. A popular method10a to use a pseudo-echo weighting function to eliminate the dispersive component, results in intensity distortions. The phasesensitive 2D J-resolved experiment proposed by Pell and Keeler10b can also provide excellent resolutions in both the dimensions with the penalty of SNR reductions. The resolutions of SSJ may not be sufficiently high for us to distinguish some close resonances in the F2 dimension. We present the experimental data of SSJ on quinine (a type of antimalarial drug; see Fig. S2, ESI†). The SSJ proves effective when providing fine structures of most of the resonances in the quinine molecule, especially in the aromatic and vinyl regions, and its performance degenerates in dealing with several crowded resonances in the aliphatic region. In higher magnetic fields, frequency differences between resonances will become larger, while scalar-coupling constants remain unchanged. Therefore, the performances of SSJ would be better under high magnetic fields. When LP is utilized for forward prediction, data points available must be at least twice the number of resonances existing in the free induction decay (FID) signals. If this condition is not fulfilled, the accuracy by which the frequencies are reproduced by the extrapolation will be unsatisfactory. Original data points of SSJ in the F2 dimension, determined by the spectral widths in both F1 and F2 dimensions (N2 = SW2/(2 SW1)) are limited. Therefore, the performances of SSJ may deteriorate when the systems with narrow frequency dispersion ranges and crowd resonances are analyzed. For ensuring the complete decay of extended SSJ FIDs, it is suggested to perform the LP extrapolation with the aid of suitable window apodizations.9a In summary, a single-scan method (SSJ) is proposed for ultrafast NMR 2D J-resolved spectroscopy. The SNRs and resolutions of the resulting SSJ spectra are similar to those of the conventional ones.
1236 | Chem. Commun., 2015, 51, 1234--1236
Communication
The SSJ can be a powerful spectroscopic tool in the areas of metabolite batch tests, organic reaction detections as well as clinical applications. In addition, repeated acquisition modules of the SSJ sequence can be combined with some existing pulse sequences to shorten experimental durations or yield extra scalar-coupling information. All the experiments were performed on an 11.7 T (proton resonance frequency of 500 MHz) Agilent NMR System (Agilent Technologies, Santa Clara, CA, USA) with a 54 mm narrow bore, using a 5 mm indirect detection probe at 298 K. Data for SSJ were processed with a software (VnmrJ 3.2) provided by the vendor. The calculation time for LP and 2D DFT is typically 1 or 2 s. The pulse sequence code, reconstruction macro, and detailed description of the data reconstruction are provided (see ESI†). This work was partially supported by the National Natural Science Foundation of China under Grants 11105114 and 11375147 and the Natural Science Foundation of Fujian Province of China under Grant 2014J05012. The authors thank Prof. Tien-Mo Shih for linguistic assistance.
