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Temperature Dependence on Reaction Rate02:55

Temperature Dependence on Reaction Rate

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The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. This premise is the basis for a theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates.
The collision theory is based on the postulates that (i) the reaction rate is proportional to the rate of reactant collisions, (ii) the reacting species collide in an orientation allowing contact between...
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The Arrhenius equation,
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Thermodynamic Potentials01:26

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Thermodynamic potentials are state functions that are extremely useful in analyzing a thermodynamic system. They have dimensions of energy. The four important thermodynamic potentials are internal energy, enthalpy, Helmholtz free energy, and Gibbs free energy. These thermodynamic potentials can be expressed using two of the following variables: pressure, volume, temperature, and entropy. These two variables are expressed as the rate of change of the thermodynamic potential with respect to other...
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Thermodynamics: Activity Coefficient01:24

Thermodynamics: Activity Coefficient

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Activity is the measure of the effective concentration of the species in solution. It can be expressed as the product of the molar concentration of the species and its activity coefficient. The activity coefficient is a dimensionless quantity and depends on the total ionic strength of the solution.
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Arrhenius Plots02:34

Arrhenius Plots

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The Arrhenius equation relates the activation energy and the rate constant, k, for chemical reactions. In the Arrhenius equation, k = Ae−Ea/RT, R is the ideal gas constant, which has a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is a constant called the frequency factor, which is related to the frequency of collisions and the orientation of the reacting molecules.
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Calculating the Equilibrium Constant02:46

Calculating the Equilibrium Constant

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The equilibrium constant for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these concentrations are known, the calculation simply involves their substitution into the Kc expression.
For example, gaseous nitrogen dioxide forms dinitrogen tetroxide according to this equation:
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Submillisecond Conformational Changes in Proteins Resolved by Photothermal Beam Deflection
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Intramolecular rate-constant calculations based on the correlation function using temperature dependent quantum

R R Valiev1, B S Merzlikin2,3, R T Nasibullin2

  • 1Department of Chemistry, Faculty of Science, University of Helsinki, P.O. Box 55 (A.I. Virtanens plats 1), FIN-00014, Finland. valievrashid@gmail.com.

Physical Chemistry Chemical Physics : PCCP
|January 17, 2024
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Summary
This summary is machine-generated.

A new theoretical method accurately calculates electronic transition rates for molecules like indocyanine green (ICG) and IR808. This approach aids in understanding molecular deactivation pathways and predicting fluorescence quantum yields.

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Area of Science:

  • Theoretical Chemistry
  • Computational Spectroscopy
  • Quantum Mechanics

Background:

  • Accurate calculation of electronic transition rates is crucial for understanding molecular photophysics.
  • Existing methods may not fully account for temperature effects or external field influences.

Purpose of the Study:

  • To present a novel theoretical method for calculating rate constants of internal conversion (IC), intersystem crossing (ISC), and radiative (R) electronic transitions.
  • To apply this method to indocyanine green (ICG) and heptamethine cyanine (IR808) molecules.

Main Methods:

  • Utilized temperature-dependent quantum Green's functions to model perturbation operators and external electromagnetic field effects.
  • Performed calculations at the Franck-Condon level and employed time-dependent density functional theory (TD-DFT) with the MN15 functional.

Main Results:

  • Determined that ICG and IR808 possess a single triplet state below the S1 state.
  • Identified internal conversion (IC) as the primary S1 state deactivation channel with high rate constants (∼109-1011 s-1).
  • Estimated fluorescence quantum yields (φfl) between 0.001–0.24, consistent with experimental data.

Conclusions:

  • The developed temperature-dependent quantum Green's function approach provides a unified methodology for calculating IC, ISC, and R rate constants.
  • This method offers a reliable tool for predicting molecular photophysical properties and validating experimental findings.