A system of physical chemistry - Index of


A system of physical chemistry - Index of


considering the circular vibration of diagram [c). Fig. 2. This point

has not yet been settled.

Molecular rotations are ascribed to collisions with other molecules.

This distinguishes molecular rotation from atomic vibration as far as

origin is concerned. Atomic vibration can indeed be affected by

collisions, but the origin of atomic vibration is more deeply seated, so

to speak, than that of molecular rotations. Presumably atomic vibra-

tion is set up as a result of absorption of radiant energy on the part of

the molecule. This might occur at a collision, but not necessarily so ;

unless the collision be very inelastic {cf. Chap. VI., the section dealing

with resonance and ionisation potentials).

When a molecule consists of more than one atom it seems reasonable

at first sight to ascribe to the molecule as a whole a certain amount

of rotational energy. The reason for the qualifying clause will not be

given now ; the point is taken up in Chap. IV. in connection with tlie

theory of the molecular heats of gases. As Kriiger has shown, in place

of true molecular rotations we may have to substitute another kind of

motion, namely, precessional vibrations} The possible kinds of rotations

and precessional vibrations in the case of a diatomic molecule

are illustrated in Fig. 3, diagrams {t>) to (d). It must be understood

that rotation or precessional vibration always refers to the molecule as

a whole. This is in contrast with the view taken of true atomic vibra-

tions which have been discussed.

As regards the rotation of a diatomic molecule the diagram [b).

Fig. 3, shows us that there are two degrees of freedom, i.e. there are two

co-ordinates at right angles defining the surface over which the rota-

tion of such a molecule can take place. The axis of rotation is at right

angles to the plane indicated in the diagram.

From the point of view of the internal molecular energy the significance

of the rotation of the molecule depends upon its moment of inertia,

i.e. the moment of inertia of each of the atoms with respect to the

axis of rotation and the number of revolutions v which the molecule

makes per second (as a result of collisions). The rotational energy is

given by the . expression 1/2 I . where I is

(27n')-', the moment of

inertia and v has already been defined.

' The term " precessional vibration " requires perhaps a word of explanation.

The type of motion represented by the term is shown in diagrams (c) and (

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