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NMR Spectroscopy: Spin–Spin Coupling01:08

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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...
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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.
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NMR Spectroscopy Of Amines

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In proton NMR spectroscopy, primary amines and secondary amines showcase their N–H protons as a broad signal in the chemical shift range between δ 0.5 and 5 ppm. The exact position in this range depends on several factors, including sample concentration, hydrogen bonding, and the type of solvent used. Since amine protons undergo fast proton exchange in solution, the protons are labile and therefore do not participate in any splitting with adjacent protons. Thus, the observed peak is...
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Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
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Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...
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When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
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Engineering spin Hamiltonians using multiple pulse sequences in solid state NMR spectroscopy.

Jiangyu Cui1, Jun Li2, Xiaomei Liu1

  • 1CAS Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China; Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|July 18, 2018
PubMed
Summary
This summary is machine-generated.

This study introduces a new method to design nuclear magnetic resonance pulse sequences for solid-state materials. The developed technique effectively suppresses proton-proton interactions, enhancing spectral clarity.

Keywords:
Average Hamiltonian theoryHeteronuclear correlationHomonuclear decouplingMultiple pulsesSolid-state NMR

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Area of Science:

  • Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Quantum Control in Spectroscopy

Background:

  • Multiple pulse sequences are crucial for manipulating spin Hamiltonians in solid-state NMR.
  • Average Hamiltonian theory is a cornerstone for analyzing these complex sequences.

Purpose of the Study:

  • To develop a general procedure for selecting sub-Hamiltonians within average Hamiltonian theory.
  • To design novel proton-proton homonuclear decoupling sequences for static solids.

Main Methods:

  • Analysis of multiple pulse sequences using average Hamiltonian theory.
  • Expansion of average Hamiltonians into reachable sub-Hamiltonians.
  • Utilizing pulse flip-angle and phase as control variables for sub-Hamiltonian selection.

Main Results:

  • A new proton-proton homonuclear decoupling sequence was designed and analyzed.
  • The sequence effectively suppresses 1H-1H homonuclear dipolar interactions with finite pulse lengths.
  • Variable scaling of heteronuclear dipolar and chemical shift interactions was achieved, dependent on pulse flip-angle.

Conclusions:

  • The newly designed decoupling scheme offers effective suppression of homonuclear dipolar interactions.
  • Tunable scaling factors for heteronuclear interactions provide flexibility in solid-state NMR experiments.
  • Experimental validation on a 15N-acetyl-valine crystal confirmed the scheme's performance.