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Related Concept Videos

NMR Spectrometers: Overview01:20

NMR Spectrometers: Overview

NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field...
¹H NMR of Labile Protons: Temporal Resolution01:10

¹H NMR of Labile Protons: Temporal Resolution

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
The –OH proton in alcohols typically appears in the range of δ 2 to 5 ppm but can vary depending on the specific...
NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
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...
NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences01:17

NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse.
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...

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Sizable concentration-dependent frequency shifts in solution NMR using sensitive probes.

Susie Y Huang1, Clemens Anklin, Jamie D Walls

  • 1Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, California 90095, USA.

Journal of the American Chemical Society
|December 9, 2004
PubMed
Summary

Radiation damping causes significant frequency shifts in solution Nuclear Magnetic Resonance (NMR) experiments. These shifts, up to +/-81 Hz, are crucial for understanding and optimizing NMR techniques like solvent suppression.

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Area of Science:

  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Physical Chemistry
  • Analytical Chemistry

Background:

  • Radiation damping is an increasingly important feedback interaction in high-field solution NMR.
  • Observed concentration-dependent frequency shifts in NMR experiments require explanation.

Purpose of the Study:

  • To investigate the cause of unexplained frequency shifts in solution NMR.
  • To elucidate the role of radiation damping in these observed shifts.
  • To explore applications of radiation damping-induced frequency shifts in NMR methodology.

Main Methods:

  • Experiments were conducted using a cryoprobe at 600 MHz.
  • Numerical simulations were employed to model the observed phenomena.
  • Analysis focused on deviations in the phase of the radiation damping field and its relation to transverse magnetization.

Main Results:

  • A time-averaged frequency shift of up to +83/-81 Hz was experimentally observed.
  • Frequency shifts were found to depend on longitudinal magnetization and probe tuning.
  • Deviations from perfect orthogonality between the radiation damping field and transverse magnetization were identified as the cause.

Conclusions:

  • The study provides a physical explanation for observed solvent precession frequency shifts.
  • Findings rationalize the empirical practice of adjusting B1 field irradiation frequency for optimal solvent presaturation.
  • Radiation damping-induced frequency shifts have potential applications in solvent suppression and broader NMR methodology.