1969), who pioneered techniques for
the safe industrial use of acetylene.
reptation A motion describing the
dynamics of a polymer in a highly
entangled state, such as a network.
Regarding the entangled state as a
set of chains between crosslinks it is
possible to regard the chain as being
in a ‘tube’, with the tube being
formed by topological constraints.
The chain is longer than the tube so
that the ‘slack’ of the chain moves
through the tube, which causes the
tube itself to change with time. This
motion was called reptation (from
the Latin reptare, to creep), by the
French physicist P. G. de Gennes,
who postulated it in 1971. Many experiments indicate that reptation
dominates the dynamics of polymer
chains when they are entangled.
resin A synthetic or naturally occurring *polymer. Synthetic resins are
used in making *plastics. Natural
resins are acidic chemicals secreted
by many trees (especially conifers)
into ducts or canals. They are found
either as brittle glassy substances or
dissolved in essential oils. Their functions are probably similar to those of
gums and mucilages.
resolution The process of separating a racemic mixture into its optically active constituents. In some
cases the crystals of the two forms
have a different appearance, and the
separation can be done by hand. In
general, however, physical methods
(distillation, crystallization, etc.) cannot be used because the optical isomers have identical physical
properties. The most common technique is to react the mixture with a
compound that is itself optically active, and then separate the two. For
instance, a racemic mixture of l-A
and d-A reacted with l-B, gives two
compounds AB that are not optical
isomers but diastereoisomers and can
be separated and reconverted into
the pure l-A and d-A. Biological techniques using bacteria that convert
one form but not the other can also
be used.
resonance The representation of
the structure of a molecule by two or
more conventional formulae. For example, the formula of methanal can
be represented by a covalent structure H 2 C=O, in which there is a double bond in the carbonyl group. It is
known that in such compounds the
oxygen has some negative charge
and the carbon some positive charge.
The true bonding in the molecule is
somewhere between H 2 C=O and the
ionic compound H 2 C
+
O
– . It is said to
be a resonance hybrid of the two, indicated by
H 2 C=O ↔ H 2 C
+ O
–
The two possible structures are called
canonical forms, and they need not
contribute equally to the actual form.
Note that the double-headed arrow
does not imply that the two forms
are in equilibrium.
resonance effect See electronic
effects.
resonance ionization spectroscopy (RIS) A spectroscopic technique in which single atoms in a gas
are detected using a laser to ionize
that atom. A sample containing the
atoms to be excited is subjected to
light from a laser, tuned so that only
that type of atom is excited by the
light. If the frequency of light at
which the atom is excited is ν, the
atoms in the excited state can be ionized if the ionization potential of the
atom is less than 2ν. In contrast to
other techniques of ionization, this
type of ionization only occurs for
atoms that are ‘in tune’ with the frequency of light. Because RIS is very
selective in determining which atom
459
resonance ionization spectroscopy
r
www.AzShimi.ir www.AzShimi.com
the safe industrial use of acetylene.
reptation A motion describing the
dynamics of a polymer in a highly
entangled state, such as a network.
Regarding the entangled state as a
set of chains between crosslinks it is
possible to regard the chain as being
in a ‘tube’, with the tube being
formed by topological constraints.
The chain is longer than the tube so
that the ‘slack’ of the chain moves
through the tube, which causes the
tube itself to change with time. This
motion was called reptation (from
the Latin reptare, to creep), by the
French physicist P. G. de Gennes,
who postulated it in 1971. Many experiments indicate that reptation
dominates the dynamics of polymer
chains when they are entangled.
resin A synthetic or naturally occurring *polymer. Synthetic resins are
used in making *plastics. Natural
resins are acidic chemicals secreted
by many trees (especially conifers)
into ducts or canals. They are found
either as brittle glassy substances or
dissolved in essential oils. Their functions are probably similar to those of
gums and mucilages.
resolution The process of separating a racemic mixture into its optically active constituents. In some
cases the crystals of the two forms
have a different appearance, and the
separation can be done by hand. In
general, however, physical methods
(distillation, crystallization, etc.) cannot be used because the optical isomers have identical physical
properties. The most common technique is to react the mixture with a
compound that is itself optically active, and then separate the two. For
instance, a racemic mixture of l-A
and d-A reacted with l-B, gives two
compounds AB that are not optical
isomers but diastereoisomers and can
be separated and reconverted into
the pure l-A and d-A. Biological techniques using bacteria that convert
one form but not the other can also
be used.
resonance The representation of
the structure of a molecule by two or
more conventional formulae. For example, the formula of methanal can
be represented by a covalent structure H 2 C=O, in which there is a double bond in the carbonyl group. It is
known that in such compounds the
oxygen has some negative charge
and the carbon some positive charge.
The true bonding in the molecule is
somewhere between H 2 C=O and the
ionic compound H 2 C
+
O
– . It is said to
be a resonance hybrid of the two, indicated by
H 2 C=O ↔ H 2 C
+ O
–
The two possible structures are called
canonical forms, and they need not
contribute equally to the actual form.
Note that the double-headed arrow
does not imply that the two forms
are in equilibrium.
resonance effect See electronic
effects.
resonance ionization spectroscopy (RIS) A spectroscopic technique in which single atoms in a gas
are detected using a laser to ionize
that atom. A sample containing the
atoms to be excited is subjected to
light from a laser, tuned so that only
that type of atom is excited by the
light. If the frequency of light at
which the atom is excited is ν, the
atoms in the excited state can be ionized if the ionization potential of the
atom is less than 2ν. In contrast to
other techniques of ionization, this
type of ionization only occurs for
atoms that are ‘in tune’ with the frequency of light. Because RIS is very
selective in determining which atom
459
resonance ionization spectroscopy
r
www.AzShimi.ir www.AzShimi.com