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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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Fermi hole - A region around an electron where the probability of finding another electron with the same spin is very small due to the action of the antisymmetry principle.The Fermi hole describes the manner in which the charge of the reference electron is spread out in space, thereby excluding the presence of an identical amount of same-spin density. McWEENY (1960).

Fermi level - In an extended structure (solids, metals, semiconductors or insulators), the average for the highest occupied and the lowest unoccupied levels. In chemistry, the Fermi level is usually considered as the highest occupied crystal orbital.

Fluxional molecules - A subclass of structurally nonrigid molecules in which all the interconverting species that are observable are chemically and structurally equivalent. The classic example of the phenomenon of fluxionality is the rapid automerization of tricyclo[,8]deca-3,6,9-triene (bullvalene), the rapid interconversion of 1 209 600 (10!/3) degenerate isomers. COTTON (1975).

Force field - Within the molecular mechanics approach, a set of potential functions defining bond stretch, bond angle (both valence and dihedral) distortion energy of a molecule as compared with its nonstrained conformation (that characterized by standard values of bond lengths and angles). A set of transferable empirical force constants is preassigned and the harmonic approximation is usually employed. Some force fields may contain terms for interactions between nonbonded atoms, electrostatic, hydrogen bond and other structural effects as well as account for inharmonicity effects.

In vibrational spectroscopy, the inverse problem is solved by determining a set of force constants and other parameters of a chosen potential energy function which would match with experimentally observed vibrational frequencies of a given series of congeneric molecules.

Fractional bond number (synonymous with the Pauling's bond order) - The formal bond order, n defined in terms of the difference between a standard single bond distance r0 and an interatomic distance r, and an empirical constant c.

c ln n = r0 - r

Fragment molecular orbital analysis - (Synonymous with reconstruction molecular orbital analysis) - The procedure based on the theory of orbital interactions for the construction of molecular orbitals of a complex molecular system from the orbitals of its fragments with the same geometry and symmetry properties as these fragments have in the molecule. HOFFMANN, ALBRIGHT, and THORN (1981).

Franck-Condon principle - Classically, the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in a molecular entity and its environment. The resulting state is called a Franck-Condon state, and the transition involved is called a vertical transition. The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states involved in the transition. IUPAC PHOTOCHEMICAL GLOSSARY (1988).

Frank-Condon State - see Frank-Condon principle.

Free radical - see Radical.

Free valence - A reactivity index applied mostly to radical reactions of conjugated systems. An atom in such a system can exert only a certain maximum of p-bonding power as measured by a sum of the p- bond orders PAB of bonds formed by it. In the Hückel molecular orbital theory, the maximum p-bonding power for a carbon atom is ˜3; hence the free valence of a carbon atom in a molecule is defined by

FA = 3 - PAB

Frontier function (synonymous with Fukui function) - The function which provides a description of electron density in the frontier orbital as the initial response in the electron density at position r, r(r), due to infinitesmal perturbation in the total number of electrons N in the molecular entity.

f(r) = r(r)/ N

FUKUI (1982); REED (1994).

Frontier orbitals - The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a given molecular entity. In the case of an odd-electron molecular entity, when its HOMO is occupied by a single electron such a molecular orbital is termed a singly occupied molecular orbital (SOMO). Depending on the properties of the reactive partner, the SOMO of a given species may function as either HOMO or LUMO. The special importance of the frontier orbitals is due to the fact that a broad variety of chemical reactions takes place at a position and in a direction where the overlap of HOMO and LUMO of the respective reactants is maximal.

Frozen core approximation - Approximation commonly used in post-SCF calculations which implies that only the contributions of the valence electrons to the correlation energy are evaluated.

Fukui function - see Frontier function

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