Notes and references 1 (a) R. Fu, E. D. Gordon, D. J. Hibbard and M. Cotton, J. Am. Chem. Soc., 2009, 131, 10830–10831; (b) S. A. Perera and R. J. Bartlett, J. Am. Chem. Soc., 2000, 122, 1231–1232; (c) Y. Miao, T. A. Cross and R. Fu, J. Biomol. NMR, 2013, 56, 265–273. 2 (a) W. P. Aue, J. Karhan and R. R. Ernst, J. Chem. Phys., 1976, 64, 4226–4227; (b) G. A. Morris, in Multidimensional NMR Methods for the Solution State, ed. G. A. Morris and J. W. Emsley, John Wiley & Sons Ltd, Chichester, 2010, ch. 11, pp. 145–159. 3 (a) M. Nilsson, A. M. Gil, I. Delgadillo and G. A. Morris, Anal. Chem., 2004, 76, 5418–5422; (b) C. Martineau, F. Fayon, C. Legein, J. Y. Buzare, G. Sillyd and D. Massiot, Chem. Commun., 2007, 2720–2722. 4 (a) C. Ludwiga and M. R. Viantb, Phytochem. Anal., 2010, 21, 22–32; (b) H. K. Kim, Y. H. Choi and R. Verpoorte, Nat. Protoc., 2010, 5, 536–549. 5 (a) J. E. Jensen, B. deB. Frederick, L. Wang, J. Brown and P. F. Renshaw, Magn. Reson. Med., 2005, 54, 783–788; (b) J. E. Jensen, B. B. Frederick and P. F. Renshaw, NMR Biomed., 2005, 18, 570–576. ˇe, T. Nishida and R. Freeman, Prog. Nucl. Magn. Reson. 6 (a) E. Kupc ˇe and R. Freeman, J. Am. Spectrosc., 2003, 42, 95–122; (b) E. Kupc Chem. Soc., 2004, 126, 6429–6440; (c) B. Jiang, F. Luo, Y. Ding, P. Sun, X. Zhang, L. Jiang, C. Li, X. Mao, D. Yang, C. Tang and M. Liu, J. Magn. Reson., 2010, 204, 165–168; (d) B. Jiang, X. Jiang, N. Xiao, X. Zhang, L. Jiang, X. Mao, D. Yang and M. Liu, Anal. Chem., 2013, 85, 2523–2528; (e) V. Y. Orekhov and V. A. Jaravine, Prog. Nucl. Magn. Reson. Spectrosc., 2011, 59, 271–292; ( f ) M. Mayzel, K. Kazimierczuk and V. Y. Orekhov, Chem. Commun., 2014, 50, 8947–8950. 7 L. Frydman, T. Scherf and A. Lupulescu, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 15858–15862. 8 (a) P. Giraudeau and S. Akoka, J. Magn. Reson., 2007, 186, 352–357; (b) P. Giraudeau and S. Akoka, J. Magn. Reson., 2008, 190, 339–345. 9 (a) J. J. Led and H. Gesmar, Chem. Rev., 1991, 91, 1413–1426; (b) Y. Y. Lin, P. Hodgkinson, M. Ernst and A. Pines, J. Magn. Reson., 1985, 62, 56l–567; (c) V. A. Mandelshtam, Prog. Nucl. Magn. Reson. Spectrosc., 2001, 38, 159–196. 10 (a) A. Bax, R. Freeman and G. A. Morris, J. Magn. Reson., 1981, 43, 333–338; (b) A. J. Pell and J. Keeler, J. Magn. Reson., 2007, 189, 293–299.
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Published on 18 November 2014. Downloaded by Selcuk University on 11/02/2015 04:01:37.
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Cite this: Chem. Commun., 2015, 51, 1234
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A single-scan method for NMR 2D J-resolved spectroscopy† Liangjie Lin, Zhiliang Wei, Yanqin Lin* and Zhong Chen
Received 1st October 2014, Accepted 18th November 2014 DOI: 10.1039/c4cc07751b www.rsc.org/chemcomm
A single-scan method is proposed to shorten the acquisition time for an NMR 2D J-resolved spectrum to 1 second. With high SNRs and acceptable resolutions, it constitutes an effective tool for obtaining decoupled proton spectra and fine scalar-coupling splitting patterns.
