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Related Concept Videos

NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

704
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...
704
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
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...
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Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

1.4K
In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the...
1.4K
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

1.4K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
1.4K
¹H NMR of Labile Protons: Temporal Resolution01:10

¹H NMR of Labile Protons: Temporal Resolution

1.2K
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...
1.2K
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

1.3K
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...
1.3K

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Residual dipolar line width in magic-angle spinning proton solid-state NMR.

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Magic-angle spinning in solid-state NMR causes residual line broadening due to dipolar interactions. Second-order terms dominate at high spinning frequencies, affecting line width and position.

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Area of Science:

  • Solid-state nuclear magnetic resonance (NMR) spectroscopy
  • Quantum mechanics and spin dynamics

Background:

  • Magic-angle spinning (MAS) is essential for averaging anisotropic interactions in solid-state NMR.
  • Homonuclear dipolar interactions in coupled spin systems lead to residual line broadening due to non-commuting Hamiltonians.

Purpose of the Study:

  • To analyze residual line broadening in coupled proton spin systems under MAS.
  • To investigate the influence of second-order and higher-order terms on spectral line shapes and positions.

Main Methods:

  • Theoretical calculations of effective Hamiltonians up to third order using Floquet theory.
  • Comparison with numerically obtained effective Hamiltonians for small spin systems.
  • Analysis of experimental data from proton spectra in model substances.

Main Results:

  • Second-order terms dominate residual line width at spinning frequencies >75 kHz, showing a specific dependence.
  • Chemical-shift truncation partially mimics third-order effects on line width.
  • Second-order contributions cause both line broadening and shifts in the center of gravity.

Conclusions:

  • Second-order dipolar interactions are the primary cause of residual line broadening and shifts at high MAS frequencies.
  • Understanding these effects is crucial for accurate spectral interpretation in solid-state NMR.
  • Experimental observations align with theoretical predictions of second-order Hamiltonian effects.