Aliphatic C-H...anion hydrogen bonds: weak contacts or strong interactions?

Electronic structure calculations, MP2/aug-cc-pVDZ, are used to determine C-H...Cl- hydrogen bond energies for a series of XCH3 donor groups in which the electron-withdrawing ability of X is varied over a wide range of values. When attached to polarizing substituents, aliphatic CH groups are moderate-to-strong hydrogen bond donors, exhibiting interaction energies comparable to those obtained with O-H and N-H groups. The results explain why aliphatic C-H donors are observed to function as competitive binding sites in solution and suggest that such C-H...anion contacts should be considered as possible contributors when evaluating the denticity of an anion receptor.