Nuclear magnetic resonance (NMR) awards us with molecular structure information because NMR parameters, such as chemical shifts and spin–spin coupling constants, possess high sensitivity to structural and conformational features of the molecules observed.1 NMR 2D J-resolved spectroscopy2 was developed to distinctly obtain the aforementioned details by separating the effects of chemical shifts and scalar couplings into two independent dimensions. It has been widely applied to chemical studies,3 metabolomics,4 and in vivo detections.5 Conventional 2D J-resolved spectroscopy (Con-J) suffers from long acquisition time due to the necessity of numerous t1 increments to construct the indirect dimension with a satisfactory resolution. To date numerous methods have been proposed to accelerate the acquisition of multidimensional NMR spectra.6 However, most of them cannot be efficiently applied to 2D J-resolved experiment. The spatially-encoded method7 replaces the conventional temporal encoding with spatial encoding and compresses the acquisition of a multidimensional spectrum into a single scan. It has been successfully applied to 2D J-resolved spectroscopy.8 A typical sequence of spatially encoded J-resolved spectroscopy (SEJ) is shown in Fig. 1(b). Low signal-to-noise ratio (SNR) is arguably the main drawback of the spatially-encoded method for broader applications. In this study, a scheme is proposed for recording 2D J-resolved spectra with high SNRs and decent spectral resolutions in a single scan (termed as SSJ, as shown in Fig. 1(c)). Every single acquisition module records evolutions of both chemical shifts and scalar couplings in the direct (F2) dimension, and it is repeated to
Department of Electronic Science, Fujian Provincial Key Laboratory of Plasma and Magnetic Resonance, State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc07751b
1234 | Chem. Commun., 2015, 51, 1234--1236
Fig. 1 (a) Conventional 2D J-resolved spectroscopic sequence, (b) SEJ sequence, and (c) SSJ sequence. The filled and open bars represent the p/2 and p pulses, respectively. The rectangles with sloping arrows indicate adiabatic frequency-swept p (chirp-p) pulses, GE the encoding gradient, GP the purge gradient, and GD the decoding gradient.
trace scalar-coupling evolutions in the indirect (F1) dimension. Phases of the repeated 1801 pulses are altered by referencing to (x, x, x, x) for alleviating pulse imperfections. Signals during even (or odd) acquisition modules are arranged in sequential order to yield a 2D array. For providing sufficient spectral widths in F1 dimensions (SW1 = 1/(2 Ta)), the acquisition time within one module (Ta) will be limited. As a result, the raw data of SSJ suffer from serious truncations in the F2 dimension. As it is known, liquid-state NMR signals can be efficiently modeled by the summation of a finite number of damped sinusoids. This character is fully used by several reported algorithms to achieve spectral resolutions beyond the restriction of the time-frequency uncertainty principle such as linear prediction9a (LP), matrix pencil method,9b and filter diagonalization method.9c The LP extrapolation, capable of alleviating truncation errors and enhancing resolutions of discrete Fourier transform (DFT) spectra, is applied to further predict the raw data of SSJ to fully decay in the F2 dimension. A sample of 0.5 M ethyl 3-bromopropionate in CDCl3 was tested with Con-J, SEJ, and SSJ sequences, and the results are
This journal is © The Royal Society of Chemistry 2015
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Published on 18 November 2014. Downloaded by Selcuk University on 11/02/2015 04:01:37.
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Fig. 2 2D J-resolved spectra of ethyl 3-bromopropionate (0.5 M in CDCl3) by SSJ (a), SSJ after LP extrapolation (b), Con-J (c) and SEJ (d). The F2 projections are given on the top of the boxed diagrams, and F1 projections for resonances located at 1.3 and 4.2 ppm are also given inside the boxed diagrams. Experimental parameters for SSJ: SW2 = 2.5 kHz, Ta = 12 ms (N2 = 30, SW1 = 42 Hz) and N = 30; for SEJ: BW = 25 kHz, encoding gradients GE = 1 G cm1, decoding gradients GD = 1.6 G cm1, encoding time TE = 60 ms, decoding time TD = 12 ms, frequency range of chirp pulses Pbw = 15 kHz and N = 30.
