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

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 (Cl(3)C-H···C(6)H(6)), fluoroform-benzene (F(3)C-H···C(6)H(6)), chloroform-fluorobenzene, as blue-shifting hydrogen-bonded complexes and cyanide acid-benzene (NC-H···C(6)H(6)), bromide and chloride acids-benzene ((Br)Cl-H···C(6)H(6)) and acetylene-benzene (C(2)H(2)···C(6)H(6)) as red-shifting complexes.