Reconciliation of Experimental and Computed Thermodynamic Properties for Methyl-Substituted 3‑Ring Aromatics. Part 1: 9‑Methylanthracene

Measurements leading to the calculation of thermodynamic properties in the ideal-gas state for 9-methylanthracene (Chemical Abstracts registry number 779-02-2) are reported. Experimental methods were adiabatic heat-capacity calorimetry, differential scanning calorimetry, comparative ebulliometry, inclined-piston manometry, vibrating-tube densitometry, and oxygen bomb calorimetry. The critical temperature, pressure, and density were estimated based on these measurements with well-established correlations. Molar thermodynamic functions for the condensed and ideal-gas states were derived from the experimental results. Statistical calculations were performed based on molecular geometry optimization and vibrational frequencies using B3LYP hybrid density functional theory with def2-QZVPD basis set. Excellent accord between computed and experimentally derived ideal-gas entropies is shown, once account is taken of coupled low-frequency vibrational modes reported in the literature. The enthalpy of formation for 9-methylanthracene in the gas phase was computed with an atom-equivalent based protocol described recently, and excellent agreement with the experimental value is seen. All experimental results are compared with property values reported in the literature. Multiple large inconsistencies in literature property values are resolved here.