Symmetric C···H···C Hydrogen Bonds Predicted by Quantum Chemical Calculations.

The symmetry of hydrogen bonds is a fundamental question regarding hydrogen bonding interactions. Although asymmetric C-H···C hydrogen bonds are known in the literature, no symmetric C···H···C hydrogen bonds have been reported. Herein, we propose the theoretical possibility of symmetric C···H···C hydrogen bonds on the basis of quantum chemical calculations. Several bridged carbanions with intramolecular symmetric C···H···C hydrogen bonds were designed computationally. The key to this design is to shorten the C···C distance to ca. 2.5 Å, which is predicted to be necessary for a single-well C···H···C hydrogen bond.