Selective Homonuclear Decoupling in 1H NMR: Application to

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J. Phys. Chem. A 2010, 114, 5551–5557

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Selective Homonuclear Decoupling in 1H NMR: Application to Visualization of Enantiomers in Chiral Aligning Medium and Simplified Analyses of Spectra in Isotropic Solutions Uday Ramesh Prabhu†,‡ and N. Suryaprakash*,‡ Solid State and Structural Chemistry Unit and NMR Research Centre, Indian Institute of Science, Bangalore 560 012, India ReceiVed: February 15, 2010; ReVised Manuscript ReceiVed: March 20, 2010

The proton NMR spectral complexity arising due to severe overlap of peaks hampers their analyses in diverse situations, even by the application of two-dimensional experiments. The selective or complete removal of the couplings and retention of only the chemical shift interactions in indirect dimension aids in the simplification of the spectrum to a large extent with little investment of the instrument time. The present study provides precise enantiodiscrimination employing more anisotropic NMR parameters in the chiral liquid crystalline medium and differentiates the overlapped peaks of many organic molecules and peptides dissolved in isotropic solvents. 1. Introduction The visualization of enantiomers and their quantification are becoming increasingly important in the pharmaceutical industry as well as in asymmetric synthesis. It is well-known that enantiomers cannot be distinguished by NMR spectra unless diastereomorphic interactions are invoked by utilizing either solvating or derivatizing agents.1 Another direction of approach is the use of chiral liquid crystal, poly-γ-benzyl-L-glutamate (PBLG) as an alignment medium for enantiodifferentiation. With helicogenic solvents such as CDCl3, CD2Cl2, dimethylformamide (DMF), and so forth, the homopolypeptide PBLG exhibits liquid crystallinity leading to differential orientations of dissolved enantiomers.2 This gives rise to different values for ordersensitive NMR parameters, such as residual dipolar couplings (Dij), chemical shift anisotropies (∆σi), and quadrupolar couplings (Qi) in the case of nuclei with spin I > 1/2. Various NMR active nuclei such as 13C, 2H, and so forth are generally employed for chiral discrimination.3,4 In spite of 1H being abundantly present in all the organic compounds and being more sensitive for detection, the utilization of proton-proton residual dipolar couplings for enantiodiscrimination is severely hindered due to enormous spectral broadening arising from the overlap of numerous unresolved dipolar split peaks, in addition to the overlap of peaks from both the enantiomers. In spite of inherent difficulties, several novel methods have been developed for the simplification of their spectral complexity.5-7 It has been demonstrated that correlation spectroscopy (COSY) and its variants can be employed for chiral discrimination.8-11 The disadvantage of these COSY-type experiments is the presence of chemical shift and coupling interactions both in F1 and F2 dimensions. The separation of interactions in twodimensional experiments is a well-documented procedure for spectral simplification, as in the proton J-resolved experiment, where only couplings evolve in the indirect dimension whereas, the 45° tilt of the spectrum provides only chemical shifts in the F2 dimension.12 Selective refocusing (SERF) types of experiments, although they simplify the spectrum by refocusing the * To whom correspondence should be addressed. † Solid State and Structural Chemistry Unit. ‡ NMR Research Center.

