Characteristics of nonconventional hydrogen bonds and stability of dimers of chalcogenoaldehyde derivatives: a noticeable role of oxygen compared to other chalcogens.

In this work, twenty-four stable dimers of RCHZ with R = H, F, Cl, Br, CH or NH and Z = O, S, Se or Te were determined. It was found that the stability of most dimers is primarily contributed by the electrostatic force, except for the dominant role of the induction term in those involving a Te atom, which has been rarely observed. Both electron-donating and -withdrawing groups in substituted formaldehyde cause an increase in the strength of nonconventional C-H⋯Z hydrogen bonds, as well as the dimers, in which the electron donating effect plays a more crucial role. The strength of nonconventional hydrogen bonds decreases in the following order: C-H⋯O ≫ C-H⋯S > C-H⋯Se > C-H⋯Te. Remarkably, a highly significant role of the O atom compared to S, Se and Te in increasing the C-H stretching frequency and strength of the nonconventional hydrogen bonds and dimers is found. A C-H stretching frequency red-shift is observed in C-H⋯S/Se/Te, while a blue-shift is obtained in C-H⋯O. When Z changes from O to S to Se and to Te, the C-H blue-shift tends to decrease and eventually turns to a red-shift, in agreement with the increasing order of the proton affinity at Z in the isolated monomer. The magnitude of the C-H stretching frequency red-shift is larger for C-H⋯Te than C-H⋯S/Se, consistent with the rising trend of proton affinity at the Z site and the polarity of the C-H bond in the substituted chalcogenoaldehydes. The C-H blue-shifting of the C-H⋯O hydrogen bonds is observed in all dimers regardless of the electron effect of the substituents. Following complexation, the electron-donating derivatives exhibit a stronger C-H blue-shift compared to the electron-withdrawing ones. Notably, the stronger C-H blue-shift turns out to involve a less polarized C-H bond and a decrease in the occupation at the σ*(C-H) antibonding orbital in the isolated monomer.