Fundamental Properties of the CH···O Interaction:  Is It a True Hydrogen Bond?

Ab initio calculations are used to analyze the CH···O interaction between FnH3-nCH as proton donor and H2O, CH3OH, and H2CO as acceptor. The interaction is quite weak with CH4 as donor but is enhanced by 1 kcal/mol with each F added to the donor. The CH···O interaction behaves very much like a conventional OH···O H-bond in most respects, including shifts in electron density that accompany the formation of the bond and the magnitudes of the various components of the interaction energy. The two sorts of H-bonds also gravitate toward a similar equilibrium geometry and are comparably sensitive to deformations from that structure. In a quantitative sense, while both CH···O and OH···O prefer a linear configuration, the former is somewhat more easily bent and is less sensitive to stretches from its equilibrium H-bond length. Whereas the OH bond has been shown to stretch and undergo a red shift in its vibrational frequency upon formation of a H-bond, the CH bond of the molecules studied here follows the opposite trend, a contraction and a blue shift. Analysis demonstrates that this opposite pattern is not due to any fundamental distinction between the two interactions, since the same sets of forces are acting on both. It is concluded that the CH···O interaction can, indeed, be categorized as a true H-bond.