Importance of water and intramolecular interaction governs substantial blue shift of Csp2–H stretching frequency in complexes between chalcogenoaldehydes and water

Abstract

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 H₂O 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_sp²–H⋯O and O–H⋯S/Se/Te H-bonds. It is found that a considerable blue-shift of C_sp²–H stretching frequency in the C_sp²–H⋯O H-bond is mainly determined by an addition of water into the complexes along with the low polarity of the C_sp²–H covalent bond in formaldehyde and acetaldehyde. The C_sp²–H stretching frequency shift as a function of net second hyperconjugative energy for the σ*(C_sp²–H) antibonding orbital is observed. Remarkably, a considerable C_sp²–H blue shift of 109 cm⁻¹ has been reported for the first time. Upon the addition of H₂O into the binary systems, halogenated complexes witness a decreasing magnitude of the C_sp²–H stretching frequency blue-shift in the C_sp²–H⋯O H-bond, whereas CH₃-substituted complexes experience the opposite trend.