chemical shift and the couplings in the F1 dimension,13 prevent the utilization of chemical shift parameter for chiral discrimination.14 Instead of selective decoupling, one can adopt broadband decoupling to obtain only the chemical shift information.15-18 Therefore, an improved experiment that utilizes both ∆σH and DHH results in better chiral discrimination. In this article, we report the applications of the phase-sensitive J-resolved experiment and the F1 decoupled COSY experiment for the enantiodiscrimination of chiral molecules dissolved in chiral liquid crystalline medium. The application of the latter method has also been demonstrated in the removal of overlapped multiplets in halogen-substituted benzanilide and in the simplification of NH and CRH regions of undecapeptide cyclosporine A in isotropic solvents.20,21 2. Experimental Section The detailed procedure for the preparation of the liquid crystalline samples has already been reported.9 The isotropic samples of benzanilide and cyclosporine A were prepared in the solvents CDCl3 and C6D6, respectively. All experiments were carried out on Bruker DRX-500 NMR spectrometer at a temperature of 301 K. The temperatures were optimized using a Bruker BVT 3000 temperature controller unit. A z-filter was applied in the two-dimensional (2D) experiments to purge out all unwanted coherences, such as zero-quantum coherences, which is essential to get a clean and phase-sensitive spectrum pertaining to methyl protons that are strongly coupled. The z-filter also suppresses the zero quantum coherences in the biselective F1-decoupled COSY experiment. Except in the case of strongly coupled methyl groups, the clean and phase-sensitive spectrum was obtained without any z-filter. For suppression of the zero-quantum coherences, the swept 180° pulses used in both the sequences are smoothed CHIRP with a bandwidth of 31 kHz and of 31 ms duration. The homospoil gradient pulse is of strength 11 G/cm applied for the duration of 2 ms. A simultaneous gradient pulse of strength 4 G/cm was applied during CHIRP pulse. Two-step phase cycling was used to suppress the axial peaks. The phase of the first pulse and receiver was cycled according to x, -x. Unless otherwise mentioned, the remaining pulses are of phase x. The experimental and

10.1021/jp1013994  2010 American Chemical Society Published on Web 04/08/2010

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Figure 1. Pulse sequences for (A) F1-decoupled COSY and (B) biselective F1-decoupled COSY. The filled and unfilled Gaussian shapes corresponds to selective 180° (ReBurp) and 90° (EBurp2) pulses, respectively. The thin and thick rectangular pulses are hard 90° and 180° pulses, respectively. tz is the zero quantum suppression period. The chemical structure and the numbering of protons for the studied molecules are given (1) (R/S)-2chloropropanoic acid, (2) (R/S)-propylene carbonate, (3) (R/S)-propylene oxide, and (4) cyclosporine A.

processing parameters for each experiment are reported in the corresponding figure captions. 3. Results and Discussion 3.1. Visualization of Enantiomers. To demonstrate the application of the above two-dimensional methods for enantiodiscrimination, we have chosen three different chiral molecules whose chemical structures are given in Figure 1. The conventional J-resolved spectra of (R/S)-propylene carbonate (2) is reported in Figure 2A. The drawback of this experiment is the phase-twisted line shapes, which necessitate the presentation of the 2D data matrix in a magnitude mode, thereby compromising the resolution achievable.19 The phase-sensitive, double absorptive spectrum has been obtained by appending a z-filter element with an adiabatic swept 180° pulse (CHIRP) to the J-resolved pulse sequence.22 Although this modification results in higher resolution, the z-filter element retains the full multiplet structure unlike in the conventional J-resolved spectrum, thereby restricting the process of tilting and obtaining a proton-decoupled proton spectrum. The pattern recognition algorithm has been reported to generate the broadband proton-decoupled proton spectrum from this multiplet pattern.22,23 We have applied the z-filter-appended J-resolved method for the visualization of enantiomers of (R/S)-propylene carbonate aligned in PBLG. It may be pointed out that, when dealing with aligned chiral molecules giving rise to spectra amenable for first-order analyses, the J-resolved experiment should be construed as both J- and D-resolved experiments. Complete multiplicity in the z-filtered version is evident in Figure 2B. As far as chiral

