Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
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You are here: Resources > Tutorials > Entropy > Methyl rotor entropy

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.

At low temperatures the harmonic oscillator is a good approximation, and at very high temperatures the free rotor is a better approximation.

The reduced moment of inertia for a methyl group varies depending on the rest of the molecule. In the ethane example it is 1.55 amu Ų. The moment of inertia for a methyl attached to an infinitely heavy mass will be about 3 amu Ų . For example, in (E)-1,3-pentadiene the methyl rotor reduced moment of inertia is 2.987 amu Ų

See also the discussion on internal rotation in section I.D. A brief description of the thermochemical quantities and methods.