effect. See also inverse compton effect.
computational chemistry The
use of computers in chemical research. With the increase in processing power of computers, calculations
on individual molecules and on
chemical systems have become important tools for research and industrial development. With simple
molecules, predictions can be made
about electronic structure and properties using *ab-initio calculations.
For more complex molecules *semiempirical calculations are used.
The Üeld has been particularly expanded by the *density-functional
method of treating large molecules
and by the availability of software for
analysing molecular behaviour and
structure. See also molecular modelling.
concentrated Describing a solution that has a relatively high concentration of solute.
concentration The quantity of dissolved substance per unit quantity of
a solution. Concentration is measured in various ways. The amount of
substance dissolved per unit volume
of the solution (symbol c) has units of
mol dm
–3 or mol l
–1
. It is now called
amount concentration (formerly molarity). The mass concentration (symbol ρ) is the mass of solute per unit
volume of solution. It has units of
kg dm
–3 , g cm
–3
, etc. The molality is
the amount of substance per unit
mass of solvent, commonly given in
units of mol kg
–1 . See also mole fraction.
concentration cell See cell.
concentration gradient (diffusion
gradient) The difference in concentration between a region of a solution or gas that has a high density of
particles and a region that has a relatively lower density of particles. By
random motion, particles will move
from the area of high concentration
towards the area of low concentration, by the process of *diffusion,
until the particles are evenly distributed in the solution or gas.
concerted reaction A type of reaction in which there is only one stage
rather than a series of steps. The S N 2
mechanism in *nucleophilic substitutions is an example. See also pericyclic reactions.
condensation The change of a
vapour or gas into a liquid. The
change of phase is accompanied by
the evolution of heat (see latent
heat).
condensation polymerization
See polymer.
condensation pump See diffusion pump.
condensation reaction A chemical reaction in which two molecules
combine to form a larger molecule
with elimination of a small molecule (e.g. H 2 O). See aldehydes; ketones.
condenser A device used to cool a
vapour to cause it to condense to a
liquid. See liebig condenser.
conducting polymer An organic
polymer that conducts electricity.
Conducting polymers have a crystalline structure in which chains of
conjugated unsaturated carbon–
carbon bonds are aligned. Examples
are polyacetylene and polypyrrole.
There has been considerable interest
in the development of such materials
because they would be cheaper and
lighter than metallic conductors.
They do, however, tend to be chemically unstable and, so far, no commercial conducting polymers have
been developed.
computational chemistry
138
c
www.AzShimi.ir www.AzShimi.com
computational chemistry The
use of computers in chemical research. With the increase in processing power of computers, calculations
on individual molecules and on
chemical systems have become important tools for research and industrial development. With simple
molecules, predictions can be made
about electronic structure and properties using *ab-initio calculations.
For more complex molecules *semiempirical calculations are used.
The Üeld has been particularly expanded by the *density-functional
method of treating large molecules
and by the availability of software for
analysing molecular behaviour and
structure. See also molecular modelling.
concentrated Describing a solution that has a relatively high concentration of solute.
concentration The quantity of dissolved substance per unit quantity of
a solution. Concentration is measured in various ways. The amount of
substance dissolved per unit volume
of the solution (symbol c) has units of
mol dm
–3 or mol l
–1
. It is now called
amount concentration (formerly molarity). The mass concentration (symbol ρ) is the mass of solute per unit
volume of solution. It has units of
kg dm
–3 , g cm
–3
, etc. The molality is
the amount of substance per unit
mass of solvent, commonly given in
units of mol kg
–1 . See also mole fraction.
concentration cell See cell.
concentration gradient (diffusion
gradient) The difference in concentration between a region of a solution or gas that has a high density of
particles and a region that has a relatively lower density of particles. By
random motion, particles will move
from the area of high concentration
towards the area of low concentration, by the process of *diffusion,
until the particles are evenly distributed in the solution or gas.
concerted reaction A type of reaction in which there is only one stage
rather than a series of steps. The S N 2
mechanism in *nucleophilic substitutions is an example. See also pericyclic reactions.
condensation The change of a
vapour or gas into a liquid. The
change of phase is accompanied by
the evolution of heat (see latent
heat).
condensation polymerization
See polymer.
condensation pump See diffusion pump.
condensation reaction A chemical reaction in which two molecules
combine to form a larger molecule
with elimination of a small molecule (e.g. H 2 O). See aldehydes; ketones.
condenser A device used to cool a
vapour to cause it to condense to a
liquid. See liebig condenser.
conducting polymer An organic
polymer that conducts electricity.
Conducting polymers have a crystalline structure in which chains of
conjugated unsaturated carbon–
carbon bonds are aligned. Examples
are polyacetylene and polypyrrole.
There has been considerable interest
in the development of such materials
because they would be cheaper and
lighter than metallic conductors.
They do, however, tend to be chemically unstable and, so far, no commercial conducting polymers have
been developed.
computational chemistry
138
c
www.AzShimi.ir www.AzShimi.com