C-H•••O Hydrogen Bonding. The Prototypical Methane-Formaldehyde System: A Critical Assessment

Distinguishing the functionality of C-H center dot center dot center dot O hydrogen bonds (HBs) remains challenging, because their properties are difficult to quantify reliably. Herein, we present a study of the model methane-formaldehyde complex (MFC). Six stationary points on the MFC potential energy surface (PES) were obtained at the CCSD(T)/ANO2 level. The CCSDT(QJ/CBS interaction energies of the conformers range from only-1.12 kcal mol(-1) to -0.33 kcal mol(-1), denoting a very flat PES. Notably, only the lowest energy stationary point (MFC1) corresponds to a genuine minimum, whereas all other stationary points-including the previously studied ideal case of a(e)(C-H center dot center dot center dot O) = 180 degrees-exhibit some degree of freedom that leads to MFC1. Despite the flat PES, we clearly see that the HB properties of MFC1 align with those of the prototypical water dimer O-H center dot center dot center dot O HB. Each HB property generally becomes less prominent in the higher-energy conformers. Only the MFC1 conformer prominently exhibits (1) elongated C-H donor bonds, (2) attractive C-H center dot center dot center dot O=C interactions, (3) n(O) -> sigma*(C-H) hyperconjugation, (4) critical points in the electron density from Bader's method and from the noncovalent interactions method, (5) positively charged donor hydrogen, and (6) downfield NMR chemical shifts and nonzero (2)J(C-M-H-M center dot center dot center dot O-F) coupling constants. Based on this research, some issues merit further study. The flat PES hinders reliable determinations of the HE-induced shifts of the C-H stretches; a similarly difficult challenge is observed for the experiment. The role of charge transfer in HBs remains an intriguing open question, although our BLW and NBO computations suggest that it is relevant to the C-H center dot center dot center dot O HB geometries. These issues notwithstanding, the prominence of the HB properties in MFC1 serves as clear evidence that the MFC is predominantly bound by a C-H center dot center dot center dot O HB.