Theoretical analysis based on X–H bonding strength and electronic properties in red- and blue-shifting hydrogen-bonded X–H⋯π complexes

Abstract

A theoretical study based on the X–H bond strength of the proton donor fragment and its concomitant classical red-shifting or improper blue-shifting of the pure stretching frequency, in weakly hydrogen-bonded X–H⋯π complexes, is presented. In this sense, the dissociation energy differences, defined as, ΔD_e = D_e^X–H[complex] − D_e^X–H [isolated], showed to be linearly connected with the change in stretching frequencies, Δν = ν_X–H[complex] − ν_X–H[isolated], of red- and blue-shifting H-bonds. This relationship allows us to define a threshold for the type of the stretching shift of the X–H bond: ΔD_e^X–H > 50.3 kcal mol⁻¹ leads to blue-shifting whereas ΔD_e^X–H < 50.3 kcal mol⁻¹ leads to red-shifting behavior. Complementarily, natural bond orbital analysis along the X–H stretching coordinate and electric dipole polarizability was performed to investigate the factors involved in red- or blue-shifting hydrogen-bonded complexes. It has been found that a high tendency to deplete the electronic population on the H atom upon X–H stretching is exhibited in blue-shifting H-bonded complexes. On the other hand, these types of complexes present a compact electronic redistribution in agreement with polarizability values. This study has been carried out taking as models the following systems: chloroform–benzene (Cl₃C–H⋯C₆H₆), fluoroform–benzene (F₃C–H⋯C₆H₆), chloroform–fluorobenzene, as blue-shifting hydrogen-bonded complexes and cyanide acid–benzene (NC–H⋯C₆H₆), bromide and chloride acids–benzene ((Br)Cl–H⋯C₆H₆) and acetylene–benzene (C₂H₂⋯C₆H₆) as red-shifting complexes.