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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...
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...
Mass Spectrum: Interpretation01:24

Mass Spectrum: Interpretation

An unknown compound can be established by identifying the molecular ion peak in the mass spectrum. The molecular ion peak is often weak or absent due to the predominance of fragmentation in high-energy electron beams. In such cases, a soft-energy electron beam can be used to scan the spectrum to enhance the intensity of the molecular ion peak. Additionally, chemical ionization, field ionization, and desorption ionization spectra are used to obtain a relatively intense molecular ion peak.To...
High-Resolution Mass Spectrometry (HRMS)01:15

High-Resolution Mass Spectrometry (HRMS)

The resolution of a mass spectrometer depends on the efficiency of separating ions with different ion masses. The mass of an atom is approximated to the sum of the masses of protons and neutrons inside, considering the masses of protons and neutrons as equal. However, the masses of the proton (1.6726 × 10−24 g) and neutron (1.6749 × 10−24 g) are not truly equal. There is a minor error in the expression of atomic masses relative to the simplest atom of hydrogen. For example, the mass of helium...
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...
Polymers: Molecular Weight Distribution01:10

Polymers: Molecular Weight Distribution

For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.

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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
14:55

Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy

Published on: September 17, 2017

Formula weight prediction by internal reference diffusion-ordered NMR spectroscopy (DOSY).

Deyu Li1, Gerald Kagan, Russell Hopson

  • 1Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.

Journal of the American Chemical Society
|March 28, 2009
PubMed
Summary
This summary is machine-generated.

This study introduces an internal reference correlated DOSY method for determining the formula weight (FW) of unknown molecules. This practical technique accurately characterizes species in various solvents and concentrations.

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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
14:55

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Published on: September 17, 2017

Application and Methodology of the Non-destructive 19F Time-domain NMR Technique to Measure the Content in Fluorine-containing Drug Products
09:24

Application and Methodology of the Non-destructive 19F Time-domain NMR Technique to Measure the Content in Fluorine-containing Drug Products

Published on: August 22, 2017

Area of Science:

  • Analytical Chemistry
  • Physical Chemistry
  • Organic Chemistry

Background:

  • Formula weight (FW) is crucial for characterizing reactive intermediates and organometallic complexes.
  • Accurate determination of FW aids in understanding composition, aggregation, and solvation states.
  • Existing methods may have limitations in diverse solvent and concentration conditions.

Purpose of the Study:

  • To develop and validate an internal reference correlated DOSY method for calculating FW.
  • To assess the method's accuracy and applicability across different solvents and concentrations.
  • To provide guidelines for the successful implementation of this novel characterization technique.

Main Methods:

  • Utilized Diffusion Ordered Spectroscopy (DOSY) with an internal reference standard.
  • Employed correlated DOSY to calculate the formula weight (FW) of unknown species.
  • Investigated the influence of solvent, concentration, and solution viscosity on accuracy.

Main Results:

  • Achieved excellent correlations for both small molecules (DIPA) and organometallic complexes (aggregate 1).
  • Demonstrated that relative diffusion rates inversely correlate with solution viscosity, aligning with the Stokes-Einstein equation.
  • Found prediction accuracy (r^2, % difference) is linked to density changes, improving with decreasing density.

Conclusions:

  • The internal reference correlated DOSY method provides a practical and accurate approach for FW determination.
  • The method is versatile, applicable to various molecules and complexes in different solution conditions.
  • This technique offers a valuable tool for characterizing unknown chemical species.