How much does a methyl rotor (internal rotation) contribute to the entropy?
The contribution to the entropy for a methyl rotor depends on the reduced moment of inertia and the barrier to internal rotation. Using ethane (CH3-CH3) as an example we can compute the contribution to entropy of the methyl internal rotation. We will use results calculated at HF/6-31G* and a temperature of 298.15K and determine the contribution to the entropy three different ways: 1. Treating the methyl internal rotation as a harmonic oscillator we have a scaled frequency of 292.6 cm-1. This leads to a contribution to the entropy of 6.1 J K-1 mol-1. 2. Treating the methyl rotation as a free internal rotor with a reduced moment of inertia of 1.55 amu 2 leads to a contribution to the entropy of 12.0 J K-1 mol-1. 3.
