A high level computational study of the CH₄/CF₄ dimer: how does it compare with the CH₄/CH₄ and CF₄/CF₄ dimers?

Abstract

The interaction within the methane–methane (CH₄/CH₄), perfluoromethane–perfluoromethane (CF₄/CF₄) methane–perfluoromethane dimers (CH₄/CF₄) was calculated using the Hartree–Fock (HF) method, multiple orders of Møller–Plesset perturbation theory [MP2, MP3, MP4(DQ), MP4(SDQ), MP4(SDTQ)], and coupled cluster theory [CCSD, CCSD(T)], as well as the PW91, B97D, and M06-2X density functional theory (DFT) functionals. The basis sets of Dunning and coworkers (aug-cc-pVxZ, x = D, T, Q), Krishnan and coworkers [6-311++G(d,p), 6-311++G(2d,2p)], and Tsuzuki and coworkers [aug(df, pd)-6-311G(d,p)] were used. Basis set superposition error (BSSE) was corrected via the counterpoise method in all cases. Interaction energies obtained with the MP2 method do not fit with the experimental finding that the methane–perfluoromethane system phase separates at 94.5 K. It was not until the CCSD(T) method was considered that the interaction energy of the methane–perfluoromethane dimer (−0.69 kcal mol⁻¹) was found to be intermediate between the methane (−0.51 kcal mol⁻¹) and perfluoromethane (−0.78 kcal mol⁻¹) dimers. This suggests that a perfluoromethane molecule interacts preferentially with another perfluoromethane (by about 0.09 kcal mol⁻¹) than with a methane molecule. At temperatures much lower than the CH₄/CF₄ critical solution temperature of 94.5 K, this energy difference becomes significant and leads perfluoromethane molecules to associate with themselves, forming a phase separation. The DFT functionals yielded erratic results for the three dimers. Further development of DFT is needed in order to model dispersion interactions in hydrocarbon/perfluorocarbon systems.

Keywords:

• ab initio
• van der Waals interactions
• intermolecular forces
• fluorous molecules

Acknowledgements

The work described in this article was partially supported by NSF grant CHE 0518112 and by the School of Pharmacy at the University of Wisconsin-Madison.