High-Level Ab initio Calculations of Interaction Energies of C₂H₄-CH₄ and C₂H₆-CH₄ Dimers: A Medel Study of CH/pi Interaction

Intermolecular interaction energies of five C₂H₄-CH₄ dimers and two C₂H₆-CH₄ dimers were calculated. Dunning's correlation consistent basis sets (cc-pVXZ, X = D, T, Q, and 5) were used to estimate the MP2 interaction energies of the dimers at the basis set limit. The dimer in which the C--H bond of CH₄ points to the C=C bond of C₂H₄ has slightly larger bonding energy than the other four C₂H₄-CH₄ dimers. The estimated CCSD(T) interaction energy of this dimer at the basis set limit is -0.49 kcal/mol, which is about 10% of the bonding energy of water dimer. The large correlation interaction energy (-0.86 kcal/mol), calculated as the difference between the HF and post-SCF interaction energies, suggests that the dispersion interaction is significantly important for the attraction between C₂H₄ and CH₄. The analysis of the electrostatic interaction using distributed multipoles shows that the attractive electrostatic interaction (-0.24 kcal/mol) is playing an important role to stabilize this dimer. On the other hand, the electrostatic interaction is negligible for the C₂H₆-CH₄ dimers. The C-H bond of CH4 does not prefer to point to the C-C bond of C₂H₆.

Fig. 1. The geometries of the dimers considered in this work. The intermolecular distance R = 3.8 �ð for the dimers B and G, and 4.2 �ð for the other dimers. The MP2/cc-pVQZ level interaction potentials had minima at these intermolecular distances.