Abstract
In this work, twenty-four stable dimers of RCHZ with R = H, F, Cl, Br, CH(3) or NH(2) 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(sp(2))-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(sp(2))-H⋯O ≫ C(sp(2))-H⋯S > C(sp(2))-H⋯Se > C(sp(2))-H⋯Te. Remarkably, a highly significant role of the O atom compared to S, Se and Te in increasing the C(sp(2))-H stretching frequency and strength of the nonconventional hydrogen bonds and dimers is found. A C(sp(2))-H stretching frequency red-shift is observed in C(sp(2))-H⋯S/Se/Te, while a blue-shift is obtained in C(sp(2))-H⋯O. When Z changes from O to S to Se and to Te, the C(sp(2))-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(sp(2))-H stretching frequency red-shift is larger for C(sp(2))-H⋯Te than C(sp(2))-H⋯S/Se, consistent with the rising trend of proton affinity at the Z site and the polarity of the C(sp(2))-H bond in the substituted chalcogenoaldehydes. The C(sp(2))-H blue-shifting of the C(sp(2))-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(sp(2))-H blue-shift compared to the electron-withdrawing ones. Notably, the stronger C(sp(2))-H blue-shift turns out to involve a less polarized C(sp(2))-H bond and a decrease in the occupation at the σ*(C(sp(2))-H) antibonding orbital in the isolated monomer.