discrimination is concerned, this is an advantage since the dipolar coupled pattern enables the filtering of the spectrum for each enantiomer. Therefore, we have not employed a pattern recognition algorithm to generate a singlet from the multiplet pattern. The differentiation achieved is depicted by the dashed horizontal lines in Figure 2B, and the corresponding cross sections for R and S enantiomers are plotted adjacent to the 2D spectrum. The advantage of the z-filtered J-resolved experiment is clearly obvious, as one can extract a complete spectrum for each enantiomer (Figure 2B) unlike a single transition in a conventional J-resolved spectrum (Figure 2A). 3.1.2. SelectiWe Refocusing to Reduce Multiplicity. In spite of achieving enantiodiscrimination, the z-filtered J-resolved spectrum is complicated due to the presence of couplings in both the dimensions. This complexity can be simplified by breaking the coupling among few selected protons or all the protons. This is achieved by using the SERF experiment, which is a selective pulse analogue of the J-resolved experiment.13 In the SERF experiment, initially the multiplet to be detected is selectively excited, and the couplings to be retained between the protons are selectively inverted by using soft 180° pulses. This implies that the application of all selective pulses on the methyl protons ensures the evolution of only the coupling among methyl protons during the t1 period while refocusing the remaining couplings. This results in only two distinct dipolar split triplets, one for each enantiomer, in the indirect dimension. Not only are the components of triplets well separated, but the line widths are also reduced as the 180° pulse applied in the middle of evolution period refocuses the magnetic field inho-

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Figure 2. (A) The 2D J-resolved spectrum of (R/S)-propylene carbonate oriented in PBLG. The size of the 2D data matrix is 256 × 4096. Spectral widths are 100 and 2248 Hz in F1 and F2 dimensions. The number of accumulations for each t1 increment is one. The relaxation delay is 1.0 s. The data was zero filled to 512 and 8192 points in F1 and F2 dimensions, respectively, and was processed with a 90° shifted sine bell window function. The spectrum is presented in magnitude mode. For B, the experimental parameters are same as those in A, except data is presented in phasesensitive mode using time-proportional phase incrementation (TPPI) and processed without any window function. The multiplets pertaining to different protons are marked. The cross sections corresponding to the enantiopure spectrum of the methyl region in both A and B are represented by a dashed line and are plotted adjacent to them: (i) the methyl region of the one-dimensional proton spectrum, (ii) R enantiomer, and (iii) S enantiomer.

Figure 3. (A) The 2D SERF spectrum corresponding to H4 proton of (R/S)-propylene carbonate oriented in PBLG. Methyl protons are selectively excited and inverted by pulses of the duration 3.13 ms. The size of the 2D data matrix is 256 × 512. Spectral widths are 70 and 200 Hz in F1 and F2 dimensions, respectively. The number of accumulations for each t1 increment is one. The relaxation delay is 1.0 s. The data were zero filled to 512 and 1024 points and processed with a 90° shifted sine bell window function. The spectrum is presented in the magnitude mode. (B) SERFPh and (C) F1-decoupled COSY all the experimental parameters are same as in A, except for the presentation of data in phase-sensitive mode using TPPI and processed without any window function and with zero-quantum suppression. Note the full multiplicity of peaks in spectrum B and C. “*” corresponds to the strong coupling artifacts.

mogeneity. An added advantage of the SERF experiment is that it can be utilized to measure the enantiomeric excess. The phase-twisted SERF spectrum is made purely absorptive by appending the z-filter element to the original pulse sequence.13 The methyl proton-excited SERF spectrum of (R/S)propylene carbonate is reported in Figure 3A. Analogous to the J-resolved spectrum (Figure 2B), the multiplet pattern in the z-filtered SERF experiment, cited in the literature as SERFPh,24 is also complex, as evident from Figure 3B. The phase-sensitive spectrum obtained with full multiplicity could be utilized to extract the enantiopure spectrum as in the case of the z-filtered

J-resolved spectrum. Quantification of enantiomeric excess is also possible provided the strong coupling artifacts do not interfere with the genuine peaks. In Figure 3B, genuine peaks and strong coupling artifacts (marked *) are not well separated, thereby limiting the precise determination of enantiomeric excess. 3.1.3. Retention of Both Chemical Shift and Dipolar Coupling Information. A major drawback of the methyl selective excited J-resolved and SERF experiments is that the central transition does not evolve, as chemical shift is refocused during the evolution period. Retention of this chemical shift

