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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
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Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
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A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
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Infrared spectroscopy is primarily used to determine the types of bonds and functional groups. In carboxylic acid derivatives, a typical carbonyl bond absorption is observed around 1650–1850 cm−1. For esters, the absorption is recorded at around 1740 cm−1, while acid halides show the absorption at about 1800 cm−1. Another acid derivative, the acid anhydrides, exhibit two carbonyl absorption around 1760 cm−1 and 1820 cm−1, arising from the symmetrical and...
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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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Enhancing computer-assisted structure elucidation with DFT analysis of J-couplings.

Alexei V Buevich1, Mikhail E Elyashberg2

  • 1Department of Discovery and Preclinical Sciences, Process Research and Development, NMR Structure Elucidation, Merck & Co., Inc, Kenilworth, NJ.

Magnetic Resonance in Chemistry : MRC
|January 10, 2020
PubMed
Summary
This summary is machine-generated.

This study introduces a new method to verify nonstandard correlations (NSCs) in computer-assisted structure elucidation (CASE) using J-coupling computations. This enhances the accuracy of molecular structure determination from NMR data.

Keywords:
13C1HDFTJ-couplingsNMRnatural productsstructure elucidation

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Area of Science:

  • Computational Chemistry
  • Organic Chemistry
  • Spectroscopy

Background:

  • Computer-assisted structure elucidation (CASE) systems primarily use 1D and 2D NMR data.
  • Standard CASE systems interpret HMBC and COSY spectra as 2- or 3-bond correlations.
  • Nonstandard correlations (NSCs), involving 4+ bonds, are often present but computationally expensive to include.

Purpose of the Study:

  • To develop and validate a method for verifying the reliability of nonstandard correlations (NSCs) in CASE.
  • To improve the efficiency and accuracy of the fuzzy structure generation (FSG) algorithm within CASE systems.
  • To leverage high-accuracy density functional theory (DFT) computed J-couplings for NSC validation.

Main Methods:

  • Verification of NSCs by comparing them with DFT-computed J-couplings (4-6JCH).
  • Analysis of strychnine to identify and quantify NSCs in HMBC spectra.
  • Application of the NSC verification method to 11 natural products and a CASE study of cleospinol A isomers.

Main Results:

  • 41% of observed 8-Hz HMBC cross-peaks in strychnine were identified as NSCs consistent with DFT-computed J-couplings (>0.3 Hz).
  • The 0.3 Hz cutoff for J-couplings was validated across 11 real-world natural product structures.
  • DFT-computed J-couplings of NSCs effectively differentiated the correct structure among six proposed isomers for cleospinol A.

Conclusions:

  • The proposed method reliably verifies NSCs by correlating them with DFT-computed J-couplings.
  • This approach enhances the robustness of CASE analysis and improves the accuracy of molecular structure elucidation.
  • The method can aid in identifying potential discrepancies in experimental NMR data.