Macrocyclic Effect: A complex formed with a closed ring ligand is more stable than its open chain analogue.

Madelung Constant: This is the factor by which the ionic charges must be scaled to allow for the different arrangements of ions in different structures. It is determined only by the geometry of the crystal.

Madelung Energy: This is the coulombic part of the lattice energy, ie. that resulting from the attractive interaction of the oppositely charged ions.

Magnetogyric ratio: The coefficient of proportionality between the z-component of a nucleus’ magnetic moment and the z-component of its spin angular momentum. It is an experimentally determined quantity.

Markownikov’s Rule: A rule for determining the regiospecificity of H-X additions to alkenes.

Mass Susceptibilty: This measures the degree of magnetism in a compound.

Maxwell distribution: A mathematical function giving the distribution of speeds within a gaseous sample. It is dependent upon the molecular mass of the gas and the temperature of the sample.

Mean Free Path: The average distance travelled by a molecule in a gas between collisions.

Mean Speed: The average speed of the particles in a gaseous sample. It can be obtained by integrating v f(v) (where f(v) is the Maxwell Boltzmann distribution) with respect to v, the molecular speed, and evaluating the integral between limits of 0 and infinity.

Mechanism: The manner in which a reaction proceeds. Mechanistic detail will include the movement of electrons, and will provide a step by step account of bonds formed and broken in a reaction.

Mesomeric Effect: The mesomeric effect occurs through π bonds, and can either add (if there are filled p orbitals – as on O or N) or remove (if there is an empty p orbital) local electron density.

Meso: (See diastereoisomer). A molecule with more than one chiral centre, which would be a diastereoisomer except that changing either of its chiral centres does not change the actual molecule overall.

Meta: A position on a phenyl ring – see ortho.

Me: Denotes a methyl (CH3) group.

Migration: The movement of some group, or even single atom, to a different site in the same molecule – a type of rearrangement.

Moiety: An often arbitrary term used to describe some sort of fragment of a molecule, or charged species.

Molality: A method of expressing the concentration as moles of solute per kilogram of solvent.

Mole fraction: The amount of a substance expressed as a fraction of the total number of molecules that are present. It ranges between 0 (none of the substance is present) to 1 (pure substance).

Molecular Orbital Diagram: (MO Diagram) This is a means of representing the energies and names of the molecular orbitals for a given molecule. It shows the relative energies of the molecular orbitals, and how they relate to the basis atomic orbitals.

Molecular Orbital: The wavefunction which defines the properties (location etc) of an electron in a molecule. We speak of an electron that obeys such a wavefunction as occupying the relevant orbital.

Molecular tumbling: The random motion of molecules in a solution, where their positions and orientations are constantly changing.

Moment of inertia: The rotational analogue of mass, a measure of the difficulty in rotationally accelerating a body.

Morse: A mathematical curve sometimes used to replace the parabola for a more accurate model of the vibration of bonds.

Most Probable Speed: The speed at which a particle picked at random from a gaseous sample is most likely to be travelling. It corresponds to the speed at which the Maxwell-Boltzmann distribution of speeds is at a maximum.

Multiplet: A characteristic splitting pattern observable in NMR spectra. Lines that are unsplit are referred to as singlets, those split into two equal peaks are doublets, etc.