shown in Fig. 2. Fig. 2(a) shows the SSJ spectrum without any post-processing other than 2D DFT. The resolution in the F2 dimension is 102.0 Hz due to the limited acquisition time (Ta = 12 ms). With the introduction of LP extrapolation, it can be improved to 7.8 Hz (Fig. 2(b)). Fig. 2(c) and (d) depict the Con-J and SEJ spectra, respectively. All these methods can provide correct multiplet patterns in F1 dimensions as F1 projections shown in Fig. 2(a)–(d). Detailed comparisons among Fig. 2(b)–(d) are summarized in Table 1. From the table, we can see that the Con-J spectrum requires an experimental duration of 10 minutes that can be shortened by SSJ and SEJ to less than one second. The Con-J spectrum shows the best resolution in the F2 dimensions because of its full sampling, while resolutions of SSJ (7.8 Hz) and SEJ (24.0 Hz) in the F2 dimensions are limited by the performance of LP extrapolation and the spatial-encoding time (TE), respectively. The SNR of the SSJ spectrum (2537) is similar to that of the Con-J spectrum (2606), whereas the SEJ spectrum suffers from a serious SNR decrease (309), which primarily results from the enlarged receiver bandwidth (BW), molecular diffusion, and long spatial-encoding time. The SSJ and
Con-J sequences were also verified to exhibit satisfactory spectra for a low-concentration sample (5 mM ethyl 3-bromopropionate in CDCl3, see Fig. S1, ESI†). The limiting solute concentration for SEJ is approximately 100 mM with a reasonable linewidth in the F2 dimension. These experimental results demonstrate the time-cost advantage of SSJ over the conventional method and the SNR and resolution advantages of SSJ over SEJ. For the spatially-encoded technique, samples detected should be sufficiently homogeneous for applying spatial encoding. Complex pulse sequences also place strict requirements on hardware and experimental skills. The intrinsic shortcomings of SEJ indicate that the SSJ can be more suitable under certain conditions. A mixture solution comprising pyridine, 2,3-dihydrofuran and n-propanol with the same concentration of 100 mM in dimethyl sulfoxide-d6 (DMSO-d6) was detected by the SSJ sequence, and the results are shown in Fig. 3(c). The Con-J spectrum is shown in Fig. 3(b). The F2 projections shown in Fig. 3(b) and (c) indicate that SSJ can provide decoupled 1H spectra as Con-J can. The F1 projections for resonances marked with blue dashed rectangles in Con-J and SSJ spectra are shown in Fig. 3(d). There are complex coupling systems in the molecules of pyridine and dihydrofuran, resulting in complicated scalar-coupling splitting patterns. The SSJ can provide similar F1 projections as Con-J can, such as the F1 projections of resonances J, J0 and F, as shown in Fig. 3(d). Conventional 1D 1H experiments can provide superior spectral resolutions for small molecules in mobile solvents (0.9 Hz for the mixture solution). However, the performances of 1D 1H spectra
Table 1 Detailed comparisons of ethyl 3-bromopropionate spectra acquired with Con-J, SSJ, and SEJ
a
LW1 (Hz) LW2b (Hz) SNRc Duration
Con-J
SSJ
SEJ
1.0 3.6 2606 10 min
1.6 7.8 2537 720 ms
2.2 24.0 310 780 ms
a
Linewidths in F1 dimensions (LW1) were measured from F1 projections of the resonance located at 1.3 ppm. b Linewidths in F2 dimensions (LW2) were measured from F2 projections of the resonance located at 1.3 ppm. c The SNRs of resulting spectra were calculated by dividing the height of the peak located at 1.3 ppm by the root-mean-square of noise ranging from 1.8 to 2.2 ppm.
This journal is © The Royal Society of Chemistry 2015
Fig. 3 (a) Molecular structures for pyridine, 2,3-dihydrofuran and, n-propanol, (b) Con-J spectrum and its F2 projection of the mixture solution, (c) SSJ spectrum and its F2 projection, and (d) F1 projections of the resonances marked with blue dashed rectangles in Con-J (first row) and SSJ (second row) spectra. Experimental parameters for SSJ: SW2 = 5 kHz, Ta = 10 ms (N2 = 50, SW1 = 50 Hz) and N = 50. The total acquisition time was 1 second.