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Figure 4. (A) The 2D SERFPh spectrum of (R/S)-2-chloropropanoic acid oriented in PBLG with zero-quantum suppression by the Thrippleton-Keeler element. Methyl protons are selectively excited and inverted by pulses of the duration 6.25 ms. The size of the 2D data matrix is 128 × 512. Spectral widths are 110 and 250 Hz in F1 and F2 dimensions, respectively. The number of accumulations for each t1 increment is 2. The relaxation delay is 1.0 s. The data were zero filled to 512 and 1024 points and processed without any window function. The spectrum is presented in phasesensitive mode using TPPI. (B) F1-decoupled COSY with the same experimental parameters as those in A. (C) The 2D SERFPh spectrum of (R/S)-2-chloropropanoic acid oriented in PBLG and with zero-quantum suppression. Methine proton is selectively excited and inverted by pulses of the duration 25.0 ms. The size of 2D data matrix is 32 × 512. Spectral widths are 10 and 200 Hz in F1 and F2 dimensions respectively. Number of accumulations for each t1 increment is 2. The relaxation delay is 1.0 s. The data were zero filled to 128 and 1024 points and processed without any window function. The spectrum is presented in phase-sensitive mode using TPPI. (D) F1-decoupled COSY with the same experimental parameters as those in C. “*” corresponds to the strong coupling artifacts.

information contributes to an additional parameter for chiral discrimination, provided there is a measurable chemical shift difference between the two enantiomers. Therefore, an experiment that permits chemical shift evolution with sufficient resolution to extract couplings would be of significant help. This condition has been achieved by a correlation experiment with the combination of soft and hard 180° pulses.16-18 We have employed the reported pulse sequence with an additional 180° pulse to achieve decoupling in the F1 dimension. The pulse sequence for this F1 decoupled COSY experiment, reported in Figure 1A, resembles the SERFPh experiment except for the employment of a hard 180° pulse in the middle of evolution period.25 This combination of soft and hard 180° pulses inverts only the out-of-band protons, hence all the couplings with this out-of-band protons are refocused, whereas the selectively excited protons experience 360° rotation, resulting in the retention of chemical shifts and couplings among them. It may be pointed out that, along with chemical shift, magnetic field inhomogeneities are also retained, giving rise to additional line broadening. Thus, as far as the resolution is concerned, the SERFPh experiment is better compared to F1-decoupled COSY. The F1-decoupled COSY spectrum of propylene carbonate is reported in Figure 3C. In this specific example, although there is chemical shift evolution in the F1 dimension, due to negligible difference in their values between the enantiomers, the central peaks are not resolved. Therefore, SERFPh (Figure 3B) and F1-decoupled COSY spectra (Figure 3C) look alike in this specific example. In situations where there is a measurable chemical shift difference to visualize enantiomers, SERF kills this interaction.

This concept is obvious in the SERFph and F1-decoupled COSY spectra of selectively excited methyl and methine groups of (R/S)-2-chloropropanoic acid (1), reported in Figure 4. Figure 4A,C corresponds to the SERFPh experiment on methyl (H4) and methine (H5) protons, respectively. The differential values of dipolar couplings enable chiral discrimination from the methyl region of the spectrum, whereas absence of dipolar coupling of the methine region prevents this discrimination. Figure 4B,D corresponds to F1-decoupled COSY spectra of H4 and H5 protons. In both these spectra, the chemical shift difference between the enantiomers will be an additional parameter, in addition to the dipolar coupling, enabling better chiral discrimination. For methyl protons selective experiment, both excitation and inversion pulses are applied on methyl protons. Consequently, only chemical shift and coupling interactions between methyl protons are observed (Figure 4B). The central transitions of the triplet are now resolved with a chemical shift difference of 1.1 Hz. Similarly, in the methine proton excited spectrum (Figure 4D), all the coupling interactions with methine proton are refocused, and only the chemical shift evolves during the t1 period. Hence, only two singlets, one at each chemical shift position of an enantiomer, are detected in the F1 dimension with a separation of 1.7 Hz. It may be pointed out that it is not possible to use this chemical shift information for chiral discrimination in the SERFph experiment. Therefore, the present experiment is a significant improvement providing better chiral visualization. This observation has been further exemplified by comparing normal 1D spectrum with the F1 projections of both SERFPh and F1-decoupled COSY experi-

