vated complex in a step of a chemical
reaction. Reactions are said to be uni-
molecular, bimolecular, or trimolecu-
laraccording to whether 1, 2, or 3
molecules are involved.molecular knot (knotane)A type
of compound in which one or more
chains of atoms in the molecule are
looped in the conÜguration of a knot.
The molecule may have only one
closed chain forming the knot. If the
knot is a trefoil knot the compound
is chiral. Alternatively, molecular
knots may have two or more sepa-
rate loops tied together in a knot. In
such cases there is no formal chemi-
cal bonding between the rings and
they are held by *mechanical bond-
ing. Molecular knots can be produced
synthetically and also occur naturally
in certain proteins.molecular modellingThe use of
computer software to produce simu-
lations of molecular structures. Vari-
ous chemical drawing programs exist
to allow a graphic representation of
chemical formulas in two dimen-
sions. More sophisticated three-
dimensional modelling programs
also exist. The information about the
molecule is stored in a dataÜle giv-
ing the numbers and types of atoms
present and the coordinates of these
atoms. The software converts this
into an on-screen three-dimensional
view of the molecular structure in
some speciÜed format (e.g. ball-and-
stick, wireframe, etc.). The structure
can be rotated on the screen and, de-
pending on the software, calculations
may be done on the molecule.
molecular orbitalSee orbital.molecular-orbital theory A
method of computational chemistry
in which the electrons are not as-
signed to individual bonds between
atoms, but are treated as moving
under the inÛuence of the nuclei inthe whole molecule. The molecule
has a set of molecular orbitals (see or-
bital). The usual technique is to ob-
tain molecular orbitals by a linear
combination of atomic orbitals
(LCAO). See also density-function
theory; valence-bond theory.molecular recognition The way
in which a molecule may have a
highly speciÜc response to another
molecule or atom. It is a feature of
host–guest chemistry. See supramol-
ecular chemistry.
molecular sievePorous crystalline
substances, especially aluminosili-
cates (see zeolite), that can be dehy-
drated with little change in crystal
structure. As they form regularly
spaced cavities, they provide a high
surface area for the adsorption of
smaller molecules.
The general formula of these sub-
stances is MnO.Al 2 O 3 .xSiO 2 .yH 2 O,
where M is a metal ion and n is twice
the reciprocal of its valency. Molecu-
lar sieves are used as drying agents
and in the separation and puriÜca-
tion ofÛuids. They can also be loaded
with chemical substances, which re-
main separated from any reaction
that is taking place around them,
until they are released by heating or
by displacement with a more
strongly adsorbed substance. They
can thus be used as cation exchange
mediums and as catalysts and cata-
lyst supports. They are also used as
the stationary phase in certain types
of *chromatography (molecular-sieve
chromatography).molecular symmetryThe set of
symmetry operations (rotations,
reÛections, etc.) that can be applied
to a molecule. This set forms the
*point group of the molecule. For an
isolated molecule the *SchoenÛies
system rather than the *Hermann–
Mauguin system is used to denote its
symmetry. Molecular symmetry ismolecular knot 358m