Importance of water and intramolecular interaction governs substantial blue shift of Csp2 -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 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 Csp2 -H⋯O and O-H⋯S/Se/Te H-bonds. It is found that a considerable blue-shift of Csp2 -H stretching frequency in the Csp2 -H⋯O H-bond is mainly determined by an addition of water into the complexes along with the low polarity of the Csp2 -H covalent bond in formaldehyde and acetaldehyde. The Csp2 -H stretching frequency shift as a function of net second hyperconjugative energy for the σ*(Csp2 -H) antibonding orbital is observed. Remarkably, a considerable Csp2 -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 Csp2 -H stretching frequency blue-shift in the Csp2 -H⋯O H-bond, whereas CH3-substituted complexes experience the opposite trend.

Theoretical Aspects of Nonconventional Hydrogen Bonds in the Complexes of Aldehydes and Hydrogen Chalcogenides.

Hydrogen bonds (H-bonds) in the complexes between aldehydes and hydrogen chalcogenides, XCHO...nH2Z with X = H, F, Cl, Br, and CH3, Z = O, S, Se, and Te, and n = 1,2, were investigated using high-level ab initio calculations. The Csp2-H...O H-bonds are found to be about twice as strong as the Csp2-H...S/Se/Te counterparts. Remarkably, the S/Se/Te-H...S/Se/Te H-bonds are 4.5 times as weak as the O-H...O ones. The addition of the second H2Z molecule into binary systems induces stronger complexes and causes a positive cooperative effect in ternary complexes. The blue shift of Csp2-H stretching frequency involving the Csp2-H...Z H-bond sharply increases when replacing one H atom in HCHO by a CH3 group. In contrast, when one H atom in HCHO is substituted with a halogen, the magnitude of blue-shifting of the Csp2-H...Z H-bond becomes smaller. The largest blue shift up to 92 cm-1 of Csp2-H stretching frequency in Csp2-H...O H-bond in CH3CHO...2H2O has rarely been observed and is much greater than that in the cases of the Csp2-H...S/Se/Te ones. The Csp2-H blue shift of Csp2-H...Z bonds in the halogenated aldehydes is converted into a red shift when H2O is replaced by a heavier analogue, such as H2S, H2Se, or H2Te. The stability and classification of nonconventional H-bonds including Csp2-H...Se/Te, Te-H...Te, and Se/Te-H...O have been established for the first time.