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Related Concept Videos

Reaction Quotient02:35

Reaction Quotient

The status of a reversible reaction is conveniently assessed by evaluating its reaction quotient (Q). For a reversible reaction described by m A + n B ⇌ x C + y D, the reaction quotient is derived directly from the stoichiometry of the balanced equation as
Qualitative Analysis01:10

Qualitative Analysis

Qualitative analysis is the process of identifying elements, ions, or compounds in an unknown sample. It is the first and most fundamental type of analysis based on the hierarchy of analytical goals. This hierarchy is significant as it provides a structured approach to scientific research, with qualitative analysis serving as the initial step, providing essential information before moving on to quantitative or other forms of analysis.
There are two main approaches to qualitative analysis:...
Qualitative Analysis03:46

Qualitative Analysis

For solutions containing mixtures of different cations, the identity of each cation can be determined by qualitative analysis. This technique involves a series of selective precipitations with different chemical reagents, each reaction producing a characteristic precipitate for a specific group of cations. Metal ions within a group are further separated by varying the pH, heating the mixture to redissolve a precipitate, or adding other reagents to form complex ions.
For instance, group IV...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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 slanted or...
2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)01:19

2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does...
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...

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Related Experiment Video

Updated: Jun 25, 2026

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy
10:40

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

Published on: June 28, 2016

Quasichemical theory with a soft cutoff.

Shaji Chempath1, Lawrence R Pratt, Michael E Paulaitis

  • 1Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.

The Journal of Chemical Physics
|February 12, 2009
PubMed
Summary

This study introduces a soft-cutoff molecular quasichemical theory, enabling molecular dynamics for solvation free energy calculations. While versatile, hard cutoffs performed better for liquid water simulations.

Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Statistical Mechanics

Background:

  • The molecular quasichemical theory of liquids is widely successful.
  • Existing hard-cutoff methods require Monte Carlo simulations for solvation free energy calculations.

Purpose of the Study:

  • To develop a soft-cutoff version of the molecular quasichemical theory.
  • To enable the use of molecular dynamics simulations for solvation free energy calculations.
  • To explore the treatment of fluids with smooth repulsive interactions.

Main Methods:

  • Development of the soft-cutoff molecular quasichemical theory.
  • Application of the theory to molecular dynamics simulations.
  • Comparison with hard-cutoff quasichemical theory and molecular-field theory.

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Studying Soft-matter and Biological Systems over a Wide Length-scale from Nanometer and Micrometer Sizes at the Small-angle Neutron Diffractometer KWS-2
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Studying Soft-matter and Biological Systems over a Wide Length-scale from Nanometer and Micrometer Sizes at the Small-angle Neutron Diffractometer KWS-2

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Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing
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Synthesis and Operation of Fluorescent-core Microcavities for Refractometric Sensing

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Main Results:

  • The soft-cutoff theory allows molecular dynamics simulations for solvation free energy.
  • Fluids with smooth repulsive interactions can be treated analogously to hard-sphere fluids.
  • For liquid water, soft-cutoff conditioning did not alter convergence but hard cutoffs performed better.

Conclusions:

  • The soft-cutoff molecular quasichemical theory offers a new computational approach.
  • Molecular dynamics can now be applied to solvation free energy calculations.
  • Hard cutoffs remain superior for specific systems like liquid water.