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Related Concept Videos

NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
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NMR Spectroscopy of Aromatic Compounds01:14

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Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
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Two-Dimensional (2D) NMR: Overview01:12

Two-Dimensional (2D) NMR: Overview

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The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse....
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NMR Spectrometers: Overview01:20

NMR Spectrometers: Overview

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NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field...
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¹³C NMR: ¹H–¹³C Decoupling01:04

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The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
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2D NMR: Overview of Heteronuclear Correlation Techniques01:18

2D NMR: Overview of Heteronuclear Correlation Techniques

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Heteronuclear correlation spectroscopy is an analytical technique that investigates the coupling between different types of nuclei, often a proton and an X-nucleus, such as carbon-13 or nitrogen-15. This method is commonly used in nuclear magnetic resonance (NMR) spectroscopy to gain insights into complex chemical compounds' structural and compositional aspects. A typical heteronuclear correlation spectrum displays X-nucleus chemical shifts on one axis and a proton spectrum on the other...
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Cross-Modal Retrieval Between 13C NMR Spectra and Structures Based on Focused Libraries.

Hanyu Sun1,2, Xi Xue1, Xue Liu1

  • 1State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia Medica, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100050, PR China.

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This summary is machine-generated.

Focused libraries improve compound identification using carbon-13 nuclear magnetic resonance (13C NMR) spectra. This approach enhances structure-elucidation accuracy compared to traditional, large spectral libraries.

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Area of Science:

  • Analytical Chemistry
  • Computational Chemistry
  • Spectroscopy

Background:

  • Compound identification relies on matching 13C NMR spectra with spectral libraries.
  • Previous deep contrastive learning systems (CReSS) faced limitations with large, redundant libraries and lack of unknown structures.

Purpose of the Study:

  • To enhance structure-elucidation accuracy in compound identification.
  • To address limitations of traditional spectral libraries in deep learning models.
  • To develop a more efficient method for cross-modal retrieval between 13C NMR spectra and chemical structures.

Main Methods:

  • Replaced traditional large libraries with focused libraries generated by CMGNet.
  • Employed a deep contrastive learning system (CReSS) with focused libraries.
  • Introduced SAmpRNN, a recurrent neural network, to amplify focused libraries.
  • Evaluated the combined model on 6,471 13C NMR spectra.

Main Results:

  • The combined model achieved a Top-10 accuracy of 54.03%, significantly outperforming CReSS with a random library (9.17%).
  • SAmpRNN amplification increased structure-identification accuracy in 70.0% of 30 random cases.
  • Focused libraries (CFLS) provided more accurate candidate structures than traditional libraries.

Conclusions:

  • Focused libraries significantly improve cross-modal retrieval accuracy for 13C NMR spectra and structures.
  • The combined CReSS and CMGNet model, enhanced by SAmpRNN, offers a powerful tool for compound identification.
  • This approach provides a more accurate and efficient alternative to traditional library matching methods.