vated complex in a step of a chemical
reaction. Reactions are said to be unimolecular, bimolecular, or trimolecular according 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 separate loops tied together in a knot. In
such cases there is no formal chemical bonding between the rings and
they are held by *mechanical bonding. Molecular knots can be produced
synthetically and also occur naturally
in certain proteins.
molecular modelling The use of
computer software to produce simulations of molecular structures. Various chemical drawing programs exist
to allow a graphic representation of
chemical formulas in two dimensions. More sophisticated threedimensional modelling programs
also exist. The information about the
molecule is stored in a data Üle giving 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-andstick, wireframe, etc.). The structure
can be rotated on the screen and, depending on the software, calculations
may be done on the molecule.
molecular orbital See orbital.
molecular-orbital theory A
method of computational chemistry
in which the electrons are not assigned to individual bonds between
atoms, but are treated as moving
under the inÛuence of the nuclei in
the whole molecule. The molecule
has a set of molecular orbitals (see orbital). The usual technique is to obtain 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 supramolecular chemistry.
molecular sieve Porous crystalline
substances, especially aluminosilicates (see zeolite), that can be dehydrated 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 substances is M n O.Al 2 O 3 .xSiO 2 .yH 2 O,
where M is a metal ion and n is twice
the reciprocal of its valency. Molecular sieves are used as drying agents
and in the separation and puriÜcation of Ûuids. They can also be loaded
with chemical substances, which remain 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 catalyst supports. They are also used as
the stationary phase in certain types
of *chromatography (molecular-sieve
chromatography).
molecular symmetry The 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 is
molecular knot
358
m
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reaction. Reactions are said to be unimolecular, bimolecular, or trimolecular according 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 separate loops tied together in a knot. In
such cases there is no formal chemical bonding between the rings and
they are held by *mechanical bonding. Molecular knots can be produced
synthetically and also occur naturally
in certain proteins.
molecular modelling The use of
computer software to produce simulations of molecular structures. Various chemical drawing programs exist
to allow a graphic representation of
chemical formulas in two dimensions. More sophisticated threedimensional modelling programs
also exist. The information about the
molecule is stored in a data Üle giving 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-andstick, wireframe, etc.). The structure
can be rotated on the screen and, depending on the software, calculations
may be done on the molecule.
molecular orbital See orbital.
molecular-orbital theory A
method of computational chemistry
in which the electrons are not assigned to individual bonds between
atoms, but are treated as moving
under the inÛuence of the nuclei in
the whole molecule. The molecule
has a set of molecular orbitals (see orbital). The usual technique is to obtain 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 supramolecular chemistry.
molecular sieve Porous crystalline
substances, especially aluminosilicates (see zeolite), that can be dehydrated 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 substances is M n O.Al 2 O 3 .xSiO 2 .yH 2 O,
where M is a metal ion and n is twice
the reciprocal of its valency. Molecular sieves are used as drying agents
and in the separation and puriÜcation of Ûuids. They can also be loaded
with chemical substances, which remain 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 catalyst supports. They are also used as
the stationary phase in certain types
of *chromatography (molecular-sieve
chromatography).
molecular symmetry The 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 is
molecular knot
358
m
www.AzShimi.ir www.AzShimi.com