Highly Efficient Determination of Complex NMR Multiplet Structures in Inhomogeneous Magnetic Fields

Efficient determination of scalar coupling networks by band selective decoupled 2D NMR spectroscopy

Scalar (J) couplings constitute one of vital features observed in NMR spectroscopy and show valuable information for molecular structure elucidation and conformation analysis. However, existing J coupling measurement techniques are generally confined by the concerns of resolution, SNR, and experimental efficiency. Herein, we exploit an efficient 2D NMR protocol to deal with the above concerns by enabling rapid, sensitive, and high-resolution J coupling extraction. This protocol delivers full-resolved pure shift 2D absorption-mode spectroscopy to gain great convenience for efficient coupling measurements on overcrowded NMR signals. Resulting from band selective signal evolution, this protocol ensures high signal intensity with full magnetization preservation to meet the demand on probing low-concentration samples. This protocol focuses on accessing coupling information between specific two coupled spin families, and it is not applicable to all possible spin systems. Besides, it adopts echo-train selective refocusing acquisition to accelerate pure shift 2D J-edited implementations into pseudo-2D acquisition, and thus holding the experimental efficiency similar to conventional SERF experiments. Therefore, this study presents a promising tool for efficient extraction of J coupling networks, and takes an important step for coupling measurement techniques with wide applications on molecular conformation elucidation and stereochemical configuration analysis.

High-resolution NMR spectroscopy for measuring complex samples based on chemical-shift-difference selection

NMR spectroscopy serves as an immensely powerful tool for component assignments and molecular structure elucidations. However, proton NMR spectra are generally trapped with spectral congestion caused by limited frequency differences and complex multiplets. 2D NMR can effectively relieve spectral congestion, but its resolution and acquisition efficiency are restricted by the broad spectral bandwidth. Herein, we introduce an NMR method based on chemical-shift-difference selection by chirp excitation to record high-resolution 2D NMR spectra for extracting coupling correlation networks and multiplet structures, suitable for measurements on complex samples. The performance of the proposed method is illustrated in determining diastereotopic methylene protons, small frequency-difference coupled proton pairs of furanose, pyranose and benzene rings. This study is expected to benefit molecular structure elucidation and composition analysis of complex samples in chemistry, biochemistry and metabonomics.

Simultaneous determination of multiple coupling networks by high-resolution 2D J-edited NMR spectroscopy

J coupling constitutes an important NMR parameter for molecular-level composition analysis and conformation elucidation. Dozens of J-based approaches have been exploited for J coupling measurement and coupling network determination, however, they are generally imposed to insufficient spectral resolution to resolve crowded NMR resonances and low measurement efficiency that a single experiment records one J coupling network. Herein, we propose a general NMR method to collect high-resolution 2D J-edited NMR spectra, which are characterized with advantages of pure absorptive lineshapes, decoupled chemical shift dimension, as well as eliminated axial peaks, thus facilitating J coupling partner assignments and J coupling constant measurements. More meaningfully, this protocol allows simultaneous determination of multiple coupling networks for highly efficient multiplet analyses via addressing multiple protons within one single experiment. Additionally, another variant is proposed for high-resolution applications under adverse magnetic field conditions. Therefore, this study provides a useful NMR protocol for configurational and structural studies with extensive applications in chemistry, biology, and material science.