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Related Concept Videos

NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

2.6K
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...
2.6K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.2K
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.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.2K
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

1.0K
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
1.0K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

3.7K
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
3.7K
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

1.7K
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
1.7K
Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

2.9K
Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
2.9K

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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
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Transfer Learning from Simulation to Experimental Data: NMR Chemical Shift Predictions.

Herim Han1,2, Sunghwan Choi1

  • 1Division of National Supercomputing, Korea Institute of Science and Technology Information, 245 Daehak-Ro, Yuseong-Gu, Daejeon 34141, Republic of Korea.

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Summary

Knowledge transfer from simulation data improves chemical shift prediction accuracy. This approach enhances machine learning model usability with limited experimental data, overcoming database limitations.

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Area of Science:

  • Computational Chemistry
  • Machine Learning
  • Spectroscopy

Background:

  • Accurate prediction of chemical shifts (δ) is crucial for molecular structure elucidation but remains challenging.
  • Current machine learning (ML) methods show promise but are hindered by the need for extensive chemical databases.

Purpose of the Study:

  • To investigate the transferability of prior knowledge from simulation databases to predict experimental chemical shifts (δ).
  • To enhance the applicability of ML models for chemical shift prediction using limited experimental data.

Main Methods:

  • Utilizing prior knowledge gained from a simulation database.
  • Applying additional training with small, randomly sampled experimental data.
  • Leveraging knowledge transfer to train models on focused chemical spaces.

Main Results:

  • Reliable accuracy for predicting experimental chemical shifts (δ) was achieved.
  • Successful training of ML models on sparsely covered experimental chemical spaces was demonstrated.
  • The approach effectively transferred knowledge despite differences between simulation and experimental databases.

Conclusions:

  • Knowledge transfer from simulation databases is a viable strategy for improving experimental chemical shift prediction.
  • This method enhances the usability of local experimental databases for ML applications.
  • The approach offers a solution to the database size limitations in ML-driven chemical structure elucidation.