Importance of water and intramolecular interaction governs substantial blue shift of C sp 2 -H stretching frequency in complexes between chalcogenoaldehydes and water.

Geometrical structure, stability and cooperativity, and contribution of hydrogen bonds to the stability of complexes between chalcogenoaldehydes and water were thoroughly investigated using quantum chemical methods. The stability of the complexes increases significantly when one or more H 2 O molecules are added to the binary system, whereas it decreases sharply going from O to S, Se, or Te substitution. The O-H⋯O H-bond is twice as stable as C sp 2 -H⋯O and O-H⋯S/Se/Te H-bonds. It is found that a considerable blue-shift of C sp 2 -H stretching frequency in the C sp 2 -H⋯O H-bond is mainly determined by an addition of water into the complexes along with the low polarity of the C sp 2 -H covalent bond in formaldehyde and acetaldehyde. The C sp 2 -H stretching frequency shift as a function of net second hyperconjugative energy for the σ*(C sp 2 -H) antibonding orbital is observed. Remarkably, a considerable C sp 2 -H blue shift of 109 cm -1 has been reported for the first time. Upon the addition of H 2 O into the binary systems, halogenated complexes witness a decreasing magnitude of the C sp 2 -H stretching frequency blue-shift in the C sp 2 -H⋯O H-bond, whereas CH 3 -substituted complexes experience the opposite trend.