Thermodynamic properties of pyrrole, 1-methylpyrrole, 2,4-dimethylpyrrole, and 2,5-dimethylpyrrole: Experimental and computational results

- Heat capacities were measured for pyrrole and three methylpyrroles.
- Densities were measured from 323 K to ∼475 K for all compounds.
- The critical temperature was determined experimentally for all compounds.
- New high-precision vapor pressures are reported for 2,4- and 2,5-dimethylpyrrole.
- Computations and experiments are in excellent accord.

New measurements of critical temperature, saturated liquid heat capacity from temperature T ≈ 315 K to T ≈ 550 K, and saturated liquid density from T = 323 K to T ≈ 475 K are reported for pyrrole (Chemical Abstracts registry number [109-97-7]), 1-methylpyrrole [96-54-8], 2,4-dimethylpyrrole [625-82-1], and 2,5-dimethylpyrrole [625-84-3]. New measurements of vapor pressure are reported for 2,4-dimethylpyrrole {338 < (T/K) < 477} and 2,5-dimethylpyrrole {341 < (T/K) < 479}, as well as the enthalpy of combustion determined with oxygen-bomb calorimetry for 2,4-dimethylpyrrole. These new measurements are combined with literature values to calculate thermodynamic properties in the ideal-gas state for extended ranges of temperature for all compounds: pyrrole {298 < (T/K) < 550}, 1-methylpyrrole {298 < (T/K) < 530}, 2,4-dimethylpyrrole {298 < (T/K) < 600}, and 2,5-dimethylpyrrole {298 < (T/K) < 560}. Molar thermodynamic functions (enthalpies, entropies, and Gibbs energies) for the liquid and ideal-gas states were derived from the experimental studies at selected temperatures. Statistical calculations were performed with optimized geometries, scaled vibrational frequencies, and methyl rotational potentials performed using B3LYP hybrid density functional theory with the def2-TZVPPD basis set. Methyl torsional barriers were evaluated at the DLPNO-CCSD(T)/def2-QZVP level of theory using the B3LYP/def2-TZVPPD geometries. Computed ideal-gas properties are shown to be in excellent accord with ideal-gas entropies derived from thermophysical property measurements, as well as with reported experimental heat capacities for the ideal-gas state. All experimental results are compared with property values available in the literature.