inductive effect The effect of a
group or atom of a compound in
pulling electrons towards itself or in
pushing them away. Inductive effects
can be used to explain some aspects
of organic reactions. For instance,
electron-withdrawing groups, such as
–NO 2 , –CN, –CHO, –COOH, and the
halogens substituted on a benzene
ring, reduce the electron density on
the ring and decrease its susceptibility to further (electrophilic) substitution. Electron-releasing groups, such
as –OH, –NH 2 , –OCH 3 , and –CH 3 ,
have the opposite effect. See also electronic effects.
industrial fermenter See bioreactor.
inelastic neutron scattering A
technique for investigating the motion of molecules by scattering neutrons. The neutrons pick up or lose
energy as they move through a sample of a liquid. The analysis of neutron scattering experiments enables
information to be obtained about the
liquid.
inert gases See noble gases.
inert-pair effect An effect seen especially in groups 13 and 14 of the
periodic table, in which the heavier
elements in the group tend to form
compounds with a valency two lower
than the expected group valency. It is
used to account for the existence of
thallium(I) compounds in group 13
and lead(II) in group 14. In forming
compounds, elements in these
groups promote an electron from a
Ülled s-level state to an empty p-level.
The energy required for this is more
than compensated for by the extra
energy gain in forming two more
bonds. For the heavier elements, the
bond strengths or lattice energies
in the compounds are lower than
those of the lighter elements. Consequently the energy compensation is
less important and the lower valence
states become favoured.
infrared chemiluminescence A
technique for studying chemical reaction mechanisms by measuring
and analysing weak infrared emissions from product molecules formed
in certain chemical reactions. If products are formed with excess energy,
this appears as excited vibrational
states of the molecules, which decay
with emission of infrared radiation.
Spectroscopic investigation of this radiation gives information about the
states in which the product molecules were formed.
infrared radiation (IR) Electromagnetic radiation with wavelengths
longer than that of red light but
shorter than radiowaves, i.e. radiation in the wavelength range 0.7 micrometre to 1 millimetre. It was
discovered in 1800 by William Herschel (1738–1822) in the sun’s spectrum. The natural vibrational
frequencies of atoms and molecules
and the rotational frequencies of
some gaseous molecules fall in the
infrared region of the electromagnetic spectrum. The infrared absorption spectrum of a molecule is highly
characteristic of it and the spectrum
can therefore be used for molecular
identiÜcation. Glass is opaque to infrared radiation of wavelength
greater than 2 micrometres and
other materials, such as germanium,
quartz, and polyethylene, have to be
used to make lenses and prisms. Photographic Ülm can be made sensitive
to infrared up to about 1.2 µm.
infrared spectroscopy (IR spectroscopy) A technique for chemical
analysis and the determination of
structure. It is based on the principles that molecular vibrations occur
in the infrared region of the electromagnetic spectrum and functional
groups have characteristic absorption
inductive effect
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group or atom of a compound in
pulling electrons towards itself or in
pushing them away. Inductive effects
can be used to explain some aspects
of organic reactions. For instance,
electron-withdrawing groups, such as
–NO 2 , –CN, –CHO, –COOH, and the
halogens substituted on a benzene
ring, reduce the electron density on
the ring and decrease its susceptibility to further (electrophilic) substitution. Electron-releasing groups, such
as –OH, –NH 2 , –OCH 3 , and –CH 3 ,
have the opposite effect. See also electronic effects.
industrial fermenter See bioreactor.
inelastic neutron scattering A
technique for investigating the motion of molecules by scattering neutrons. The neutrons pick up or lose
energy as they move through a sample of a liquid. The analysis of neutron scattering experiments enables
information to be obtained about the
liquid.
inert gases See noble gases.
inert-pair effect An effect seen especially in groups 13 and 14 of the
periodic table, in which the heavier
elements in the group tend to form
compounds with a valency two lower
than the expected group valency. It is
used to account for the existence of
thallium(I) compounds in group 13
and lead(II) in group 14. In forming
compounds, elements in these
groups promote an electron from a
Ülled s-level state to an empty p-level.
The energy required for this is more
than compensated for by the extra
energy gain in forming two more
bonds. For the heavier elements, the
bond strengths or lattice energies
in the compounds are lower than
those of the lighter elements. Consequently the energy compensation is
less important and the lower valence
states become favoured.
infrared chemiluminescence A
technique for studying chemical reaction mechanisms by measuring
and analysing weak infrared emissions from product molecules formed
in certain chemical reactions. If products are formed with excess energy,
this appears as excited vibrational
states of the molecules, which decay
with emission of infrared radiation.
Spectroscopic investigation of this radiation gives information about the
states in which the product molecules were formed.
infrared radiation (IR) Electromagnetic radiation with wavelengths
longer than that of red light but
shorter than radiowaves, i.e. radiation in the wavelength range 0.7 micrometre to 1 millimetre. It was
discovered in 1800 by William Herschel (1738–1822) in the sun’s spectrum. The natural vibrational
frequencies of atoms and molecules
and the rotational frequencies of
some gaseous molecules fall in the
infrared region of the electromagnetic spectrum. The infrared absorption spectrum of a molecule is highly
characteristic of it and the spectrum
can therefore be used for molecular
identiÜcation. Glass is opaque to infrared radiation of wavelength
greater than 2 micrometres and
other materials, such as germanium,
quartz, and polyethylene, have to be
used to make lenses and prisms. Photographic Ülm can be made sensitive
to infrared up to about 1.2 µm.
infrared spectroscopy (IR spectroscopy) A technique for chemical
analysis and the determination of
structure. It is based on the principles that molecular vibrations occur
in the infrared region of the electromagnetic spectrum and functional
groups have characteristic absorption
inductive effect
284
i
www.AzShimi.ir www.AzShimi.com