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Relative Stabilities of Alkenes01:59

Relative Stabilities of Alkenes

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The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable alkene.  The three main factors determining the relative stability of alkenes are i) the number of substituents attached to the double-bond carbon atoms, ii) hyperconjugation, and iii) the stereochemistry of the double bond.
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Understanding the stability of equilibrium configurations is a fundamental part of mechanical engineering. In any system, there are three distinct types of equilibrium: stable, neutral, and unstable.
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This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
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Nuclear Stability

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Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10−15 meters, a nucleus is quite small compared to the radius of the entire atom, which is about 10−10 meters. Nuclei are extremely dense compared to bulk matter, averaging 1.8 × 1014 grams per cubic centimeter. If the earth’s density were equal to the average nuclear density, the earth’s radius would be only about 200 meters.
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Stability of Conjugated Dienes

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Introduction
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A Practical Computational Approach to Study Molecular Instability Using the Pseudo-Jahn-Teller Effect.

Pablo García-Fernández1, Jose Antonio Aramburu1, Miguel Moreno1

  • 1Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria , Avenida de los Castros s/n, 39005 Santander, Spain.

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Summary

The pseudo-Jahn-Teller effect explains molecular instability by mixing electronic states. This study reformulates the theory and develops a computational method to quantify it, revealing the crucial role of electron kinetic energy in molecular distortions.

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Area of Science:

  • Theoretical Chemistry
  • Computational Chemistry
  • Quantum Chemistry

Background:

  • Vibronic coupling theory identifies the pseudo-Jahn-Teller (pJT) effect as the cause of spontaneous instability in nondegenerate ground states.
  • Previous studies on pJT effect have limitations in fully accounting for all operators in the molecular Hamiltonian and quantitative computational appraisal.

Purpose of the Study:

  • To reformulate the pseudo-Jahn-Teller theory by explicitly including all operators from the molecular Hamiltonian.
  • To develop a practical computational approach for quantifying the pJT effect.
  • To elucidate the microscopic origins of structural instabilities in molecules and solids.

Main Methods:

  • Reformulation of the pseudo-Jahn-Teller theory.
  • Development of a computational method using Hartree-Fock and density functional theory.
  • Application to study pyramidal distortion in ammonia and its absence in borane.

Main Results:

  • The reformulated theory explicitly shows contributions from all operators, including electron-electron repulsions and electron kinetic energy.
  • A practical computational method for quantifying the pJT effect was successfully developed.
  • The study revealed the significant role of the kinetic energy of electrons in the lowest a2″ orbital in triggering pyramidal distortion in ammonia.

Conclusions:

  • The developed computational tool provides a means to explore the microscopic origins of structural instabilities.
  • The reformulated pJT theory connects vibronic coupling with other chemical theories relating electron distribution and molecular geometry.
  • The kinetic energy of electrons is a critical factor in molecular structural instabilities.