Glossary – C

Cahn-Ingold-Prelog Rules: Rules which use the idea of priority of groups to define stereochemical configurations.

Calomel Electrode: A reference electrode based on the Hg/HgCl redox couple, which is more convenient to use in standard reduction potential determination than the standard hydrogen electrode.

Carbanion: A molecular ion with a negative charge localised on a carbon atom.

Carbide: A Carbide is an compound containing carbon in a formally negative oxidation state.

Carbocations: A molecular ion with a positive charge localised on a carbon atom.

Carbonium: A molecular ionic intermediate with a positive charge not necessarily on a carbon.

Carbonyl: The functional group C=O. Found in ketones and aldehydes for example.

Carboxylate: A species containing the group: CO2. The conjugate base of a carboxylic acid.

Carboxylic Acid: A species containing the group: CO2H.

Cartesian axes: The three axes x, y and z, which are all at right angles to one another.

Catalyst: A species that increases the rate of a reaction by providing an alternative (lower energy) pathway for the reaction to follow. It is only needed in very small concentration because it is regenerated after use (this is part of the definition of a catalyst).

Catenation: The formation of chains of bonded atoms.

Cation: A positively charged ion.

Centrifugal Distortion Constant: A measure of the degree of distortion that occurs to a particular bond in a rotating molecule. (The action of centrifugal force can cause the bonds in molecules to stretch, significantly distorting the molecular shape.) The larger the constant, the more easily a bond stretches under the influence of centrifugal force.

Centrosymmetric Molecule: A molecule that possesses a center of inversion.

Chair Form: The lowest energy conformation of cyclohexane. So called because it resembles a chair.

Charge Transfer Spectra: These are the spectra of electronic transitions involving the motion of an electron between orbitals on different species, such as from the metal to the ligand in a complex.

Chelate Effect: A polydentate ligand will form a more stable complex with a metal ion than the equivalent number of monodentate ligands.

Chemical shift: The difference between the resonance frequencies of a given nucleus and a standard reference nucleus. It arises due to interactions of electrons with the applied magnetic field that generate local magnetic fields around atomic nuclei.

Chiral: A chiral molecule is one without a plane of symmetry. A chiral molecule is one which cannot be superimposed on its mirror image. A chiral molecule and its mirror image are enantiomers of each other. e.g. CHFClBr.

Cis: Two substituents on a double bond are said to have cis geometry if they are on the same side.

Class: Symmetry operations which are of the same type and can be interconverted by a symmetry operation of the point group are in the same class.

Clebsh-Gordon series: This gives the range of values which the total orbital angular momentum for a set of electrons, determined from the orbital angular momenta for the individual electrons.

Close-Packing: The structure of many compounds is based on the stacking of spheres in arrangements where the occupied volume of the structure is maximized. This is known as close packing of spheres.

Closed System: A system that can exchange energy but not matter with its surroundings.

Collision Frequency: The number of collisions made by a molecule in a gas per unit time.

Colour Center: This is a defect which has a large effect on the UV, visible, or IR absorption spectra of a compound.

Combination differences: A method of simultaneous equations which is used to identify the rotational constants for different rotational energy levels.

Complex Mechanism: The mechanism by which the reaction of a metal ion complex proceeds.

Complex: A species made up of a metal ion center bonded to a number of ligand species.

Compressibility factor: Given the symbol Z, a quantity defined as Z = pVm / RT. Z = 1 for any perfect gas, so the deviation of Z from 1 is a measure of the deviation of the actual behaviour of a gas from ideal behaviour.

Comproportionation: The tendency of a mixture of species in different oxidation states to react to give a product which is in a more stable intermediate oxidation state.

Concerted: Applied to a reaction mechanism, if it is concerted, all the arrows happen at once. See also Step-Wise.

Condensation Reaction: Originally meant a reaction which gave off a molecule of water for each molecule of product, but now extended to mean a reaction that gives off a molecule of any kind of ‘by-product’ per molecule of product.

Conduction Band: The band in extended molecular orbital theory made up from the low lying vacant molecular orbitals on the individual species. It is the lowest unoccupied extended molecular orbital.

Conformation: A structural state of a molecule – different conformations may be reached only without breaking bonds.

Conjugate Acid: When a base, B, reacts with a proton donor, AH, the product BH+ is said to be the conjugate acid of B.

Conjugate Base: When a acid, AH, reacts with a proton acceptor, B, the product A is said to be the conjugate base of AH.

Conjugation: When π electrons of one functional group can be delocalised (shown by resonance structures) into neighbouring groups, they are conjugated.

Constituent: A chemical species present in a mixture.

Coordination Chemistry: The chemistry of complex compounds made up of a metal ion surrounded by ligands.

Correlation Diagram: See Walsh Diagram.

Correspondence Principle: The idea that at large quantum numbers, quantum mechanical results tend towards the classical description of the situation.

Coulombic: An electrostatic interaction between charged particles. Particles of like charge repel each other, particles of opposite charges attract each other.

Cp: The heat capacity of a system at constant pressure. It is defined as the partial derivative of the enthalpy with respect to the temperature.

Critical Pressure: The vapour pressure at the critical temperature.

Critical Temperature: The temperature above which the liquid phase of a substance cannot exist.

Cryptate Ligands: A range of cyclic organic compounds which contain nitrogen and oxygen donor atoms which act as ligands towards metal ions.

Crystal Field Theory: The description of the effect on the metal ion energy levels when in a complex, based on the electrostatic interaction of point charges representing the ligands with the metal center.

Crystal Structure: This describes the arrangement of ions in the crystal.

Cv: The heat capacity of a system at constant volume. It is defined as the partial derivative of the internal energy with respect to temperature.