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