**Glossary of terms used in theoretical
organic chemistry **

[A] [B]
[C] [D] [E]
[F] [G] [H]
[I] [J-K] [L]
[M]

[N] [O] [P]
[Q-R] [S] [T]
[U-V] [W-Z]

# **J-K**

**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*.

**H**^{eff}y_{i}(*r*)
= e_{i}
y(*r*) |
*i* = 1, 2, ...*n* |

where **H**^{eff} is the *effective*
one-electron *hamiltonian*, generally expressed as a functional
of electron density, r(*r*) and
e_{i} are the energies associated
with the y_{i}(*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 y_{i}’s
according to

r(*r*) = |y_{i}(*r*)|^{2}

The Kohn-Sham orbitals y_{i}
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, -e_{i}, as calculated
within the *Hartree-Fock *approximation. The theorem is not applied
to* localized molecular orbitals*, which are not eigenfunctions
of the* effective* *hamiltonian**.*