High-Level ab Initio Calculations of Interaction Energies of C2H4−CH4 and C2H6−CH4 Dimers: A Model Study of CH/π Interaction

Intermolecular interaction energies of five C 2 H 4 −CH 4 dimers and two C 2 H 6 -CH 4 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 CH 4 points to the C C bond of C 2 H 4 has slightly larger bonding energy than the other four C 2 H 4 −CH 4 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 C 2 H 4 and CH 4 . The analysis of the electrostatic interaction using distributed multipoles shows that the attractive electrostatic interaction (−0.24 kcal/mol) is playing important role to stabilize this dimer. On the other hand, the electrostatic interaction is negligible for the C 2 H 6 −CH 4 dimers. The C−H bond of CH 4 does not prefer to point to the C−C bond of C 2 H 6 .