Medium-Dependent Crossover from the Red to Blue Shift of the Donor’s Stretching Fundamental in the Binary Hydrogen-Bonded Complexes of CDCl3 with Ethers and Ketones

Factors contributing to halogen bond strength and stretch or contraction of internal covalent bond

The halogen bond formed by a series of Lewis acids TF₃X (T = C, Si, Ge, Sn, Pb; X = Cl, Br, I) with NH₃ is studied by quantum chemical calculations. The interaction energy is closely mimicked by the depth of the σ-hole on the X atom as well as the full electrostatic energy. There is a first trend by which the hole is deepened if the T atom to which X is attached becomes more electron-withdrawing: C > Si > Ge > Sn > Pb. On the other hand, larger more polarizable T atoms are better able to transmit the electron-withdrawing power of the F substituents. The combination of these two opposing factors leaves PbF₃X forming the strongest XBs, followed by CF₃X, with SiF₃X engaging in the weakest bonds. The charge transfer from the NH₃ lone pair into the σ*(TX) antibonding orbital tends to elongate the covalent TX bond, and this force is largest for the heavier X and T atoms. On the other hand, the contraction of this bond deepens the σ-hole at the X atom, which would enhance both the electrostatic component and the full interaction energy. This bond-shortening effect is greatest for the lighter X atoms. The combination of these two opposing forces leaves the T-X bond contracting for X = Cl and Br, but lengthening for I.

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

Stable binary complexes of RCZOH⋯CH3CHZ (R = CH3, H, F; Z = O, S) are due to contributions from the O-H⋯O/S and Csp2 -H⋯O/S hydrogen bonds. The strength of Csp2 /O-H⋯O is 1.5 to 2 times greater than that of the Csp2 /O-H⋯S bond. The substitution of H(Csp2 ) of HCZOH by CH3 causes a decrease in complex stability, while the opposite trend occurs for the F atom. A very large red shift of the O-H stretching frequency in O-H⋯O/S bonds was observed. A surprising Csp2 -H blue shift up to 104.5 cm-1 was observed for the first time. It is found that the presence of O-H⋯O/S hydrogen bonds and a decisive role of intramolecular hyperconjugation interactions in the complex cause a significant blue shift of the Csp2 -H covalent bonds. A striking role of O compared to the S atom in determining the blue shift of Csp2 -H stretching vibration and stability of binary complexes is proposed. The obtained results show that the ratio of deprotonation enthalpy and proton affinity could be considered as an index for the classification of the non-conventional hydrogen bond. SAPT2+ results show that the strength of RCSOH⋯CH3CHS complexes is dominated by electrostatic and induction energies, while a larger contribution to the stability of remaining complexes is detected for the electrostatic component.

Understanding noncovalent bonds and their controlling forces

The fundamental underpinnings of noncovalent bonds are presented, focusing on the σ-hole interactions that are closely related to the H-bond. Different means of assessing their strength and the factors that control it are discussed. The establishment of a noncovalent bond is monitored as the two subunits are brought together, allowing the electrostatic, charge redistribution, and other effects to slowly take hold. Methods are discussed that permit prediction as to which site an approaching nucleophile will be drawn, and the maximum number of bonds around a central atom in its normal or hypervalent states is assessed. The manner in which a pair of anions can be held together despite an overall Coulombic repulsion is explained. The possibility that first-row atoms can participate in such bonds is discussed, along with the introduction of a tetrel analog of the dihydrogen bond.