Chem. Commun., 2015, 51, 1234--1236 | 1235
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can be readily influenced by inhomogeneous magnetic fields. In addition, it is difficult for 1D 1H spectroscopy to interpret complicated molecules or mixtures due to its narrow chemicalshift dispersion range and scalar-coupling splitting. The time required for SSJ is identical to that for the conventional singlepulse experiment. The spin echo modules within the SSJ sequence suppress the effects from static magnetic field inhomogeneity, rendering F1 linewidths of the resultant J-resolved spectra approaching the natural limit of 1/(pT2). Moreover, by separating chemical shifts and scalar-coupling splitting into orthogonal dimensions, SSJ can present molecular structural information more clearly. Therefore, SSJ can serve as a complement for 1D spectroscopy when we analyze some complicated samples. Similar to the case of Con-J spectra, SSJ spectra need to be shown in the absolute-value mode due to the presence of dispersive antiphase contributions, which result in broadened linewidths. Therefore, J values measured may be unreliable. There are several ways to obtain phase-sensitive J-resolved spectra for more accurate J values, but they suffer from corresponding drawbacks. A popular method10a to use a pseudo-echo weighting function to eliminate the dispersive component, results in intensity distortions. The phasesensitive 2D J-resolved experiment proposed by Pell and Keeler10b can also provide excellent resolutions in both the dimensions with the penalty of SNR reductions. The resolutions of SSJ may not be sufficiently high for us to distinguish some close resonances in the F2 dimension. We present the experimental data of SSJ on quinine (a type of antimalarial drug; see Fig. S2, ESI†). The SSJ proves effective when providing fine structures of most of the resonances in the quinine molecule, especially in the aromatic and vinyl regions, and its performance degenerates in dealing with several crowded resonances in the aliphatic region. In higher magnetic fields, frequency differences between resonances will become larger, while scalar-coupling constants remain unchanged. Therefore, the performances of SSJ would be better under high magnetic fields. When LP is utilized for forward prediction, data points available must be at least twice the number of resonances existing in the free induction decay (FID) signals. If this condition is not fulfilled, the accuracy by which the frequencies are reproduced by the extrapolation will be unsatisfactory. Original data points of SSJ in the F2 dimension, determined by the spectral widths in both F1 and F2 dimensions (N2 = SW2/(2 SW1)) are limited. Therefore, the performances of SSJ may deteriorate when the systems with narrow frequency dispersion ranges and crowd resonances are analyzed. For ensuring the complete decay of extended SSJ FIDs, it is suggested to perform the LP extrapolation with the aid of suitable window apodizations.9a In summary, a single-scan method (SSJ) is proposed for ultrafast NMR 2D J-resolved spectroscopy. The SNRs and resolutions of the resulting SSJ spectra are similar to those of the conventional ones.
1236 | Chem. Commun., 2015, 51, 1234--1236
Communication
The SSJ can be a powerful spectroscopic tool in the areas of metabolite batch tests, organic reaction detections as well as clinical applications. In addition, repeated acquisition modules of the SSJ sequence can be combined with some existing pulse sequences to shorten experimental durations or yield extra scalar-coupling information. All the experiments were performed on an 11.7 T (proton resonance frequency of 500 MHz) Agilent NMR System (Agilent Technologies, Santa Clara, CA, USA) with a 54 mm narrow bore, using a 5 mm indirect detection probe at 298 K. Data for SSJ were processed with a software (VnmrJ 3.2) provided by the vendor. The calculation time for LP and 2D DFT is typically 1 or 2 s. The pulse sequence code, reconstruction macro, and detailed description of the data reconstruction are provided (see ESI†). This work was partially supported by the National Natural Science Foundation of China under Grants 11105114 and 11375147 and the Natural Science Foundation of Fujian Province of China under Grant 2014J05012. The authors thank Prof. Tien-Mo Shih for linguistic assistance.
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