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 center dot center dot center dot nH(2)Z 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 C-sp2-H center dot center dot center dot O H-bonds are found to be about twice as strong as the C-sp2-H center dot center dot center dot S/Se/Te counterparts. Remarkably, the S/Se/Te-H center dot center dot center dot S/Se/Te H-bonds are 4.5 times as weak as the O-H center dot center dot center dot 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 center dot center dot center dot 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 C-sp2-H center dot center dot center dot Z H-bond becomes smaller. The largest blue shift up to 92 cm(-1) of C-sp2-H stretching frequency in C-sp2-H center dot center dot center dot H-bond in CH3CHO center dot center dot center dot 2H(2)O has rarely been observed and is much greater than that in the cases of the C-sp2-H center dot center dot center dot S/Se/Te ones. The C-sp2-H blue shift of C-sp2-H center dot center dot center dot 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 C-sp2-H center dot center dot center dot Se/Te, Te-H center dot center dot center dot Te, and Se/Te-H center dot center dot center dot O have been established for the first time.