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Pure Appl. Chem., Vol. 71, No. 10, pp. 1919-1981, 1999

Glossary of terms used in theoretical organic chemistry

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Jahn-Teller (JT) effect - Deals with molecular distortions due to an electronically degenerate ground state. For nonlinear molecular entities in a geometry described by a symmetry point group possessing degenerate irreducible representations there always exists at least one nontotally symmetric vibration that makes the electronic states with orbital degeneracy unstable at this geometry The nuclei are displaced to new equilibrium positions of lower symmetry causing a splitting of the originally degenerate electronic states (first-order Jahn-Teller effect). In the case of molecules with a nondegenerate ground electron state, but with a very low lying excited state, distortions of proper symmetry arise which mix ground and excited states, and thereby lower the ground state energy ( pseudo Jahn-Teller or second-order Jahn-Teller effect). The closer the states in energy, the more effective is the mixing. The pseudo Jahn-Teller effect manifests itself in fluxional behaviour (see fluxional molecules) and stereochemical nonrigidity of molecules and ions. BERSUKER (1984); PEARSON (1983).
See also Vibronic coupling, Peierls distortion, and Renner effect.

Kinetic stability - The propensity of a molecular system not to undergo chemical changes in a reasonable period of time even in the presence of small external perturbations, owing to a high activation barrier.

Kohn-Sham orbitals - The functions y(r) in a set of one-electron equations derived by Kohn and Sham, from which one can obtain the exact electron density and hence the total energy.

Heffyi(r) = ei y(r) i = 1, 2, ...n

where Heff is the effective one-electron hamiltonian, generally expressed as a functional of electron density, r(r) and ei are the energies associated with the yi(r) . The Kohn-Sham equations are fundamental in density functional theory as they serve as a starting point for approximate methods. The electron density r(r) can be calculated from the yi’s according to

r(r) = |yi(r)|2                                                                   

The Kohn-Sham orbitals yi should not be confused with the molecular orbitals obtained in Hartree-Fock method. They have no physical significance other than in allowing the exact r(r) to be calculated by the above equation. KOHN and SHAM (1965); PARR and YANG (1989); WEBER, HUBER and WEBER (1993).

Koopmans’ theorem - Directly relates experimental ionization potentials with energy levels of molecular orbitals. The theorem states that the ionization potential required to remove an electron from the orbital yi is given by the negative value of the energy of the orbital, -ei, as calculated within the Hartree-Fock approximation. The theorem is not applied to localized molecular orbitals, which are not eigenfunctions of the effective hamiltonian.

[A] [B] [C] [D] [E] [F] [G] [H] [I] [J-K] [L] [M]
[N] [O] [P] [Q-R] [S] [T] [U-V] [W-Z]

> Abstract
> General remarks
> Arrangement

> Fundamental physical constants used in the glossary
> References
> Appendix. Glossary of acronyms of terms used in theoretical organic chemistry

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