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.