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Figure 5. (A) The 2D SERFPh spectrum of (R/S)-propylene oxide recorded oriented in PBLG. The diastereotopic proton H7 is selectively excited, and diastereotopic protons H7 and H6 are selectively inverted by pulses of the duration 62.50 ms. The size of the 2D data matrix is 128 × 4096. Spectral widths are 50 and 2042 Hz in F1 and F2 dimensions. The number of accumulations for each t1 increment is 2. The relaxation delay is 1.0 s. The data were zero filled to 512 and 8192 points and processed without any window function. (B) Biselective F1-decoupled COSY where all the experimental parameters are same as (A). (C) F1-decoupled COSY of H7. The size of 2D data matrix is 32 × 1024. Spectral widths are 10 and 250 Hz in F1 and F2 dimensions. The number of accumulations for each t1 increment is 1. The relaxation delay is 1.0 s. The data was zero filled to 64 and 2048 points and processed without any window function. The chemical shift difference between the enantiomers measured by the spectrum (A) and (C) is 0.9 Hz. Results are compared with the F1 projections of each experiment (ii, iii and iv) and the normal one-dimensional proton spectrum (i).

ments, which are plotted adjacent to the 2D spectra and denoted as i, ii, and iii, respectively (Figure 4). There are other modifications of SERF experiments, where coupling between a particular pair of protons is retained while the coupling to other protons is removed. In such situations, one of the proton resonances is excited and subsequently detected, whereas the inversion pulse is applied to both the protons whose coupling information is to be retained. Recently an improved version of SERFPh was developed,25 where an additional soft 180° pulse and Keeler-Thrippleton filter was applied to improve the quality of the spectrum by suppressing the evolution of unwanted coherences.26 But none of these experiments provide an additional chemical shift parameter for chiral discrimination. 3.1.4. BiselectiWe F1-Decoupled COSY. Additional improvement has been carried out in the present study to reintroduce the chemical shift interaction, which was lost in SERFPh and improved SERFPh experiments, by the combined use of soft and hard 180° pulses. This pulse sequence, shown in Figure 1B, is similar to the SERFPh pulse sequence except for the addition of a hard 180° pulse. We prefer to term this pulse sequence as the biselective F1-decoupled COSY experiment, where chemical shift difference and coupling between only two protons are allowed to evolve. The spectra for SERFPh and F1-decoupled COSY of (R/S)-propylene oxide (3) aligned in PBLG are reported in Figure 5A,C, respectively, where diastereotopic protons H6 and H7 are employed for experimental demonstration. The proton H7 is detected while retaining the coupling to proton H6 and chemical shift interaction. It is clearly evident that there is only coupling evolution in SERFPh (Figure 5A) and there is no chemical shift evolution. Therefore a distinct dipolar split doublet is observed for each enantiomer in F1

