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Magnetic Tweezers for the Measurement of Twist and Torque
Published on: May 19, 2014
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Counterintuitive torsional barriers controlled by hydrogen bonding.
Héctor Barbero1, Antoine Meunier1, Kondalarao Kotturi1
1Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, USA. masson@ohio.edu.
Physical Chemistry Chemical Physics : PCCP
|September 23, 2020
Summary
Intramolecular hydrogen bonds in biphenyls unexpectedly lowered torsional barriers, contrary to expectations. Solvent polarity and polarizability significantly influenced these barriers, revealing complex solute-solvent interactions.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Spectroscopy
Background:
- Torsional barriers in biphenyls are crucial for understanding molecular conformation and reactivity.
- Intramolecular hydrogen bonding can significantly influence molecular properties, including rotational barriers.
- Solvent effects play a critical role in modulating chemical processes and molecular interactions.
Purpose of the Study:
- To investigate the impact of intramolecular N-HO=C hydrogen bonding on the torsional barriers of isosteric disubstituted biphenyls.
- To determine the influence of solvent properties, specifically hydrogen bond accepting ability, on these torsional barriers.
- To elucidate the interplay between intramolecular interactions, solvent effects, and conformational dynamics using computational and experimental methods.
Main Methods:
- Variable temperature 1H NMR spectroscopy was employed to measure torsional barriers in three different deuterated solvents.
- Density functional theory (DFT) calculations were utilized to model the system and understand the underlying forces.
- The Hunter solvation model was applied to analyze solvent effects on the observed torsional barriers.
Main Results:
- A biphenyl with N'-acylcarbohydrazide substituents exhibited unexpectedly low torsional barriers, despite intramolecular hydrogen bonding.
- The torsional barrier was found to be lower in solvents with higher hydrogen bond accepting abilities.
- Computational studies revealed that polarizable solvents also contribute to solute stabilization, in addition to hydrogen bond acceptors.
Conclusions:
- Intramolecular hydrogen bonds in this specific biphenyl system do not significantly increase the torsional barrier, and can even lower it under certain conditions.
- Solvent polarity and polarizability are key factors influencing the torsional isomerization pathway of substituted biphenyls.
- A model was developed to accurately extrapolate activation energies in pure solvents from data obtained in solvent mixtures.
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