Decalin: An example of a fused ring. A decalin is effectively two cyclohexane rings joined through two carbon atoms. They occur in cis and trans formations.

Defect: This is a discontinuity in the otherwise ideal lattice in a solid.

Degeneracy: The existence in a system of different wavefunctions that correspond to the same energy.

Degree of freedom: A term for potential or kinetic energy which appears as the square of some coordinate or velocity. Thus there are three degrees of translational freedom for a molecule, corresponding to the three direction x, y and z in which it may travel. (The kinetic energy term in each of these directions contains a v2 term.)

Delocalization: In Lewis Representation it is the ability to put the formal charge in different locations. An electron is delocalized if it can occupy more than one molecular orbital, and hence which is free to move throughout a compound.

Denticity: The number of atoms through which a ligand may form bonds to the metal ion in a complex.

Deprotonation: The loss of H+.

Deuterium: This is an isotope of hydrogen. It is chemically identical to hydrogen except that it has a neutron in its nucleus where hydrogen does not.

Diastereoisomer: Also diastereomer. A diastereoisomer is a molecule with more than one chiral centre in it, where if the chirality of one of the centres is changed, a different molecule is produced.

Diathermic: A barrier that allows the transfer of heat through it.

Dimensionless: Of a value, having no units.

Disproportionation: The tendency of a species in a given unstable oxidation state to react to give a mixture of species with both higher and lower oxidation states.

Dissociative Mechanism: The mechanism for the substitution of ligands of a complex where the rate determining step is the loss of the leaving ligand from the complex, where the coordination number of the metal ion is decreased.

Doping: The substitution of atoms in a semiconductor by species of a different valence to the host species, such that extra charge carriers, electrons or positive holes, are introduced and the conductivity of the semiconductor is increased.

Double Bond: A double bond is a combination of a σ and a π bond, caused by overlap of two s orbitals and two p orbitals.

de Broglie Relation: A link between momentum (a property of particles) and wavelength (a property of waves), which states that p =h / λ.