Thermodynamic properties of the methylpyridines. Part 1. Heat capacity measurements for 4-methylpyridine between T = 6.4 K and T = 18.6 K, resolutionof 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 numb er 108-89-4) by adiabatic calorimetry. Results for temperatures greater tha n or equal to 20 K were found to be in excellent accord with literature val ues; however, accord for < 20 K was poor. The new experimental heat capacit ies for temperatures <20 K are used with results of inelastic neutron scatt ering (i.n.s.) studies from the literature to resolve the contributions to the heat capacities. Contributions arising from rotation of the methyl grou p 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 suggesti ons in the literature of a six-fold potential are not supported. Revised th ermodynamic functions for the condensed phases of 4-methylpyridine are deri ved. Revised thermodynamic functions are corroborated through accord betwee n calorimetric and statistically derived standard entropies. (C) 1999 Acade mic Press.