dimension. On the other hand, in F1-decoupled COSY (Figure 5C), there is only chemical shift evolution, and couplings are removed. Thus two distinct singlets at the chemical shift positions of each enantiomer are observed in the F1 dimension. It implies that one has to employ two different experiments to obtain both chemical shift and dipolar coupling information. Furthermore, the chiral discrimination is achieved in these experiments by utilizing one of the two parameters. The immediate advantage of the present biselective F1 decoupled COSY experiment is obvious from the spectrum reported in Figure 5B, which provides both sets of information in a single experiment. For clarity, the F1 cross sections corresponding to these spectra are plotted adjacent to Figure 5C. The cross section iii is clearly distinct from ii and iv and contains both chemical shift and dipolar coupling information. These experiments also have potential applications in diverse chemical problems. Such situations are encountered in, viz., the removal of the multiplet overlap and also for the simplified analysis of the spectra in isotropic solutions. Applications in combating such problems are discussed in the following sections taking specific examples. 3.2. Separation of Overlapped Multiplets in Organic Molecules. The overlap of the peaks is inevitable in the 1H NMR spectra of many organic molecules. For example, the onedimensional 1H NMR spectra of several dihalogen-substituted benzanilides are very complex.27,28 A typical one-dimensional spectrum of 3-bromo-N-(2-fluorophenyl)benzamide in the solvent CDCl3 and its chemical structure with numbering of protons are reported in Figure 6A. The analysis of the spectrum has already been reported.20 The particular region of the spectrum marked with a dashed rectangle pertains to protons numbered 2 and 7. The severe overlap of the peaks hampers the

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Figure 6. (A,B) The chemical structure of 2-bromo-N-(2-fluorophenyl) benzamide and its one-dimensional proton spectrum in the solvent CDCl3. (C) The 2D F1-decoupled COSY spectrum of the molecule in the isotropic solvent CDCl3. Overlapped region pertaining to protons H2 and H7 are selectively excited and inverted by pulses of the duration 250.0 ms. The size of the 2D data matrix is 32 × 2048. Spectral widths are 10 and 500 Hz in F1 and F2 dimensions. The number of accumulations for each t1 increment is one. The relaxation delay is 1.0 s. The data was zero filled to 128 and 4096 points and processed without any window functions. The data is presented in phase-sensitive mode using TPPI. The one-dimensional proton spectrum marked with dashed rectangle is expanded and reported in (i). The F2 cross sections denoted by two arrows are plotted in (ii) and (iii), and the F1 cross section is plotted in (iv) showing a chemical shift difference of 2.1 Hz.

Figure 7. (A) The 2D F1-decoupled COSY of the NH region of cyclosporine A in the solvent CDCl3. The NH and CRH are selectively excited and inverted by pulses of the duration 12.50 and 8.064 ms, respectively. The size of the 2D data matrix is 512 × 1024. the spectral width is 500 Hz in both F1 and F2 dimensions. The number of accumulations for each t1 increment is two. The relaxation delay is 1.0 s. The data was zero filled to 512 and 8192 points and processed without any window function. The F1 and F2 cross sections are plotted below the 2D spectrum. (B) F1decoupled COSY of the CRH region of cyclosporine A. The size of the 2D data matrix is 1024 × 4096. Spectral widths are 750 and 1250 Hz in F1 and F2 dimensions. The number of accumulations for each t1 increment is two. The relaxation delay is 1.0 s. The data was zero filled to 1024 and 4096 points and processed without any window function. The F1 and F2 cross sections are plotted below the 2D spectrum.

identification of multiplet pattern for these protons. Similar problems have been encountered in many substituted benzanilides studied.20,27,28 In combating the problem of overlap, higher quantum correlation and resolved experiments have been

adopted previously. In the present work we demonstrate the usefulness of the F1-decoupled COSY experiment in unraveling these overlapped multiplets without resorting to complicated higher quantum experimental methodology. Figure 6C contains

