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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

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...
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
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Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...
Atomic Nuclei: Nuclear Spin State Population Distribution01:14

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Near absolute zero temperatures, in the presence of a magnetic field, the majority of nuclei prefer the lower energy spin-up state to the higher energy spin-down state. As temperatures increase, the energy from thermal collisions distributes the spins more equally between the two states. The Boltzmann distribution equation gives the ratio of the number of spins predicted in the spin −½ (N−) and spin +½ (N+) states.
Atomic Spectroscopy: Effects of Temperature01:27

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Atomization, converting samples into gas-phase atoms and ions, is essential for atomic spectroscopy. The flame temperature required for atomization affects the efficiency of the atomic spectroscopic methods by increasing the atomization efficiency and the relative population of the excited and ground states.
At thermal equilibrium, the relative populations of excited and ground state atoms can be estimated using the Maxwell–Boltzmann distribution. For example, an increase in temperature from...

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Exploring Caspase Mutations and Post-Translational Modification by Molecular Modeling Approaches
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Temperature dependent single molecule rotational dynamics in PMA.

Subhasis Adhikari1, Markus Selmke, Frank Cichos

  • 1Molecular Nanophotonics Group, Institute of Experimental Physics I, University Leipzig, 04103 Leipzig, Germany.

Physical Chemistry Chemical Physics : PCCP
|December 25, 2010
PubMed
Summary
This summary is machine-generated.

Single dye molecule rotational diffusion in poly(methyl acrylate) (PMA) closely matches predictions based on shear viscosity. A dynamic heterogeneity model explains relaxation dynamics and distributions, linking them to polymer properties.

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Area of Science:

  • Polymer physics
  • Single-molecule spectroscopy
  • Rheology

Background:

  • Understanding polymer dynamics is crucial for material science.
  • Rotational diffusion of molecules within polymers provides insights into local segmental motion.
  • Existing models often simplify the complex, heterogeneous nature of polymer dynamics.

Purpose of the Study:

  • To investigate the rotational diffusion of single dye molecules in poly(methyl acrylate) (PMA).
  • To compare experimental single-molecule data with bulk shear viscosity measurements and numerical simulations.
  • To develop and validate a model explaining the observed relaxation dynamics and distributions.

Main Methods:

  • Temperature-dependent single-molecule rotational diffusion measurements.
  • Rheological measurements of shear viscosity.
  • Numerical simulations incorporating a dynamic heterogeneity model with Vogel-Fulcher-Tammann-Hesse (VFTH) viscosity dependence.

Main Results:

  • Single-molecule rotational diffusion accurately follows Debye-Stokes-Einstein predictions based on shear viscosity.
  • A dynamic heterogeneity model, based on a Gaussian distribution of activation energies, successfully explains stretched exponential relaxation dynamics.
  • The model demonstrates that observed distributions reflect intrinsic polymer properties rather than instantaneous local viscosity variations.

Conclusions:

  • The Debye-Stokes-Einstein relation holds well for single-molecule rotational diffusion in PMA, correlating directly with shear viscosity.
  • Dynamic heterogeneity in polymers, modeled by a distribution of activation energies, is key to understanding single-molecule relaxation behavior.
  • Single-molecule dynamics are governed by fundamental polymer properties, not solely by transient local viscosity fluctuations.