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Related Concept Videos

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
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Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

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Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
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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...
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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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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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¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

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The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
1.2K
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...
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Updated: Aug 22, 2025

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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Proton-transfer spectroscopy beyond the normal-mode scenario.

Florian N Brünig1, Paul Hillmann1, Won Kyu Kim2

  • 1Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany.

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Summary

A new stochastic theory predicts proton transfer spectra. It identifies three key spectral contributions, with two not captured by normal-mode analysis, offering insights into proton transfer kinetics.

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Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Spectroscopy

Background:

  • Proton-transfer processes are fundamental in chemistry and biology.
  • Predicting spectral signatures of these processes is challenging.
  • Existing methods like normal-mode analysis have limitations.

Purpose of the Study:

  • To develop a stochastic theory for predicting proton-transfer spectral signatures.
  • To apply this theory to infrared spectra of H5O2+ cation.
  • To model proton transfer with variable barrier heights.

Main Methods:

  • Developed a stochastic theory for spectral signature prediction.
  • Computed infrared spectra using ab initio molecular-dynamics simulations.
  • Constrained oxygen atoms in H5O2+ to model variable barrier heights.

Main Results:

  • Identified three distinct spectral contributions: quasi-harmonic motion, transfer paths, and transfer-waiting-time.
  • The transfer-waiting-time contribution's frequency depends exponentially on barrier height.
  • The transfer-path contribution's frequency is largely insensitive to barrier height.

Conclusions:

  • The stochastic theory successfully predicts spectral signatures of proton transfer.
  • Two key spectral contributions are exclusively related to proton-transfer kinetics.
  • The model provides a tunable system for studying general proton-transfer processes.