Towards perfect NMR: Spin‐echo versus perfect‐echo building blocks

Abstract

The development of new tools to improve the quality of nuclear magnetic resonance (NMR) spectra is a challenging task. The concept of "perfect NMR" includes the design of robust pulse sequences that allow an investigator to obtain undistorted pure in-phase signals, with pure absorption lineshapes that are free of phase anomalies derived from undesired J modulations. Here, alternative NMR building blocks to the spin-echo that are based on a general double SE module, known as a perfect-echo, are reviewed. Several implementations to minimize/remove unwanted dispersive contributions in homonuclear and heteronuclear NMR experiments are described and illustrated with some examples of broad interest for small molecules.

t1-Noise Eliminated Dipolar Heteronuclear Multiple-Quantum Coherence Solid-State NMR Spectroscopy

“…Analogous ideas but in different contexts have been reported previously; for instance, p pulses have been used to refocus the residual effects of dipolar coupling in continuous wave decoupling sequences 52 and the CSA contributions in T1r measurements, 53 whereas the perfect echo sequences in solution NMR use p/2 pulses to refocus anti-phase components. 54 Brinkmann et al have proposed the use of dual channel symmetry-based recoupling sequences to selectively recouple heteronuclear dipolar interactions while suppressing CSA terms. 55 TONE D-HMQC-2 is similar to TONE D-HMQC-1, except a p/2±y pulse is appended at the end of the first refocused dipolar recoupling block ( Figure 1C).…”

“…This result is unexpected because additional pulses typically reduce the efficiency of NMR experiments. With the addition of the p/2 flip-back pulse, the symmetry of the TONE D-HMQC-2 pulse sequence is reminiscent of the perfect echo pulse sequences, 54 which are known to refocus homonuclear 1 35 Cl trace improves by a factor 1.74 (0.94/0.54), while there is only a modest gain in 35 Cl sensitivity by a factor 1.2 (3.6 min -1/2 /3.1 min -1/2 ) ( Figure 3H). As t1-noise is multiplicative, meaning it is proportional to the signal intensity, the t1-noise is likely also higher in the case of TONE D-HMQC-2 and this leads to a smaller gain in sensitivity with respect to TONE D-HMQC-1.…”

t1-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

“…Analogous ideas but in different contexts have been reported previously; for instance, p pulses have been used to refocus the residual effects of dipolar coupling in continuous wave decoupling sequences 55 and the CSA contributions in T 1r measurements, 56 whereas the perfect echo sequences in solution NMR use p/2 pulses to refocus anti-phase components. 57 Brinkmann et al have proposed the use of dual channel symmetrybased recoupling sequences to selectively recouple heteronuclear dipolar interactions while suppressing CSA terms. 58 TONE D-HMQC-2 is similar to TONE D-HMQC-1, except a p/2 AEy pulse is appended at the end of the first refocused dipolar recoupling block (Fig.…”

“…This result is unexpected because additional pulses typically reduce the efficiency of SSNMR experiments. With the addition of the p/2 flip-back pulse, the symmetry of the TONE D-HMQC-2 pulse sequence is reminiscent of the perfect echo pulse sequences, 57 which are known to refocus homonuclear 1 H couplings. 2-Spin 1 H-1 H SIMPSON numerical simulations and experiments with dipolar recoupled perfect echo pulse sequences show increased 1 H signal intensity, suggesting that the introduction of purge pulses (p/2 or LG spin-lock) may refocus 1 H homonuclear dipolar couplings (Fig.…”

A community-built calibration system: The case study of quantification of metabolites in grape juice by qNMR spectroscopy

“…[5,6] Since the intensity of a signal is linearly proportional to the number of NMR active nuclei generating the signal, the response factor (ratio between the signal produced by the analyte and the quantity of analyte which produces the signal) is independent on the molecule and the quantification of the analyte can be achieved directly by calculating the area under the signal (integral). [7] Moreover, the design of new pulse sequences for FIDs acquisition [8][9][10][11][12][13] and novel algorithms for data processing [14][15][16][17] enhanced the capability of NMR for discriminating among very similar compounds contained in complex mixtures, as pharmaceutical, natural products, agrochemicals, foodstuff, and biological fluids. Nevertheless, to date few official protocols have been reported which employ NMR technique for purity assessment and quantification purposes.…”