Selective Homonuclear Decoupling in 1H NMR the selectively excited F1-decoupled COSY spectrum of this overlapped region marked in Figure 6B. The F1 dimension of the 2D spectrum reveals two independent chemical shifts for protons 2 and 7 with a separation of 2.0 Hz. The cross section taken along the F2 dimension at the chemical shift position of each proton identifies the multiplet pattern from which the couplings can be easily extracted. The one-dimensional spectrum (i), F2 cross sections at F1 chemical shifts (ii and iii), and the F1 projection (iv) are plotted adjacent to the 2D spectrum. The significant advantage of the present experiment is clearly obvious from these plots. 3.3. Application to Cyclosporine A. The application of this method for bigger molecules is demonstrated by taking the example of an 11 residue peptide cyclosporine A dissolved in the solvent C6D6.21 Consequent to the methylation of seven NH protons, the 1H NMR spectra of this undecapeptide has only four doublets in the NH region due to vicinal (3JCRH-NH) coupling. Measurement of the doublet separations in the F2 dimension provide vicinal coupling from which the dihedral angle, a useful parameter for the structure calculation, can be measured. The application of the selective F1-decoupled COSY experiment of the amide region (Figure 7A) provides singlets for each NH proton in the F1 dimension and a doublet in the F2 dimension. The corresponding F1 and F2 projections (i and ii) are plotted below Figure 7A. In this particular example, NH peaks are well isolated. When severe overlap of peaks is encountered, the measure of these doublet separations is a challenging task. Thus the present experiment will be useful in situations when many NH peaks or peaks in other regions of the spectrum are overlapped. For demonstrating such an application, the selective F1-decoupled COSY experiment has been carried out for the CRH region of the molecule. The CRH region contains 11 individual proton chemical shifts. Of these, 10 CR protons are well isolated, and we have carried out F1 decoupling for this selectively excited region of the spectrum. This COSY spectrum is reported in Figure 7B along with F1 and F2 projections (iii and iv) plotted below the corresponding 2D spectrum. The experiment reveals all 10 CRH chemical shifts in the F1 dimension. The F2 cross sections extracted at each singlet position correspond to the multiplet pattern from which different couplings to particular CRH can be determined. Thus the present experiment not only enables the unraveling of overlap but also permits the precise measure of chemical shift and couplings, even in situations when there is severe overlap of peaks. Since only the region of interest is selectively excited, the spectral width in the F1 dimension has been drastically reduced, giving better resolution and little investment of instrumental time compared to their respective hard pulse analog experiments. In addition to several advantages, the experiment also has some limitations. The primary requirement is the availability of isolated multiplets for selective excitation. To extract the complete information in a bigger molecule, many selective 2D experiments have to be performed. In such situations, a broadband proton decoupled proton spectrum can be obtained by a recently published method, which gives the complete information in a single experiment.15 The disadvantage of broadband decoupling is the poor sensitivity, as the slice selection technique is employed. This requires instrumental time of several hours compared to a few minutes in selective experiments. 4. Conclusions It is demonstrated that F1-decoupled COSY type experiments can be applied to combat many problems that are generally

