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Related Concept Videos

Catalytically Perfect Enzymes01:07

Catalytically Perfect Enzymes

4.9K
The theory of catalytically perfect enzymes was first proposed by W.J. Albery and J. R. Knowles in 1976. These enzymes catalyze biochemical reactions at high-speed. Their catalytic efficiency values range from 108-109 M-1s-1. These enzymes are also called 'diffusion-controlled' as the only rate-limiting step in the catalysis is that of the substrate diffusion into the active site. Examples include triose phosphate isomerase, fumarase, and superoxide dismutase.
 
Most enzymes...
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Echo01:06

Echo

911
The human ear cannot distinguish between two sources of sound if they happen to reach within a specific time interval, typically 0.1 seconds apart. More than this, and they are perceived as separate sources.
Imagine the sound is reflected back to the ears. Assuming that the source is very close to the human, the difference between hearing the two sounds—the emitted sound and the reflected sound—may be more than the minimum time for perceiving distinct sounds. If this is the case,...
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Titration Calculations: Strong Acid - Strong Base02:28

Titration Calculations: Strong Acid - Strong Base

33.8K
Calculating pH for Titration Solutions: Strong Acid/Strong Base
A titration is carried out for 25.00 mL of 0.100 M HCl (strong acid) with 0.100 M of a strong base NaOH. The pH at different volumes of added base solution can be calculated as follows:
(a) Titrant volume = 0 mL. The solution pH is due to the acid ionization of HCl. Because this is a strong acid, the ionization is complete and the hydronium ion molarity is 0.100 M. The pH of the solution is then:
33.8K
Strong Acid and Base Solutions03:22

Strong Acid and Base Solutions

35.3K
A strong acid is a compound that dissociates completely in an aqueous solution and produces a concentration of hydronium ions equal to the initial concentration of acid. For example, 0.20 M hydrobromic acid will dissociate completely in water and produces 0.20 M of hydronium ions and 0.20 M of bromide ions.
35.3K
Titration of a Strong Acid with a Strong Base01:23

Titration of a Strong Acid with a Strong Base

10.1K
During the titration of a strong acid with a strong base, pH calculations are primarily based on the concentration of residual hydronium or hydroxide ions. Initially, a strong acid like hydrochloric acid fully dissociates, creating hydronium and chloride ions, resulting in a low pH. The addition of a strong base like sodium hydroxide alters the concentration of hydronium ions by neutralizing them. As more base is added, the pH gradually increases. At the equivalence point, all hydronium ions...
10.1K
Titration Calculations: Weak Acid - Strong Base03:55

Titration Calculations: Weak Acid - Strong Base

49.1K
Calculating pH for Titration Solutions: Weak Acid/Strong Base
For the titration of 25.00 mL of 0.100 M CH3CO2H with 0.100 M NaOH, the reaction can be represented as:
49.1K

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Updated: Jan 21, 2026

Gradient Echo Quantum Memory in Warm Atomic Vapor
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Gradient Echo Quantum Memory in Warm Atomic Vapor

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Is perfection ever attainable? Strong coupling effects in the Perfect Echo.

Peter W A Howe1

  • 1Jealott's Hill Reseach Centre, Syngenta, Berkshire, UK.

Magnetic Resonance in Chemistry : MRC
|July 27, 2019
PubMed
Summary

The Perfect Echo sequence in NMR spectroscopy can distort signal intensities in strongly coupled spin systems. Adding an orthogonal pulse minimizes these distortions, ensuring accurate analysis of small-molecule proton NMR spectra.

Keywords:
1HNMRperfect echospin echostrong coupling

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Last Updated: Jan 21, 2026

Gradient Echo Quantum Memory in Warm Atomic Vapor
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Study of Protein Dynamics via Neutron Spin Echo Spectroscopy
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Area of Science:

  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Organic Chemistry
  • Spectroscopic Analysis

Background:

  • The Perfect Echo sequence is widely used in small-molecule proton NMR spectroscopy.
  • It is known to refocus homonuclear J-couplings in AX spin systems and in-phase magnetization for weakly coupled systems.

Purpose of the Study:

  • To investigate and theoretically analyze intensity distortions observed with the Perfect Echo sequence in strongly coupled spin systems.
  • To identify the origin of these distortions and propose a method to mitigate them.

Main Methods:

  • Experimental acquisition of NMR spectra using the Perfect Echo sequence.
  • Theoretical analysis of spin systems, focusing on strongly coupled spins.
  • Comparison of spectra with and without an added orthogonal excitation pulse.

Main Results:

  • The Perfect Echo sequence causes intensity distortions in strongly coupled spin systems, contrary to previous assumptions.
  • These distortions arise from magnetization transfer due to the final inversion pulse.
  • The common 'roofing' method for identifying coupling partners is unreliable with the Perfect Echo sequence.

Conclusions:

  • Intensity distortions in strongly coupled systems are a real phenomenon with the Perfect Echo sequence.
  • An orthogonal excitation pulse effectively reduces these distortions.
  • The findings necessitate caution when interpreting NMR spectra acquired with the Perfect Echo sequence, particularly regarding resonance intensities.