Active Thermochemical Tables: the thermophysical and thermochemical properties of methyl, CH₃, and methylene, CH₂, corrected for nonrigid rotor and anharmonic oscillator effects

Abstract

The thermophysical properties (isobaric heat capacity, entropy, enthalpy increment) of two prominent radicals, methyl, CH₃, and methylene, CH₂, were computed using the Nonrigid Rotor Anharmonic Oscillator (NRRAO) approach and compared to their RRHO counterparts, demonstrating significant differences between the results from the two approaches. Methylene presents a typical case in which the NRRAO thermophysical properties have significantly higher values than their RRHO counterparts at higher temperatures. In the case of methyl, the positive anharmonicity of the umbrella motion causes an opposite effect, and the NRRAO corrected thermophysical properties have lower values than their RRHO counterparts. The NRRAO corrected thermophysical properties, in turn, affect the resulting thermochemical properties. Two reactions important in combustion modelling were tested: the recombination of methyl radical with hydrogen atoms to form methane, and the recombination of two methyl radicals to form ethane. The related NRRAO equilibrium constants differ significantly from their RRHO analogs, and the consequences for chemical modelling are discussed. Also reported are the most current ATcT enthalpies of formation for CH_n (n = 4-0) species and for C₂H₆, together with the tightly related sequential bond dissociation enthalpies along with the CH_n series.

GRAPHICAL ABSTRACT

KEYWORDS:

• Thermochemistry
• thermophysical properties
• equilibrium constant
• heat of formation

Acknowledgements

The authors express their gratitude to John F. Stanton (University of Florida), who is the driving force behind our periodic ATcT Task Force One virtual meetings, for his enthusiastic support of the ATcT project and for numerous fascinating discussions related to high-accuracy computational thermochemistry. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘Argonne’). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. The (additional) data that support the findings of this study are also available upon request from the corresponding author. The supplementary material contains spectroscopic constants for CH₃ and CH₂ that were used to obtain NRRAO properties, extended tables of thermophysical and thermochemical properties for methyl, equilibrated methylene, triplet methylene, singlet methylene, methane, and ethane, extended tables of thermochemical properties (including the equilibrium constant) for recombination of methyl with a hydrogen atom to form methane and for recombination of methyl radicals to form ethane, visual comparison of NRRAO and RRHO properties for methane and ethane, and NRRAO NASA polynomials (both CHEMKIN style and new style) for ethane, methane, methyl, equilibrated methylene, triplet methylene, singlet methylene, and hydrogen atom.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division, under grant number DE-AC02-06CH11357 (Argonne National Laboratory), through the Gas-Phase Chemical Physics Program (BR) and the Computational Chemical Sciences Program (DHB).