High-resolution 2D NMR spectra in inhomogeneous fields based on intermolecular multiple-quantum coherences with efficient acquisition schemes

Spatially encoded ultrafast high-resolution 2D homonuclear correlation spectroscopy in inhomogeneous fields

Two-dimensional (2D) NMR spectroscopy shows strong vitality and unsubstitutable significance among NMR techniques. In many cases, however, it is virtually impossible to obtain high-resolution 2D spectra in inhomogeneous fields. Furthermore, conventional 2D NMR spectroscopy suffers from long acquisition time. In this paper, two new pulse sequences tracking the differences of the precession frequencies of two coupled spins are proposed to ultrafast achieve high-resolution 2D correlation spectroscopy (COSY and TOCSY) in inhomogeneous fields in a single scan. The spectral width of the indirect dimension can be reduced to improve the spectral resolution without loss of the correlative information for reconstructing the COSY and TOCSY spectra with mathematical manipulations. The theoretical analysis was given and experiments were performed to verify theoretical predictions. The results show that the proposed sequences can give correct 2D COSY and TOCSY spectra if the field inhomogeneity is linear along the orientation of encoding and decoding gradients.

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.

Reconstruction of self-sparse 2D NMR spectra from undersampled data in the indirect dimension

Reducing the acquisition time for two-dimensional nuclear magnetic resonance (2D NMR) spectra is important. One way to achieve this goal is reducing the acquired data. In this paper, within the framework of compressed sensing, we proposed to undersample the data in the indirect dimension for a type of self-sparse 2D NMR spectra, that is, only a few meaningful spectral peaks occupy partial locations, while the rest of locations have very small or even no peaks. The spectrum is reconstructed by enforcing its sparsity in an identity matrix domain with ℓ(p) (p = 0.5) norm optimization algorithm. Both theoretical analysis and simulation results show that the proposed method can reduce the reconstruction errors compared with the wavelet-based ℓ(1) norm optimization.

Fast high-resolution 2D correlation spectroscopy in inhomogeneous fields via Hadamard intermolecular multiple quantum coherences technique

Recently, a method based on intermolecular multiple quantum coherences (iMQCs) has been proposed to obtain high-resolution 2D COSY spectra in inhomogeneous fields via 3D acquisitions. However, the very long acquisition time prevents its practical application. To overcome this shortage, the Hadamard technique was applied for the iMQC method in this paper. For the new pulse sequence, the direct frequency-domain excitation is used in the first indirect detection dimension, so the 3D acquisition was replaced by an array of 2D acquisitions. The acquisition time can be reduced to 10 min. The resulting spectra retain useful structural information including chemical shifts and multiplet patterns of J coupling even when the inhomogeneous line broadening leads to overlap of neighboring diagonal resonances in the conventional COSY spectrum. The experimental results are consistent with the theoretical predictions and computer simulations. The new sequence may provide a time-efficient way for the studies of chemical solution in inhomogeneous fields.