Hydrogen bonds (H-bonds) in the complexes between aldehydes and hydrogen chalcogenides, XCHO...nH 2 Z with X = H, F, Cl, Br, and CH 3 , Z = O, S, Se, and Te, and n = 1,2, were investigated using high-level ab initio calculations. The C sp2 − H...O H-bonds are found to be about twice as strong as the C sp2 − 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 H 2 Z molecule into binary systems induces stronger complexes and causes a positive cooperative effect in ternary complexes. The blue shift of C sp2 −H stretching frequency involving the C sp2 −H...Z H-bond sharply increases when replacing one H atom in HCHO by a CH 3 group. In contrast, when one H atom in HCHO is substituted with a halogen, the magnitude of blueshifting of the C sp2 −H...Z H-bond becomes smaller. The largest blue shift up to 92 cm −1 of C sp2 −H stretching frequency in C sp2 −H...O H-bond in CH 3 CHO...2H 2 O has rarely been observed and is much greater than that in the cases of the C sp2 −H...S/Se/Te ones. The C sp2 −H blue shift of C sp2 −H...Z bonds in the halogenated aldehydes is converted into a red shift when H 2 O is replaced by a heavier analogue, such as H 2 S, H 2 Se, or H 2 Te. The stability and classification of nonconventional H-bonds including C sp2 −H...Se/Te, Te−H...Te, and Se/Te−H...O have been established for the first time.

The considerable blue shift of Csp2–H stretching frequency.

Six stable geometrical structures of the interaction between CH3CHS and H2O were observed on potential energy surface at the MP2/ aug-cc-pVDZ level of theory. Interaction energies corrected by both ZPE and BSSE for all the complexes at the MP2/aug-cc-pVTZ//MP2/aug-cc-pVDZ high level of theory range from -12.0 to -61.4 kJ.mol -1 . When adding water molecule to CH3CHS, stability of interacting system and cooperative capacity of complexes are increased.

Twenty stable geometrical structures of RCHSe∙∙∙nH2Z (R = H, F, Cl, Br, CH3; n = 1, 2; Z = O, S) were observed on potential surface energy. The strength of complexes increases in the order of substituted derivatives H < F < Cl < Br < CH3. The O–H∙∙∙O H‐bond is ca. four times stronger than the S–H∙∙∙S counterpart while Csp2–H∙∙∙S bond strength is about half of the Csp2–H∙∙∙O bond strength. This work reveals that a contraction of the Csp2–H bond length and an increase of its stretching frequency upon complexation are induced as replacing an H atom in H2CSe by CH3, and an inverse trend is observed in the case of F/Cl/Br halogen substitution. In addition, magnitude of Z–H bond elongation accompanied by a decrease of its stretching frequency increase in the order of substitution of F ~ Cl ~Br < H < CH3, following complexation. For RCHSe∙∙∙H2Z binary system, when H2O molecule is substituted by H2S, the Csp2–H blue‐shift in Csp2–H∙∙∙Z H‐bond is decreased, while the Z–H red‐shift in the Z–H∙∙∙Se H‐bond is increased, and an inverse change is detected in the case of ternary system. When adding one H2O molecule to the binary system, the Csp2–H blue‐shift of Csp2–H∙∙∙Z for H/CH3‐substituted derivatives is increased, while an increase in the Csp2–H red‐shift of Csp2–H∙∙∙Z H‐bond is observed for F/Cl/Br‐substituted derivatives.