readily than in one-dimensional
NMR, particularly for complex spec-
tra.
corrosion Chemical or electro-
chemical attack on the surface of a
metal. See also electrolytic corro-
sion; rusting.corundumA mineral form of alu-
minium 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 el-
ements gives rise to a variety of
colours. *Ruby is a red variety con-
taining chromium; *sapphire is a
blue variety containing iron and
titanium. Corundum occurs as a
rock-forming mineral in both meta-
morphic and igneous rocks. It is
chemically resistant to weathering
processes and so also occurs in allu-
vial (placer) deposits. The second
hardest mineral after diamond (it has
a hardness of 9 on the Mohs’ scale), it
is used as an abrasive.COSYSee correlation spec-
troscopy.COTSee cyclo-octatetraene.Cotton effect The wavelength de-
pendence of the optical rotary disper-
sion curve or the *circular dichroism
curve in the neighbourhood of an ab-
sorption band, both having charac-
teristic shapes. If the wavelength is
decreased, the rotation angle in-
creases 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 λis the wave-length; this is called the negative
Cotton effect. These effects occur for
coloured substances and for colour-
less substances with bands in the ul-
traviolet. It is named after the French
physicist Aimé Cotton (1859–1951).coulombSymbol 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 explosionThe 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 mol-
ecules is produced byÜrst adding
electrons, accelerating the ions in an
electricÜeld, 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 spa-
tial arrangement of the original mol-
ecule to be derived.coumarin(1,2-benzopyrone) A
pleasant-smelling colourless crys-
talline compound, C 9 H 6 O 2 , m.p. 70°C.
It occurs naturally in tonka (or ton-
quin) beans, and is synthesized from
salicylaldehyde. It forms coumarinic
acidon hydrolysis with sodium hy-
droxide. Coumarin is used in makingcorrosion 146c