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A method of calculating atomic and molecular structure directly from the first principles of quantum mechanics, without using quantities derived from experiment (such as ionization energies found by spectroscopy) as parameters. Ab-initio calculations require a large amount of numerical computation; the amount of computing time required increases rapidly as the size of the atom or molecule increases. The development of computing power has enabled the properties of both small and large molecules to be calculated accurately, so that this form of calculation can now replace ...

1. Not dependent on or relative to anything else, e.g. *absolute zero.

2. Denoting a temperature measured on an ...

A way of denoting the absolute structure of an optical isomer (see optical activity). Two conventions are in use: The ...

Zero of thermodynamic *temperature (0 kelvin) and the lowest temperature theoretically attainable. It is the temperature at which the kinetic energy of atoms and molecules is minimal. It is equivalent to –273.15°C or –459.67°F. ...

1. (in chemistry) The take up of a gas by a solid or liquid, or the take up of a liquid by a solid. Absorption differs from ...

1. (in spectroscopy) The molar absorption coefficient (symbol ϵ) is a quantity that characterizes the absorption of light (or any other type of electromagnetic radiation) as it passes through a sample of the absorbing material. It has the dimensions of 1/(concentration ...

A long vertical column used in industry for absorbing gases. The gas is introduced at the bottom of the column and the absorbing liquid, often water, passes in at the top and falls down against the countercurrent of gas. The towers are also known as ...

Any of a class of plastics based on acrylonitrile–butadiene–styrene copolymers.

A chemical reaction that involves bimolecular removal of an atom or ion from a molecule. An example is the abstraction of hydrogen from methane by reaction with a radical:...

1. The ratio of the total mass of a specified element in the earth’s crust to the total mass of the earth’s crust, often expressed as a percentage. For example, the abundance of aluminium in the earth’s crust is about 8%....

Anticlinal. See torsion angle.

The symbol for the *acetylacetonato ligand, used in formulae.

A flammable material used to start and spread a fire in cases of arson. Petrol and paraffin are the substances commonly used. Traces of accelerant are detectable by gas chromatography in forensic work....

A substance that increases the rate of a chemical reaction, i.e. a catalyst.

1. (in chemistry and biochemistry) A compound, molecule, ion, etc., to which electrons are donated in the formation of a coordinate bond....

A *photosynthetic pigment that traps light energy and channels it to chlorophyll a, the primary pigment, which initiates the reactions of photosynthesis. Accessory pigments include the carotenes and chlorophylls ...

A type of *voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the opposite direction to that in which the cell supplies current, reverses the chemical reactions in the cell. The common types are the ...

A colourless crystalline aromatic compound, C₁₂H₁₀; m.p. 95°C; b.p. 278°C. It is an intermediate in the production of some dyes....

Organic compounds formed by addition of alcohol molecules to aldehyde molecules. If one molecule of aldehyde (RCHO) reacts with one molecule of alcohol (R...

A white crystalline primary amide of ethanoic acid, CH₃CONHC₆H₅; r.d. 1.2; m.p. 114.3°C; b.p. 304°C. It is made by reacting phenylamine (aniline) with excess ethanoic acid or ethanoic anhydride and is used in the manufacture of dyestuffs and rubber. The full systematic name is N-...