1D NMR Homodecoupled ¹H Spectra with Scalar Coupling Constants from 2D NemoZS‐DIAG Experiments

Abstract

A two-dimensional liquid-state NMR experiment cleanly separating chemical shifts and scalar couplings information is introduced. This DIAG experiment takes advantage of a drastic reduction of the spectral window in the indirect dimension to be quickly recorded and of a new non-equidistant modulation of the selective pulse to improve the sensitivity of the broadband homodecoupling Zangger-Sterk sequence element by one order of magnitude. A simple automatic analysis results in 1D spectra displaying singlets and lists of the scalar couplings for first-order multiplets. This facilitates the analysis of 1D spectra by resolving multiplets based on their differences in chemical shifts and coupling structures. The complexity of 1D1 H spectra is a long-standing problem in many liquid-state NMR applications. This is mostly because of the presence of scalar coupling making it difficult to identify the individual resonances because the multiplet structure causes serious signal overlap (see Figure 1 a).Ideally, these complex multiplets should be separated in an additional dimension. If the classic J-resolved experiment was close to this goal, [1] problems with the signal lineshape and the complexity of the multiplet structures in the presence of second-order effects found partial solutions [2] but not satisfactory enough to become generally applicable. One solution consists in taking advantage of the large chemical shift dispersion of 13 C and use decoupled HSQC [3] or more recent variants [4] of this experiment to resolve signals and facilitate the determination of coupling constants.We developed an approach based on a 2D homonuclear sequence (see Figure 2) producing spectra where multiplets that are overlapping in 1D spectra (Figure 1 a) are spread along the indirect dimension (Figure 1 b). We called this experiment "DIAG" because it generates signals only along the diagonal (dashed line in Figure 1 b). Parallels could be drawn with earlier attempts to exploit the diagonal of COSY spectra.[5] But our approach, which is based on in-phase magnetization, avoids the difficulties of dealing with the numerous anti-phase magnetizations and the risk of signal cancellation because of the presence of positive and negative peaks. In order to simplify the structure of signals, the indirect F1 dimension is decoupled using the Zangger-Sterk sequence element (ZS) [6] because the constant-time incrementation used in many NMR pulse sequences cannot be applied to high-resolution homonuclear 1 H experiments. The first part of the nemoZS-DIAG sequence (see Figure 2) is quite similar to the pure shift experiment.[7] But instead of using the ZS element to generate series of spin echos, it is used to decouple only the indirect dimension of a 2D spectrum. In this manner, the precious scalar coupling information is retained in the detected dimension. The dresolved [8] experiment developed by Giraud et al. was aiming at the same goal, but it is based on version of the ZS sequence using more than one selective pulse which is detrimental. T...

High Resolution for Chemical Shifts and Scalar Coupling Constants: The 2D Real‐Time J‐Upscaled PSYCHE‐DIAG

“…Although it is possible to selectively excite certain protons to cope with overlap, the use of a DIAG experiment enables a route to an experiment, where all signals can be J‐upscaled and recorded well‐resolved simultaneously. A regular DIAG experiment yields a 2D spectrum where coupling constants can be read from horizontal traces. However, the resolution of the signals in each trace is often not sufficient for the identification of coupling patterns or the determination of small J ‐values.…”

“…(soft pulse excited Correlated SpectroscopY) The latter two separate chemical shift in the direct dimension from J‐coupling information in the indirect dimension. More recently, the DIAG (DIAGonal correlation) experiment was proposed by Cotte and Jeannerat . It generates spectra with signals only along the diagonal, where multiplets that would overlap in a standard 1D experiment are separated in the indirect dimension.…”