Rotational Freedom of Ammonium Ions and Methyl Groups by Cross‐Section Measurements with Slow Neutrons Available to Purchase

The slope Δσ_s/Δλ_n of a plot of the scattering cross section per proton vs the wavelength of subthermal neutrons is shown to increase with increasing freedom of rotation of ammonium ions. For ammonium iodide, the slope increases from 6.2 b/Å—H in phase II to 11.2 b/Å—H in phase I, where the ammonium ions are rotating relatively freely. Measurements on five liquid methylbenzenes indicate greater hindrance to rotation of methyl groups in o‐xylene (slope=9.5±0.3 b/Å—H), than in toluene, m‐xylene, p‐xylene, and mesitylene (slopes=11.4±0.3 b/Å—H).

From an empirical plot of slopes vs barriers to rotation of ammonium ions in several ammonium salts, slopes of 5.5 and 5.8 b/Å—H for (NH₄)₂CrO₄ and NH₄CNS indicate a barrier of about 4 kcal mole⁻¹ for both these salts, while a high slope of 13 b/Å—H for NH₄ClO₄ predicts a barrier of only about 0.1 to 0.2 kcal mole⁻¹. Assuming a similar type curve for methyl groups, the barrier to rotation of the methyl groups in o‐xylene is estimated to be about 1 kcal mole⁻¹, and a value of 0.1 to 0.4 kcal mole⁻¹ is predicted for the other methylbenzenes.