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

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

937
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...
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Dynamic Equilibrium02:20

Dynamic Equilibrium

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A reversible chemical reaction represents a chemical process that proceeds in both forward (left to right) and reverse (right to left) directions. When the rates of the forward and reverse reactions are equal, the concentrations of the reactant and product species remain constant over time and the system is at equilibrium. A special double arrow is used to emphasize the reversible nature of the reaction. The relative concentrations of reactants and products in equilibrium systems vary greatly;...
55.3K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.1K
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...
1.1K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.4K
In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
1.4K
Equilibrium Conditions for a Particle01:23

Equilibrium Conditions for a Particle

1.6K
When an object is in equilibrium, it is either at rest or moving with a constant velocity. There are two types of equilibrium: static and dynamic. Static equilibrium occurs when an object is at rest, while dynamic equilibrium occurs when an object is moving with a constant velocity. In both cases, there must be a balance of forces acting on the object.
To understand the concept of equilibrium, let us first consider the forces acting on an object. When different forces act on an object, they can...
1.6K
IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

1.8K
A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...
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Related Experiment Video

Updated: Sep 27, 2025

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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Equilibrium-nonequilibrium ring-polymer molecular dynamics for nonlinear spectroscopy.

Tomislav Begušić1, Xuecheng Tao1, Geoffrey A Blake1

  • 1Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.

The Journal of Chemical Physics
|April 9, 2022
PubMed
Summary
This summary is machine-generated.

We developed a new computational method, equilibrium-nonequilibrium ring-polymer molecular dynamics (RPMD), to accurately simulate molecular dynamics. This approach accounts for nuclear quantum effects in spectroscopy, overcoming limitations of previous methods.

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Area of Science:

  • Computational Chemistry
  • Spectroscopy
  • Quantum Dynamics

Background:

  • Two-dimensional Raman and terahertz-Raman spectroscopy reveal molecular insights.
  • Simulating condensed-phase systems is computationally expensive, hindering theoretical validation.
  • Incorporating quantum mechanics in simulations is crucial but challenging.

Purpose of the Study:

  • To present a practical computational method for simulating nonlinear optical spectroscopy.
  • To account for nuclear quantum effects in molecular dynamics simulations.
  • To provide a computationally feasible alternative to existing methods.

Main Methods:

  • Equilibrium-nonequilibrium ring-polymer molecular dynamics (RPMD).
  • Calculation of the two-time response function for nonlinear optical spectroscopy.
  • Comparison with classical and double Kubo transformed (DKT) methods.

Main Results:

  • The equilibrium-nonequilibrium RPMD method accurately includes nuclear quantum effects.
  • The method is exact in the classical limit, reducing to established classical dynamics.
  • Benchmark calculations show advantages over classical and DKT approaches.

Conclusions:

  • Equilibrium-nonequilibrium RPMD offers a practical way to incorporate quantum effects in spectroscopy simulations.
  • The method simplifies theoretical analysis by avoiding the need for specific Kubo transformed correlation functions.
  • This work enables the application of real-time path-integral techniques to multidimensional spectroscopy.