Racemate: A racemic mixture of two stereoisomers – i.e. a 50/50 mixture so that no optical rotation of light occurs.

Racemic: See Racemate.

Radial Distribution Function: This is the probability of finding an electron at a given distance from the nucleus, and is different for different atomic orbitals.

Radial Node: This is the distance from the nucleus where the sign of the radial wavefunction changes.

Radial Wavefunction: This describes the part of the electronic wavefunction which depends only on the distance of the electron from the nucleus.

Radical: A reactive species with an unpaired electron.

Radius Ratio Rules: These can be used to predict the structure which will be adopted by a given set of ions, based on the ratio of the ionic radius of the small ion to that of the large ion.

Rate Determining Step: Just as a chain is only as weak as its weakest link, a reaction is only as fast as its slowest step – the Rate Determining Step (often abbreviated RDS).

Rate Limiting Step: See Rate Determining Step.

Reaction Quotient: A measure of how far advanced a reaction is, usually expressed in terms of the activities or fugacities of the chemical species involved.

Reagents: The chemicals used to carry out a reaction, usually excluding the starting molecule itself.

Rearrangement: See the rearrangements section.

Redox Couple: The oxidized and reduced species which are the reactant and product for a given half reaction.

Reduction: In general terms, the gain of electrons. Also commonly defined as the gain of hydrogen or the loss of oxygen.

Reference state: The most stable state of a particular element at one bar and a specified temperature. NB: The one exception to this is phosphorous for which the reference state is taken to be white phosphorous.

Regioselectivity: If a reagent displays regioselectivity, this means that it reacts with a specific region of the molecule in preference to other regions – e.g. with one double bond rather than another – for a variety of possible reasons.

Resonance Condition: The condition of resonance is fulfilled when the frequency of incoming radiation is such that the photons are of energy equal to the energy gap between two energy levels. Transitions between these two levels then occur, and the radiation may be absorbed.

Resonance: The ability of electrons to move around parts of molecules without changing the basic structure is resonance. If resonance is possible, then the actual molecule will be a ‘hybrid’ of all possible resonance structures – e.g. benzene.

Resonance: Resonance may be described as the transfer of energy from one system to another, when the energy transferred is equal to some resonant energy which iscommon to both systems. In the case of NMR, this is transferring energy from electromagnetic radiation to promoting the state of a nucleus.

Retention: If a chiral centre retains its configuration through the course of a reaction, then retention has occurred. See also inversion.

Reversible Reaction: A reaction which can go in both directions.

Rigid rotor: A rotating body that is not subject to centrifugal distortion.

Root Mean Square Speed: This quantity is the square root of the mean value of the molecular speeds squared. i.e. the molecular speeds are measured, squared and then their mean value is determined. The square of this value is the root mean square speed.

Russell-Saunders Coupling: This is a scheme for treating the interaction of the spin and orbital angular momenta in a multielectron species. It is based on the order of magnitude of interaction being spin-spin > orbit-orbit > spin-orbit.