Does the methyl group form a hydrogen bond? Ab initio post-Hartree–Fock study on ethane–hydrogen cyanide complex

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Ab initio LCAO-MO methods at the second- and fourth-order Møller–Plesset levels and coupled cluster CCSD and CCSD(T) approximations have been applied to investigate a formation of a hydrogen bond by a methyl group in the ethane–hydrogen cyanide complex. Geometrical parameters obtained from an MP2/6-311++G ∗∗ optimization are reported. The optimized geometry characterizes a minimum energy structure. Stability of the complex is evaluated using both the supermolecular counterpoise approach and the symmetry-adapted perturbation theory. The depth of the minimum is about 0.6 kcal/mol. Thus, one may consider this dimer as an example of a very weak hydrogen-bonded system.