I  U  P  A  C

News & Notices

Organizations & People

Standing Committees



..By Year
..By Division
..Other Committees




Links of Interest

Search the Site

Home Page




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

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]


Thermochemical resonance energy - see Resonance energy, various types of.

Thermodynamic stability - Those species are considered to be thermodynamically stable whose free energies are sufficiently low compared to all reasonable decomposition products so that detectable amounts of the former may exist in equilibrium.

Three-center, four-electron (3c-4e) bond - see Hypervalency.

Three-center, two-electron (3c - 2e) bond - A kind of multicenter s-bond in which three atomic cores are held together by two electrons. The 3c-2e bonds are particularly common in the boron hydrides, carbocations (for example, the bond in the CH2 fragment of the methanium cation, see Hypercoordination), bridged metal alkyls and metal halides. The simplest example of the 3c-2e bond is in the H3+ ion.

Through-bond interaction - An intramolecular orbital interaction of spatially separated orbitals, where the orbitals interact through their mutual mixing with s-orbitals of the intervening framework.

Through-space interaction - The orbital interaction that results from direct spatial overlap of two orbitals.

Tight binding method - Approximation of crystal orbitals in solids by a Linear Combination of Atomic Orbitals. In general, tight binding is approximation in quantum chemical methods by retaining resonance integrals only for neighboring atoms.

Topochemical principle - Reactions in the crystalline phase proceed under minimal atomic and molecular displacement.

Topological distance matrix - see Topological index.

Topological electron distribution theory - The theory of molecular structure based on the analysis of the topological properties of scalar fields of the electron density function r(r) and the Laplacian of the electron density [2r(r)], which are summarized in terms of their critical points, rc, i.e. the points where r(r) and (-2r(r)) are equal to zero. The critical points are classified according to their rank (number of non-zero eigenvalues) and signature (algebraic sum of their signs) of the Hessian matrix of r(r). Thus, a (3, -3) critical point corresponds to a maximum  of r(r) related to an atomic nucleus, whereas a (3, -1) critical point (bond point) is a saddle point that exists between every pair of neighboring bonded atoms. BADER (1990); BADER, POPELIER, and KEITH (1994).
See also Atoms in molecules, theory of .

Topological index - A numerical value associated with chemical constitution purporting for correlation of chemical structure with various physical properties, chemical reactivity or biological activity. The numerical basis for topological indices is provided (depending on how a molecular graph is converted into a numerical value) by either the adjacency matrix or the topological distance matrix. In the latter the topological distance between two verices is the number of edges in the shortest path between these.

Topological resonance energy (TRE) - see Resonance energy, various types of.

Topomerization - see Automerization.

Total energy of a molecular system - The sum of the total electronic energy, Eee and the energy of internuclear repulsion, Enr. In the Hartree-Fock (SCF) method, the value of Eee for a closed-shell molecular system is given by

Eee = 2 ei + (2Jij - Kij)

where ei stands for orbital energies and Jij and Kij stand for respectively Coulomb and exchange integrals, the summation being done over all occupied molecular orbitals. The repulsion between the nuclei (A, B ...) is defined by the expression:


Transition state (TS) (also transition state structure, transition structure) - The structure corresponding to the highest point on the minimum energy reaction path, i.e. first order saddle point on the potential energy surface. In transition state spectroscopy, the definition of the transition state is broader. It embraces the entire process of bond breaking and bond making and is used to denote the full family of configurations through which the reacting particles evolve along the route from reagents to products.
See also Hessian matrix.

Transition State Theory (TST) - The theory which provides a conceptual framework for understanding all chemical reactivity. It serves as a powerful computational tool for translating molecular structures and energetics into prediction of chemical reaction rates. EYRING (1935), GARRET and TRUHLAR (1998).

Transition state spectroscopy (TSS) - The femtosecond temporal resolution (10-14 - 10-13 s = 10 - 100 fs) laser spectroscopy that enables one to detect the intermediate configurations (transition states) of a reacting system in real time and follow the nuclear motion and energy redistribution. POLANYI and ZEWAIL (1995).

Transition vector - The eigenvector associated with the only negative eigenvalue of the Hessian matrix representing the single direction in which the potential energy surface is a maximum. The transition vector is also a path of the intrinsic reaction coordinate through the saddle point.

Transmission factor - In the equation of the theory of absolute reaction rates that relates the reaction constant k with the height of the energy barrier DETS, the value of the coefficient g is equal to 1 when the conversion of reactants to products always occurs upon attaining the height of energy barrier, and less than 1 when this condition is not met.

k = gt (kBT/h) (QTS/QR) exp(-DETS/RT)

where k is the rate constant for the reaction of conversion of reactants R into products by proceeding through transition state TS; Q are partition functions per unit volume, and t is the quantum mechanical tunnelling coefficient. In some descriptions k = gt is referred to as the transmission factor.

Tunnelling effect - The passage of a particle through a potential energy barrier the height of which is larger than the energy of that particle. The effect is important for some processes involving the transfer of electrons and light atoms, particularly hydrogen atoms.

[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

Page last modified 21 August 2000.
Copyright © 2000 International Union of Pure and Applied Chemistry.
Questions or comments about IUPAC, please contact, the Secretariat.
Questions regarding the website, please contact Web Help.