Dictionary of Chemistry [6th Ed.]

to describe unimolecular reactions.

The RRK theory, put forward by O. K.

Rice, H. C. Ramsperger, and indepen-

dently by L. S. Kassel in the late

1920s, assumes that a molecule is a

system of loosely coupled oscillators.

Regarding the coupling of the oscilla-

tors as loose enables calculations to

be made on the statistical distribu-

tion of energy freelyÛowing between

the vibrational modes. In RRK theory

the rate constant for the decomposi-

tion 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 individ-

ual vibrational modes of the system

are taken into account.

ricinA highly toxic protein present

in the castor oil plant (Ricinus commu-

nis), 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 mil-

ligram per kilogram of body weight.

ringA 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 chemi-

cal reactions in which one part of a

chain reacts with another to form a

ring, as in the formation of *lactams

and *lactones.

ring conformationsShapes that

can be taken by nonplanar rings and

can be interconverted by formal rota-

tions about single bonds. In cyclo-

hexane, for example, the most stable

conformation is the chairconforma-

tion 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 boatconformation, the

1 and 4 carbon atoms lie on the same

side of the plane. Other conformers

are the twistconformation and the

half chair conformation (see dia-

gram). The energy difference be-

tween 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 con-

formations. In aÜve-membered ring,

such as that of cyclopentane, the

twistconformation has three adja-

cent carbon atoms in a plane with

one of the carbons above this plane

and the other below it. The less sta-

ble envelopeconformation has four

atoms in a plane with one atom out

of the plane. In an eight-membered

ring, such as that of cyclooctane, the

tubconformation has four atoms cor-

responding 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 alter-

natively above and below the average

plane of the molecule.

Various names are given to posi-

tions attached to atoms in different

ring conformations. In the chair con-

formation 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-axialand pseudo-

equatorialare used. In the boat con-

formation of cyclohexane the bond

roughly parallel to the plane of four

atoms is the bowsprit, the other

bond being the Ûagpole.

RISSee resonance ionization spec-

troscopy.

ricin 462

r