readily than in one-dimensional
NMR, particularly for complex spectra.
corrosion Chemical or electrochemical attack on the surface of a
metal. See also electrolytic corrosion; rusting.
corundum A mineral form of aluminium oxide, Al 2 O 3 . It crystallizes
in the trigonal system and occurs as
well-developed hexagonal crystals. It
is colourless and transparent when
pure but the presence of other elements gives rise to a variety of
colours. *Ruby is a red variety containing chromium; *sapphire is a
blue variety containing iron and
titanium. Corundum occurs as a
rock-forming mineral in both metamorphic and igneous rocks. It is
chemically resistant to weathering
processes and so also occurs in alluvial (placer) deposits. The second
hardest mineral after diamond (it has
a hardness of 9 on the Mohs’ scale), it
is used as an abrasive.
COSY See correlation spectroscopy.
COT See cyclo-octatetraene.
Cotton effect The wavelength dependence of the optical rotary dispersion curve or the *circular dichroism
curve in the neighbourhood of an absorption band, both having characteristic shapes. If the wavelength is
decreased, the rotation angle increases until it reaches a maximum
and then decreases, passing through
zero at the wavelength at which the
maximum of absorption occurs. As
the wavelength is decreased further
the angle becomes negative, until it
reaches a minimum after which it
rises again. This pattern is called the
positive Cotton effect. A mirror
image of this pattern can also occur
about the λ-axis, where λ is the wavelength; this is called the negative
Cotton effect. These effects occur for
coloured substances and for colourless substances with bands in the ultraviolet. It is named after the French
physicist Aimé Cotton (1859–1951).
coulomb Symbol C. The *SI unit of
electric charge. It is equal to the
charge transferred by a current of
one ampere in one second. The unit
is named after Charles de Coulomb.
Coulomb explosion The sudden
disruption of a molecule from which
the electrons have been stripped to
leave only the nuclei, which repel
each other because of their electric
charge. The technique of Coulomb
explosion imaging uses this effect to
investigate the shape of molecules. A
beam of high-energy neutral molecules is produced by Ürst adding
electrons, accelerating the ions in an
electric Üeld, and then removing the
electrons. The beam collides with a
thin metal foil having a thickness of
about 30 atoms. As the molecules
pass through this foil their electrons
are scattered and only the nuclei of
the molecules emerge. The process
occurs within a very short period of
time, shorter than the time required
for a complete molecular vibration,
and consequently the nuclei retain
the molecular shape until they are
suddenly repulsed by the like
charges. The nuclei then impinge on
a detector that records their velocity
and direction, thus enabling the spatial arrangement of the original molecule to be derived.
coumarin (1,2-benzopyrone) A
pleasant-smelling colourless crystalline compound, C 9 H 6 O 2 , m.p. 70°C.
It occurs naturally in tonka (or tonquin) beans, and is synthesized from
salicylaldehyde. It forms coumarinic
acid on hydrolysis with sodium hydroxide. Coumarin is used in making
corrosion
146
c
www.AzShimi.ir www.AzShimi.com
NMR, particularly for complex spectra.
corrosion Chemical or electrochemical attack on the surface of a
metal. See also electrolytic corrosion; rusting.
corundum A mineral form of aluminium oxide, Al 2 O 3 . It crystallizes
in the trigonal system and occurs as
well-developed hexagonal crystals. It
is colourless and transparent when
pure but the presence of other elements gives rise to a variety of
colours. *Ruby is a red variety containing chromium; *sapphire is a
blue variety containing iron and
titanium. Corundum occurs as a
rock-forming mineral in both metamorphic and igneous rocks. It is
chemically resistant to weathering
processes and so also occurs in alluvial (placer) deposits. The second
hardest mineral after diamond (it has
a hardness of 9 on the Mohs’ scale), it
is used as an abrasive.
COSY See correlation spectroscopy.
COT See cyclo-octatetraene.
Cotton effect The wavelength dependence of the optical rotary dispersion curve or the *circular dichroism
curve in the neighbourhood of an absorption band, both having characteristic shapes. If the wavelength is
decreased, the rotation angle increases until it reaches a maximum
and then decreases, passing through
zero at the wavelength at which the
maximum of absorption occurs. As
the wavelength is decreased further
the angle becomes negative, until it
reaches a minimum after which it
rises again. This pattern is called the
positive Cotton effect. A mirror
image of this pattern can also occur
about the λ-axis, where λ is the wavelength; this is called the negative
Cotton effect. These effects occur for
coloured substances and for colourless substances with bands in the ultraviolet. It is named after the French
physicist Aimé Cotton (1859–1951).
coulomb Symbol C. The *SI unit of
electric charge. It is equal to the
charge transferred by a current of
one ampere in one second. The unit
is named after Charles de Coulomb.
Coulomb explosion The sudden
disruption of a molecule from which
the electrons have been stripped to
leave only the nuclei, which repel
each other because of their electric
charge. The technique of Coulomb
explosion imaging uses this effect to
investigate the shape of molecules. A
beam of high-energy neutral molecules is produced by Ürst adding
electrons, accelerating the ions in an
electric Üeld, and then removing the
electrons. The beam collides with a
thin metal foil having a thickness of
about 30 atoms. As the molecules
pass through this foil their electrons
are scattered and only the nuclei of
the molecules emerge. The process
occurs within a very short period of
time, shorter than the time required
for a complete molecular vibration,
and consequently the nuclei retain
the molecular shape until they are
suddenly repulsed by the like
charges. The nuclei then impinge on
a detector that records their velocity
and direction, thus enabling the spatial arrangement of the original molecule to be derived.
coumarin (1,2-benzopyrone) A
pleasant-smelling colourless crystalline compound, C 9 H 6 O 2 , m.p. 70°C.
It occurs naturally in tonka (or tonquin) beans, and is synthesized from
salicylaldehyde. It forms coumarinic
acid on hydrolysis with sodium hydroxide. Coumarin is used in making
corrosion
146
c
www.AzShimi.ir www.AzShimi.com