Role of O-H⋯O/S conventional hydrogen bonds in considerable C sp 2 -H blue-shift in the binary systems of acetaldehyde and thioacetaldehyde with substituted carboxylic and thiocarboxylic acids.

Stable binary complexes of RCZOH⋯CH 3 CHZ (R = CH 3 , H, F; Z = O, S) are due to contributions from the O-H⋯O/S and C sp 2 -H⋯O/S hydrogen bonds. The strength of C sp 2 /O-H⋯O is 1.5 to 2 times greater than that of the C sp 2 /O-H⋯S bond. The substitution of H(C sp 2 ) of HCZOH by CH 3 causes a decrease in complex stability, while the opposite trend occurs for the F atom. A very large red shift of the O-H stretching frequency in O-H⋯O/S bonds was observed. A surprising C sp 2 -H blue shift up to 104.5 cm -1 was observed for the first time. It is found that the presence of O-H⋯O/S hydrogen bonds and a decisive role of intramolecular hyperconjugation interactions in the complex cause a significant blue shift of the C sp 2 -H covalent bonds. A striking role of O compared to the S atom in determining the blue shift of C sp 2 -H stretching vibration and stability of binary complexes is proposed. The obtained results show that the ratio of deprotonation enthalpy and proton affinity could be considered as an index for the classification of the non-conventional hydrogen bond. SAPT2+ results show that the strength of RCSOH⋯CH 3 CHS complexes is dominated by electrostatic and induction energies, while a larger contribution to the stability of remaining complexes is detected for the electrostatic component.