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Related Concept Videos

IR Spectroscopy: Molecular Vibration Overview01:24

IR Spectroscopy: Molecular Vibration Overview

1.8K
When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
1.8K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

796
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
796
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

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

995
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...
995
IR Spectrum Peak Broadening: Hydrogen Bonding01:23

IR Spectrum Peak Broadening: Hydrogen Bonding

805
The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. The stretching vibration of the strong O–H bond in alcohols and phenols (very dilute solution or gas phase) appears as a sharp peak at 3600–3650 cm−1.
However, the extent of hydrogen bonding influences the observed stretching frequency and band broadening. Intermolecular or intramolecular...
805
IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations01:08

IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations

884
Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single...
884
¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

993
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...
993

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Proton Transfer and Protein Conformation Dynamics in Photosensitive Proteins by Time-resolved Step-scan Fourier-transform Infrared Spectroscopy
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Coherent Vibrational Dynamics in an Isolated Peptide Captured with Two-Dimensional Infrared Spectroscopy.

Zifan Ma1, Laura M McCaslin2, Joseph A Fournier1

  • 1Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri 63130, United States.

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Ultrafast 2D IR spectroscopy of gas-phase glutathione reveals long-lived vibrational coherence. Deviations from theory suggest novel coherence transfer pathways, offering insights into open quantum systems.

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Area of Science:

  • Physical Chemistry
  • Chemical Physics
  • Spectroscopy

Background:

  • Quantum mechanical vibrational coherence transfer is crucial for chemical and biological processes.
  • Traditional condensed-phase nonlinear spectroscopies struggle to directly measure these coherence pathways.
  • A new experimental capability for gas-phase two-dimensional infrared (2D IR) spectroscopy allows direct measurement.

Purpose of the Study:

  • To report ultrafast 2D IR spectroscopy of the peptide glutathione (GSH) in the gas phase.
  • To investigate vibrational coherence transfer processes in an isolated molecular system.
  • To provide benchmarks for modeling coherence transfer dynamics and system-bath interactions.

Main Methods:

  • Gas-phase isolation and cryogenic cooling of glutathione (GSH).
  • Excitation of six vibrational modes within the amide I and II region using ultrafast 2D IR spectroscopy.
  • Analysis of spectral dynamics, including diagonal and off-diagonal cross-peak features.
  • Quantum chemistry calculations to determine anharmonic couplings and system-bath interactions.

Main Results:

  • Observed long-lived oscillatory behavior in spectral dynamics, indicating coherent vibrational dynamics.
  • Deviations in oscillatory signatures from standard nonlinear response theories, suggesting additional nonlinear pathways.
  • Identification of coherence transfer processes mediated by coupling to low-frequency bath modes.
  • Quantum chemistry calculations confirmed significant anharmonic couplings within GSH and strong system-bath coupling.

Conclusions:

  • The study demonstrates the capability of gas-phase 2D IR spectroscopy to probe vibrational coherence transfer.
  • Deviations from theoretical predictions highlight the complexity of coherence transfer mechanisms in open quantum systems.
  • The findings provide critical experimental data for refining theoretical models of coherence transfer and system-bath interactions, free from solvent effects.