J. Phys. Chem. A, Vol. 114, No. 17, 2010 5557 encountered in solving chemical problems such as visualization of enantiomers and extraction of couplings in overlapped and crowded regions of 1H NMR spectra of many organic molecules and peptides. The F1-decoupled experiments utilize the difference in proton chemical shifts between the enantiomers as an additional parameter for enantiodifferentiation, in addition to residual proton-proton dipolar couplings. The potential application of the experiment in spectral simplification is also demonstrated on dihalogen substituted benzanilide and on undecapeptide cyclosporine A. Although the study has been demonstrated taking specific examples, the experiments have potential applications in many such problems generally encountered in organic chemistry. The selective excitation makes the experiment faster and enhances the resolution. Acknowledgment. U.R.P. would like to thank CSIR for a senior research fellowship. We would like to thank Prof. T. N. Guru Row and Mr. Susanta Kumar Nayak for providing the sample of benzanilide. N.S. gratefully acknowledges the financial support by Board of Research in Nuclear Sciences, Mumbai, for the Grant No. 2009/37/38/BRNS. References and Notes (1) Parker, D. Chem. ReV. 1991, 91, 1441–1457. (2) Sarfati, M.; Lesot, P.; Merlet, D.; Courtieu, J. Chem. Commun. 2000, 2069–2081. (3) Meddour, A.; Berdague´, P.; Hedli, A.; Courtieu, J.; Lesot, P. J. Am. Chem. Soc. 1997, 119, 4502–4508. (4) Lesot, P.; Courtieu, J. Prog. Nucl. Magn. Reson. Spectrosc. 2009, 55, 128–159. (5) Farjon, J.; Ziani, L.; Beguin, L.; Merlet, D.; Courtieu, J. Annu. Rep. NMR Spectrosc. 2007, 61, 283–293. (6) Bikash, B.; Uday, R. P.; Suryaprakash, N. Annu. Rep. NMR Spectrosc. 2009, 67, 331–423. (7) Ziani, L.; Courtieu, J.; Merlet, D. J. Magn. Reson. 2006, 183, 60– 67. (8) Nath, N.; Suryaprakash, N. J. Magn. Reson. 2010, 202, 34–37. (9) Uday, R. P.; Bikash, B.; Suryaprakash, N. J. Phys. Chem. A 2008, 112, 5658–5669. (10) Uday, R. P.; Suryaprakash, N. J. Magn. Reson. 2008, 195, 145– 152. (11) Uday, R. P.; Suryaprakash, N. J. Magn. Reson. 2010, 202, 217– 222. (12) Aue, W. P.; Karhan, J.; Ernst, R. R. J. Chem. Phys. 1976, 64, 4226– 4227. (13) Farjon, J.; Merlet, D.; Lesot, P.; Courtieu, J. J. Magn. Reson. 2002, 158, 169–172. (14) Uday, R. P.; Bikash, B.; Suryaprakash, N. J. Magn. Reson. 2008, 191, 259–266. (15) Giraud, N.; Joos, M.; Courtieu, J.; Merlet, D. Magn. Reson. Chem. 2009, 47, 300–306. (16) Bruschweiler, R.; Griesinger, C.; Sorensen, O. W.; Ernst, R. R. J. Magn. Reson. 1988, 78, 178–185. (17) Straus, S. K.; Bremi, T.; Ernst, R. R. Chem. Phys. Lett. 1996, 262, 709–715. (18) Zangger, K.; Sterk, H. J. Magn. Reson. 1997, 124, 486–489. (19) Pell, A. J.; Keeler, J. J. Magn. Reson. 2007, 189, 293–299. (20) Manjunatha Reddy, G. N.; Susanta Kumar, N.; Guru Row, T. N.; Suryaprakash, N. Magn. Reson. Chem. 2009, 47, 684–692. (21) Kessler, H.; Loosli, H. R.; Oschkinat, H.; Ltd, S. HelV. Chim. Acta 1985, 68, 661–681. (22) Luy, B. J. Magn. Reson. 2009, 201, 18–24. (23) Woodley, M.; Freeman, R. J. Magn. Reson., Ser. A. 1994, 109, 103–112. (24) Beguin, L.; Courtieu, J.; Ziani, L.; Merlet, D. Magn. Reson. Chem. 2006, 44, 1096–1101. (25) Beguin, L.; Giraud, N.; M Ouvrard, J.; Courtieu, J.; Merlet, D. J. Magn. Reson. 2009, 199, 41–47. (26) Thrippleton, M. J.; Keeler, J. Angew. Chem., Int. Ed. 2003, 42, 3938–3941. (27) Manjunatha Reddy, G. N.; Guru Row, T. N.; Suryaprakash, N. J. Magn. Reson. 2009, 196, 119–126. (28) Bikash, B.; Manjunatha Reddy, G. N.; Uday, R. P.; Guru Row, T. N.; Suryaprakash, N. J. Phys. Chem. A. 2008, 112, 10526–10532.

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