Reconciliation of calorimetrically and spectroscopically derived standard entropies for the six dimethylpyridines between the temperatures 250 K and 650 K : a stringent test of thermodynamic consistency

Abstract Reconciliation of standard entropies Δ 0 T S m °(cal) derived from calorimetric and thermophysical property studies with standard entropies Δ 0 T S m °(stat) derived with assigned vibrational spectra and the methods of statistical mechanics is used to demonstrate consistency between thermophysical properties for the six dimethylpyridines (Chemical Abstracts registry numbers: 2,3-dimethylpyridine, 583-61-9; 2,4-dimethylpyridine, 108-47-4; 2,5-dimethylpyridine, 589-93-5; 2,6-dimethylpyridine, 108-48-5; 3,4-dimethylpyridine, 583-58-4; 3,5-dimethylpyridine, 591-22-0). Properties considered include the critical temperature, critical pressure, vapor pressure, heat capacities of the solid and liquid, second and third virial coefficients, enthalpies of vaporization, vibrational assignment, and methyl group rotational barrier. The temperature-dependent properties are shown to be consistent over the entire temperature range from near T =250 K to T =650 K (≈0.95· T c , where T c denotes the critical temperature). The analyses validate the methods and results reported previously, which provided the information required to derive the temperature-dependent properties to near T c , i . e . into the temperature and pressure range typical of petroleum processing conditions. Sensitivities of Δ 0 T S m °(stat) to errors in the vibrational assignment and to the size of methyl group rotational barriers are discussed. Vibrational assignments for vapor-phase fundamentals at low wave number for 2,3-dimethylpyridine and 3,4-dimethylpyridine are shown to be in error and are corrected.