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

摘要

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, Δv - v_(X-H)[complex] - v_(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.