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Related Concept Videos

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

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

1.8K
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.
1.8K
Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

2.6K
Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred...
2.6K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

1.4K
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...
1.4K
Chirality at Nitrogen, Phosphorus, and Sulfur02:30

Chirality at Nitrogen, Phosphorus, and Sulfur

7.3K
Chirality is most prevalent in carbon-based tetrahedral compounds, but this important facet of molecular symmetry extends to sp3-hybridized nitrogen, phosphorus and sulfur centers, including trivalent molecules with lone pairs. Here, the lone pair behaves as a functional group in addition to the other three substituents to form an analogous tetrahedral center that can be chiral.
A consequence of chirality is the need for enantiomeric resolution. While this is theoretically possible for all...
7.3K
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

3.6K
The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
3.6K
Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

11.8K
In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
11.8K

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Related Experiment Video

Updated: Mar 21, 2026

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
06:24

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal

Published on: October 31, 2019

7.0K

Dipolar Reorientations in Amorphous Nimesulide: A TSDC and DSC Study.

Joaquim J Moura Ramos, Hermínio P Diogo1

  • 1CQFM - Centro de Química- Física Molecular and IN - Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal.

Current Drug Delivery
|May 11, 2016
PubMed
Summary

Amorphous nimesulide exhibits moderate glass-forming ability and stability. Thermally Stimulated Depolarization Currents (TSDC) effectively characterized its molecular mobility and relaxation dynamics, crucial for enhancing drug stability.

Keywords:
AgingDSCTSDCdynamic fragilitysecondary relaxations

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Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid
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Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

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Last Updated: Mar 21, 2026

High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
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Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
10:35

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Published on: May 29, 2018

9.3K

Area of Science:

  • Physical Chemistry
  • Materials Science
  • Pharmaceutical Sciences

Background:

  • Amorphous solid forms of Active Pharmaceutical Ingredients (APIs) improve solubility and dissolution rates of poorly crystalline drugs.
  • Molecular mobility in amorphous solids can lead to instability; kinetic characterization of relaxations is key for glassy pharmaceutical stability.
  • Understanding molecular mobility is essential for developing stable amorphous pharmaceutical formulations.

Purpose of the Study:

  • To investigate the thermal behavior and molecular mobility of the pharmaceutical drug nimesulide in its amorphous solid state.
  • To utilize differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) for kinetic characterization of nimesulide's relaxations.
  • To determine the conditions for enhanced stability of glassy amorphous nimesulide.

Main Methods:

  • Differential Scanning Calorimetry (DSC) for general thermal behavior and kinetic characterization of glass transition relaxation.
  • Thermally Stimulated Depolarization Currents (TSDC) to isolate and analyze individual molecular motion modes between -150°C and +15°C.
  • Kinetic parameter determination from TSDC experiments to characterize the distribution of relaxation times.

Main Results:

  • No molecular mobility was observed below approximately -30°C.
  • A secondary relaxation (slow β-relaxation, Johari-Goldstein type) was identified between -15°C and +7°C, influenced by physical aging.
  • DSC and TSDC analyses revealed nimesulide as a moderately fragile glass former, with dynamic fragility indices mDSC = 52 and mTSDC = 70.

Conclusions:

  • Nimesulide exhibits moderate glass-forming ability and limited glass stability, with cold crystallization occurring near the glass transition temperature.
  • TSDC proved effective for studying molecular mobility in amorphous nimesulide, complementing DSC analysis.
  • Spectroscopic dielectric relaxation studies may be challenging under the investigated conditions.