Rotation

In classical mechanics, the energy of an object that rotates around some defined axis is given with E = ½ Iω2. Here, ω is the angular velocity in the unit of radian per second. In turn, I = mr2 is the moment of inertia I which is dependent on mass m and distance r from the axis. Having replaced m for I and v for ω, the analogy to kinetic energy E = ½ mv2 gets obvious. An object that rotates freely around three axes (A, B, C), i.e. so-called principal axes of rotation, therefore has an rotational energy of:

E = ½ IAωA2 +½ IBωB2+ ½ ICωC2

with Ji = Ii ⋅ ωi  (J = angular momentum) we obtain:

E = JA2/(2IA) + JB2/(2IB) + JC2/(2IC)

Some formulas to calculate a moment of inertia are found here. To begin with, we assume the shape of the molecule not to be dependent on the rotation, i.e. the length of bonds not to be affected by centrifugal forces. The molecule is thus regarded as rigid rotor in contrast to an elastic rotor. We have to deal with the following cases
 

Spherical top   IA = IB = IC = I  e.g.
  CH4, CCl4, SF6
Symmetric top  IA = IB = I and IC = I||
          I||>I oblate
          I||<I prolate 
e.g.
  Benzene 
  CH3I
Linear rotor
(a special case of symmetric top)
IA = 0, IB = IC all diatomic molecules, e.g.
  NO, C2N2, CO2
Asymmetric top  IA ≠ IB ≠ IC,    IA ≠ IC
IA < IB < IC
e.g.
  H2O


  Contents