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

Distinguishing the functionality of C–H···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 (<b>MFC</b>). Six stationary points on the <b>MFC</b> potential energy surface (PES) were obtained at the CCSD­(T)/ANO2 level. The CCSDT­(Q)/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 (<b>MFC1</b>) corresponds to a genuine minimum, whereas all other stationary pointsincluding the previously studied ideal case of <i>a</i>_e(C–H···O) = 180°exhibit some degree of freedom that leads to <b>MFC1</b>. Despite the flat PES, we clearly see that the HB properties of <b>MFC1</b> align with those of the prototypical water dimer O–H···O HB. Each HB property generally becomes less prominent in the higher-energy conformers. Only the <b>MFC1</b> conformer prominently exhibits (1) elongated C–H donor bonds, (2) attractive C–H···OC interactions, (3) <i>n</i>(O) → σ*­(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<i>J</i>(C_M–H_M···O_F) coupling constants. Based on this research, some issues merit further study. The flat PES hinders reliable determinations of the HB-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···O HB geometries. These issues notwithstanding, the prominence of the HB properties in <b>MFC1</b> serves as clear evidence that the <b>MFC</b> is predominantly bound by a C–H···O HB