William V. Steele

Thermodynamic properties of the methylpyridines. Part 1. Heat capacity measurements for 4-methylpyridine betweenT = 6.4 K andT = 18.6 K, resolution of low-temperature contributions, and reconciliation of calorimetrically and spectroscopically derived standard entropies

Heat capacities and enthalpy increments between the temperatures 6.4 K and 320 K were determined for 4-methylpyridine (Chemical Abstract registry number 108-89-4) by adiabatic calorimetry. Results for temperatures > 20 K were found to be in excellent accord with literature values; however, accord for T < 20 K was poor. The new experimental heat capacities for temperatures <20 K are used with results of inelastic neutron scattering (i.n.s.) studies from the literature to resolve the contributions to the heat capacities. Contributions arising from rotation of the methyl group are discussed in detail. Results are interpreted in terms of a three-fold barrier to rotation in accord with recent i.n.s. studies. Earlier suggestions in the literature of a six-fold potential are not supported. Revised thermodynamic functions for the condensed phases of 4-methylpyridine are derived. Revised thermodynamic functions are corroborated through accord between calorimetric and statistically derived standard entropies.