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Related Concept Videos

Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

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...
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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...
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

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...
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...
Carbon-13 (¹³C) NMR: Overview01:10

Carbon-13 (¹³C) NMR: Overview

Carbon-13 is a naturally occurring NMR-active isotope of carbon with a low natural abundance of 1.1%. In contrast, carbon-12 is the most abundant isotope of carbon with zero nuclear spin. Therefore, it is NMR inactive. The gyromagnetic ratio of carbon-13 is smaller than that of protons. As a result, carbon-13 resonance is about 6000 times weaker than proton resonance. For a given magnetic field strength, the resonance frequency of carbon-13 is about one-fourth of the resonance frequency for...

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Related Experiment Video

Updated: May 14, 2026

Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
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Published on: September 17, 2017

Layer selection effect on solid state 13C and 15N chemical shifts calculation using ONIOM approach.

Hoora Shaghaghi1, Hossein Pasha Ebrahimi, Niloufar Bahrami Panah

  • 1Department of Chemistry, Sharif University of Technology, Tehran, Iran.

Solid State Nuclear Magnetic Resonance
|February 19, 2013
PubMed
Summary

Calculating solid-state chemical shifts for uracil and imidazole using the ONIOM method requires careful consideration of electron correlation. The PBEPBE/6-311+G(d,p) level of theory is recommended for accurate results.

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Area of Science:

  • Computational Chemistry
  • Solid-State NMR Spectroscopy
  • Quantum Chemistry

Background:

  • Solid-state nuclear magnetic resonance (NMR) spectroscopy is crucial for characterizing materials.
  • Accurate calculation of chemical shifts provides insights into molecular structure and dynamics.
  • The two-layer ONIOM (Our Own N-layered Integrated molecular Orbital and Molecular Mechanics) method is a computational approach for studying complex systems.

Purpose of the Study:

  • To investigate the accuracy of the two-layer ONIOM approach for calculating solid-state (13)C and (15)N chemical shifts.
  • To determine the optimal level of theory and layer selection for these calculations.
  • To analyze the impact of electron correlation between ONIOM layers.

Main Methods:

  • Calculations were performed using a 2-layer ONIOM approach across 32 levels of theory.
  • Factorial design was employed as a multivariate technique to analyze computational parameters.
  • The study focused on uracil and imidazole molecules in the solid state.

Main Results:

  • The PBEPBE/6-311+G(d,p) level of theory was identified as optimal for the high layer in both ONIOM models.
  • Considering electron correlation between the two ONIOM layers is essential for accurate solid-state (15)N chemical shift calculations.
  • Excellent agreement was observed between calculated and experimental chemical shifts for both uracil and imidazole.

Conclusions:

  • The selected wave functions and layer selection within the ONIOM framework are reliable for predicting solid-state chemical shifts.
  • The study validates the use of ONIOM (PBEPBE/6-311+G(d,p):AM1) for accurate solid-state (13)C and (15)N chemical shift predictions.
  • This computational strategy enhances the understanding of solid-state properties through accurate chemical shift calculations.