Effect of substituents on complex stability and characteristics of C sp2 ‐H ∙∙∙O/S and O/ S‐H ∙∙∙S/Se hydrogen bonds in the systems of monosubstituted selenoformaldehyde with H 2 O and H 2 S

Twenty stable geometrical structures of RCHSe∙∙∙nH 2 Z (R = H, F, Cl, Br, CH 3 ; 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 < CH 3 . The O–H∙∙∙O H‐bond is ca. four times stronger than the S–H∙∙∙S counterpart while C sp2 –H∙∙∙S bond strength is about half of the C sp2 –H∙∙∙O bond strength. This work reveals that a contraction of the C sp2 –H bond length and an increase of its stretching frequency upon complexation are induced as replacing an H atom in H 2 CSe by CH 3 , 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 < CH 3 , following complexation. For RCHSe∙∙∙H 2 Z binary system, when H 2 O molecule is substituted by H 2 S, the C sp2 –H blue‐shift in C sp2 –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 H 2 O molecule to the binary system, the C sp2 –H blue‐shift of C sp2 –H∙∙∙Z for H/CH 3 ‐substituted derivatives is increased, while an increase in the C sp2 –H red‐shift of C sp2 –H∙∙∙Z H‐bond is observed for F/Cl/Br‐substituted derivatives.