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, CH3 or NH2 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 Csp2-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: Csp2-H⋯O ≫ Csp2-H⋯S > Csp2-H⋯Se > Csp2-H⋯Te. Remarkably, a highly significant role of the O atom compared to S, Se and Te in increasing the Csp2-H stretching frequency and strength of the nonconventional hydrogen bonds and dimers is found. A Csp2-H stretching frequency red-shift is observed in Csp2-H⋯S/Se/Te, while a blue-shift is obtained in Csp2-H⋯O. When Z changes from O to S to Se and to Te, the Csp2-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 Csp2-H stretching frequency red-shift is larger for Csp2-H⋯Te than Csp2-H⋯S/Se, consistent with the rising trend of proton affinity at the Z site and the polarity of the Csp2-H bond in the substituted chalcogenoaldehydes. The Csp2-H blue-shifting of the Csp2-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 Csp2-H blue-shift compared to the electron-withdrawing ones. Notably, the stronger Csp2-H blue-shift turns out to involve a less polarized Csp2-H bond and a decrease in the occupation at the σ*(Csp2-H) antibonding orbital in the isolated monomer.

Importance of water and intramolecular interaction governs substantial blue shift of Csp2–H stretching frequency in complexes between chalcogenoaldehydes and water

Geometrical structure, stability and cooperativity, and contribution of hydrogen bonds to the stability of complexes between chalcogenoaldehydes and water were thoroughly investigated using quantum chemical methods. The stability of the complexes increases significantly when one or more H₂O molecules are added to the binary system, whereas it decreases sharply going from O to S, Se, or Te substitution. The O–H⋯O H-bond is twice as stable as C_sp²–H⋯O and O–H⋯S/Se/Te H-bonds. It is found that a considerable blue-shift of C_sp²–H stretching frequency in the C_sp²–H⋯O H-bond is mainly determined by an addition of water into the complexes along with the low polarity of the C_sp²–H covalent bond in formaldehyde and acetaldehyde. The C_sp²–H stretching frequency shift as a function of net second hyperconjugative energy for the σ*(C_sp²–H) antibonding orbital is observed. Remarkably, a considerable C_sp²–H blue shift of 109 cm⁻¹ has been reported for the first time. Upon the addition of H₂O into the binary systems, halogenated complexes witness a decreasing magnitude of the C_sp²–H stretching frequency blue-shift in the C_sp²–H⋯O H-bond, whereas CH₃-substituted complexes experience the opposite trend.

The considerable blue shift of C_sp²–H stretching frequency.

Role of O–H⋯O/S conventional hydrogen bonds in considerable Csp2–H blue-shift in the binary systems of acetaldehyde and thioacetaldehyde with substituted carboxylic and thiocarboxylic acids

The presence of O–H⋯O/S conventional hydrogen bonds in the complex governs a significant blue shift of Csp2–H bonds.

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...nH₂Z 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...O H-bonds are found to be about twice as strong as the C_sp2-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 H₂Z 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...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...Z H-bond becomes smaller. The largest blue shift up to 92 cm^-1 of C_sp2-H stretching frequency in C_sp2-H...O H-bond in CH₃CHO...2H₂O has rarely been observed and is much greater than that in the cases of the C_sp2-H...S/Se/Te ones. The C_sp2-H blue shift of C_sp2-H...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...Se/Te, Te-H...Te, and Se/Te-H...O have been established for the first time.