Ab initio studies of the van der Waals complex CH4–O2. CH···O and CX···O interactions in halomethane XnCH4−n–O2 complexes (X = F, Cl; n = 1, 2, 3)

Using the explicitly correlated CCSD(T)-F12b method with cc-pVXZ-F12 basis sets up to X = 4, geometries for various configurations of the triplet CH4–O2 van der Waals complex were optimized. Counterpoise-corrected geometries and energies were obtained, and extrapolations to the complete basis set (CBS) limit were performed. Accordingly, the most stable isomer of the complex has O2 T-shaped facing CH3 (De = 183 cm−1, CBS value), followed by O2 T-shaped in plane with CH2 (157 cm−1), being more stable than lying perpendicular to this plane (142 cm−1). The linear HCH3–OO structure is similarly stable (138 cm−1), whereas linear H2CH2–OO is less stable (115 cm−1). Structures with O2 facing CH have the lowest stability. Harmonic frequencies point to a stable HCH3–O2 complex, with CH4 and O2 frequencies slightly red shifted. Binding energies of the various structures can be related to the number of weak intermolecular C–H···O interactions. Comparisons with the literature results for CH4–N2, CH4–NO, and CH4–CO complexes are made. Complexes of fluoromethanes and chloromethanes with O2 in linear C···OO arrangements were studied at the cc-pVDZ-F12 level. Any substitution increases the stability of the complex over the methane values. Complexes of CHF3–OO and CHCl3–OO, with three halogens facing O2, are most stable.