Profound importance of the conventional O–H⋯O hydrogen bond versus a considerable blue shift of the Csp2–H bond in complexes of substituted carbonyls and carboxyls†

Using quantum chemical approaches, we investigated the conventional O–H⋯O and nonconventional C_sp²–H⋯O hydrogen bonds between carboxylic acids and aldehydes in 21 stable complexes. The strength of complexes is determined by the conventional O–H⋯O bond together with the nonconventional C_sp²–H⋯O hydrogen bond, in which the former one is 4–5 times as strong as the latter one. Proportional linear correlations of the interaction energy with both individual energies of the O–H⋯O and C_sp²–H⋯O hydrogen bonds are proposed. Different impacts of electron-donating and electron-withdrawing groups in substituted formaldehyde and formic acid on characteristics of conventional and nonconventional hydrogen bonds, as well as the strength of both hydrogen bond types and complexes, are also evaluated. Following complexation, it is noteworthy that the largest blue shift of the C_sp²–H stretching frequency in the C_sp²–H⋯O bond up to 105.3 cm⁻¹ in CH₃CHO⋯FCOOH is due to a decisive role of the O–H⋯O hydrogen bond, which has been rarely reported in the literature. The obtained results show that the conventional O–H⋯O hydrogen bond plays a pivotal role in the significant blue shift of the C_sp²–H stretching frequency in the nonconventional C_sp²–H⋯O hydrogen bond. Remarkably, the considerable blue shift of the C_sp²–H stretching frequency is found to be one H of C–H in formic acid substituted by the electron-withdrawing group and one H in formaldehyde substituted by the electron-donating group. In addition, the change in the C_sp²–H stretching frequency following complexation is proportional to both changes of electron density in σ*(C_sp²–H) and σ*(O–H) orbitals, in which a dominant role of σ*(O–H) versus σ*(C_sp²–H) is observed.