to describe unimolecular reactions.
The RRK theory, put forward by O. K.
Rice, H. C. Ramsperger, and independently by L. S. Kassel in the late
1920s, assumes that a molecule is a
system of loosely coupled oscillators.
Regarding the coupling of the oscillators as loose enables calculations to
be made on the statistical distribution of energy freely Ûowing between
the vibrational modes. In RRK theory
the rate constant for the decomposition of a molecule increases with the
energy of the molecule. The RRK
theory has been applied with success
to some chemical reactions but is
limited in its predictive power. The
unsatisfactory features of the RRK
theory are improved by the *RRKM
theory (Rice–Ramsperger–Kassel–
Marcus theory), in which the individual vibrational modes of the system
are taken into account.
ricin A highly toxic protein present
in the castor oil plant (Ricinus communis), in particular in the seeds of the
plant (castor beans). It is probably the
most poisonous substance present in
plants – the fatal dose is about 1 milligram per kilogram of body weight.
ring A closed chain of atoms in a
molecule. In compounds, such as
naphthalene, in which two rings
share a common side, the rings are
fused rings. Ring closures are chemical reactions in which one part of a
chain reacts with another to form a
ring, as in the formation of *lactams
and *lactones.
ring conformations Shapes that
can be taken by nonplanar rings and
can be interconverted by formal rotations about single bonds. In cyclohexane, for example, the most stable
conformation is the chair conformation in which the 2, 3, 5 and 6 atoms
lie in a plane and the 1 and 4 carbon
atoms lie on opposite sides of the
plane. In this form, there is no ring
strain. In the boat conformation, the
1 and 4 carbon atoms lie on the same
side of the plane. Other conformers
are the twist conformation and the
half chair conformation (see diagram). The energy difference between the chair conformation and
the (least stable) half chair is 10.8
kcal/mol and the forms interconvert
quickly at normal temperatures
(about 99% of molecules are in the
chair form).
Other rings also have different conformations. In a Üve-membered ring,
such as that of cyclopentane, the
twist conformation has three adjacent carbon atoms in a plane with
one of the carbons above this plane
and the other below it. The less stable envelope conformation has four
atoms in a plane with one atom out
of the plane. In an eight-membered
ring, such as that of cyclooctane, the
tub conformation has four atoms corresponding to a pair of diametrically
opposite bonds in one plane, with
the other four atoms on the same
side of this plane. It is analogous to
the boat form of cyclohexane. The
crown conformation has atoms alternatively above and below the average
plane of the molecule.
Various names are given to positions attached to atoms in different
ring conformations. In the chair conformation of cyclohexane, an axial
bond is one that makes a large angle
with the average plane of the ring.
An equatorial bond is one that makes
a small angle. In a cyclopentane ring,
the terms pseudo-axial and pseudoequatorial are used. In the boat conformation of cyclohexane the bond
roughly parallel to the plane of four
atoms is the bowsprit, the other
bond being the Ûagpole.
RIS See resonance ionization spectroscopy.
ricin
462
r
www.AzShimi.ir www.AzShimi.com
The RRK theory, put forward by O. K.
Rice, H. C. Ramsperger, and independently by L. S. Kassel in the late
1920s, assumes that a molecule is a
system of loosely coupled oscillators.
Regarding the coupling of the oscillators as loose enables calculations to
be made on the statistical distribution of energy freely Ûowing between
the vibrational modes. In RRK theory
the rate constant for the decomposition of a molecule increases with the
energy of the molecule. The RRK
theory has been applied with success
to some chemical reactions but is
limited in its predictive power. The
unsatisfactory features of the RRK
theory are improved by the *RRKM
theory (Rice–Ramsperger–Kassel–
Marcus theory), in which the individual vibrational modes of the system
are taken into account.
ricin A highly toxic protein present
in the castor oil plant (Ricinus communis), in particular in the seeds of the
plant (castor beans). It is probably the
most poisonous substance present in
plants – the fatal dose is about 1 milligram per kilogram of body weight.
ring A closed chain of atoms in a
molecule. In compounds, such as
naphthalene, in which two rings
share a common side, the rings are
fused rings. Ring closures are chemical reactions in which one part of a
chain reacts with another to form a
ring, as in the formation of *lactams
and *lactones.
ring conformations Shapes that
can be taken by nonplanar rings and
can be interconverted by formal rotations about single bonds. In cyclohexane, for example, the most stable
conformation is the chair conformation in which the 2, 3, 5 and 6 atoms
lie in a plane and the 1 and 4 carbon
atoms lie on opposite sides of the
plane. In this form, there is no ring
strain. In the boat conformation, the
1 and 4 carbon atoms lie on the same
side of the plane. Other conformers
are the twist conformation and the
half chair conformation (see diagram). The energy difference between the chair conformation and
the (least stable) half chair is 10.8
kcal/mol and the forms interconvert
quickly at normal temperatures
(about 99% of molecules are in the
chair form).
Other rings also have different conformations. In a Üve-membered ring,
such as that of cyclopentane, the
twist conformation has three adjacent carbon atoms in a plane with
one of the carbons above this plane
and the other below it. The less stable envelope conformation has four
atoms in a plane with one atom out
of the plane. In an eight-membered
ring, such as that of cyclooctane, the
tub conformation has four atoms corresponding to a pair of diametrically
opposite bonds in one plane, with
the other four atoms on the same
side of this plane. It is analogous to
the boat form of cyclohexane. The
crown conformation has atoms alternatively above and below the average
plane of the molecule.
Various names are given to positions attached to atoms in different
ring conformations. In the chair conformation of cyclohexane, an axial
bond is one that makes a large angle
with the average plane of the ring.
An equatorial bond is one that makes
a small angle. In a cyclopentane ring,
the terms pseudo-axial and pseudoequatorial are used. In the boat conformation of cyclohexane the bond
roughly parallel to the plane of four
atoms is the bowsprit, the other
bond being the Ûagpole.
RIS See resonance ionization spectroscopy.
ricin
462
r
www.AzShimi.ir www.AzShimi.com