The question as to whether the CH⋯O interaction constitutes a true H-bond is examined from the perspective of calculated NMR chemical shieldings. Fluorinated methane derivatives FnH3−nCH are taken as proton donors; H2O, CH3OH and H2CO serve as acceptors. The shifts in the isotropic chemical shieldings of the bridging proton that occur upon formation of the complex are all within the fairly narrow range of −1.0 to −1.5 ppm. These shifts become more negative as the donor is made more acidic, following the order H3CH2CH2HCH. The carbonyl oxygen of the H2CO acceptor induces the most negative shift, followed in order by H2O and then the CH3OH. The magnitudes of the anisotropic shifts are much larger, indicating that the tensor components perpendicular to the H-bond axis are more sensitive to formation of an H-bond than is the parallel analog. The isotropic shift of the donor atom is quite sensitive to its separation from the acceptor molecule, even reversing sign as this distance changes. The carbonyl and hydroxyl acceptor O atoms exhibit shifts of their isotropic shielding of opposite sign, a distinction due to the perpendicular component. The behavior of the bridging proton in these CH⋯O complexes is quite similar to the water dimer with its conventional H-bond. On the other hand, the classification of the CH⋯O interaction on the basis of the donor and acceptor atoms, is less conclusive, as these atoms undergo more complex behavior, and are more sensitive to the specific level of theory applied.
- hydrogen bonding,