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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]


Imaginary frequency - The frequency of the normal vibration related to a negative eigenvalue (force constant) of the Hessian matrix. Transition state structure possesses only a single imaginary frequency.

Individual Gauge for Localized Orbitals (IGLO) - A method of calculation of nuclear shieldings, in which localized molecular orbitals associated with inner shell, bonding orbitals, and lone pairs have unique origins for the calculation of diamagnetic and paramagnetic terms. With this method, satisfactory estimates of NMR chemical shifts for elements in the first and second rows can be achieved in ab initio calculations with basis sets of moderate size provided sufficiently accurate molecular geometries are used. KUTZELNIGG (1980); SCHINDLER and KUTZELNIGG (1983).

Inductive effect - The polarization of chemical bonds due to shifts of their electron pairs in the direction of the electronegative group. Within a molecule, the inductive effect is transmitted through space and is determined by electrostatic forces between the interacting sites. Another original (G. Lewis) model of inductive effect relates to through-bond transmission by successive polarization of the bonds between a dipolar or charged substituent and the reaction site. A quantitative assessment of the inductive effects of groups (substituents) is given by the Taft s*-constants (for aliphatic) and the inductive sI-constants (for aromatic) compounds.

Instability of Hartree-Fock solution - Existence of additional solutions to the equations of the Hartree-Fock method occurring usually in the case when potential energy surfaces of different electronic states are drawn close together. Within the spin-restricted Hartree- Fock method (RHF), singlet and triplet instabilities are distinguished. The former involves the existence of another solution with lower energy and the electron distribution of lower symmetry. It may be regarded as an indication that the initially assumed molecular shape needs correction. The triplet instability involves rejection of the condition of double occupancy of molecular orbitals and thus transition to the spin-unrestricted Hartree-Fock method (UHF). The triplet instability is a necessary, but insufficient, condition for the conclusion as to the biradical character of the ground state of a given system. MESTECHKIN (1990); YAMAGUCHI (1981).

Internal conversion - Isoenergetic radiationless transition between two electronic states of the same multiplicity.

Internal rotation - The interconversion of stereoisomers through rotation of groups of atoms about a single bond. The internal rotation is called "free" when the energy barrier is so low that different rotational isomers are not perceptible as individual chemical species (i.e. the characteristic time of the method of observation is longer than the lifetime of the rotational isomers). The inhibition of internal rotation by a sufficiently high rotational barrier makes the phenomenon observable on the time scale of the experiment, and is termed restricted or hindered rotation.

Intersystem crossing - Isoenergetic radiationless transition between two electronic states of different multiplicity.

Intrinsic barrier - The activation free-energy barrier associated with a reaction whose free-energy change is zero.

Intrinsic reaction coordinate - A minimum-energy reaction path on a potential energy surface in mass-weighted coordinates, connecting reactants to products via the transition state. FUKUI (1981).

Ionic bond - The bond between atoms with sharply different electronegativities. In strict terms, an ionic bond refers to the electrostatic attraction experienced between the electric charges of a cation and an anion, in contrast with a purely covalent bond. In practice, it is preferable to consider the amount of ionic character of a bond rather than referring to purely ionic or purely covalent bonds. The relationship was proposed (L.Pauling) for the estimation of ionic character of a bond between atoms A and B:

Amount of ionic character = 1 - e-1/4 (cA -cB)

where cA and cB are the Pauling electronegativities of atoms A and B. This type of bonding is realized mostly in solids.

Ionization potential of an atom or a molecule - The minimal energy (Ix) needed for the detachment of an electron, i.e. X + Ix X+ + e- . When the ion is produced in its most stable state, (which is the case in e.g. photoionization or photoelectron spectroscopic techniques), the values Ix correspond to adiabatic ionization potentials. When a technique such as fast electron bombardment is used, where the ionization occurs during the period of collision, so that the ion X+ retains the geometry of the initial X, the value of IX is referred to as the vertical ionization potential.
See also Koopmans’ theorem

Isoconjugate systems - Molecular entities with similar conjugate networks which have the same number of p-electrons, e.g. cyclopentadienide and pyrrole or benzene and pyridine.

Isodesmic reaction - A reaction (sometimes hypothetical) in which the number of bonds of each given formal type, e.g. C-H, C-C, C=C, is conserved, but the relationship among the bonds is altered. An example is given for a formally conceivable reaction

F3C-C(=O)H + CH4 CH2F2 + H3C-C(=O)F

where three C-F, one C=O, and C-C and five C-H bonds are present in both reactants and products; only the environment in which bonds are located has changed. Due to the conservation of the number of electron pairs in the reactants and products the changes in their correlation energies are usually small, so that the energies of isodesmic reactions are generally well reproduced even by simple computational methods. This makes isodesmic reactions important interpretive tools and a means of providing thermochemical data. GEORGE, BOCK, and TRACHTMAN (1988); HEHRE, DITCHFIELD, RADOM, and POPLE (1970).

Isogyric reaction - A reaction in which the number of electron pairs is conserved, for example

CH4(1A1) + H(2S) CH3(2A2") + H2(1S+g)


Isolobal analogy - The concept allowing one to establish relationships between the isolobal groups of organic, main-group and transition metal inorganic and organometallic compounds. The utility of the isolobal analogy is that, if an MLn fragment is isolobal with a particular AL'm arrangement, then one should be able to replace the former in a molecule with the latter to form a new compound possessing a very similar valence electron shell and even displaying similar reactivity. The isolobal analogy makes it possible to establish qualitative correspondence between the energy and the spatial (nodal) characteristics of orbitals of the groups formed by the main-group and transition elements as well as to analyze in an approximate manner the structure and reactivity of inorganic and organometallic compounds by analogy with those of simple organic compounds. HOFFMANN (1982).

Isolobal groups - Molecular moieties for which the number, symmetry properties, occupation by electrons and approximate energy of frontier orbitals are similar. The isolobal relationship between the groups is denoted by a double-headed arrow with a tear-drop, for example

CH2 Fe(CO)4 Ni(PPh3)2 Co(h5-C5H5)CO Cu(h5-C5H5)


Isostructural reaction - A ligand exchange reaction, where the structural type of the metal complex remains the same.

MLn + X MLn-1X + L +DE

Although these types of reactions are not necessarily isodesmic reactions, a substantial degree of error cancellation for the calculated energy, DE, is achieved because the coordination sphere of the metal atom M is conserved. DAPPRICH, PIDUN, EHLERS, and FRENKING (1995).

Isotopologues - Species that differ only in the isotopic composition of their molecules or ions. SEEMAN, SECOR, DISSLKAMP, and BERNSTEIN (1992).
See also isotopomers.

Isotopomers - Isomers due to the positions of nuclear isotopes, for example CH2DOH and CH3OD.

[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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