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?? complexes, is presented. In this sense, the dissociation energy differences, defined as, ?D e = D e X-H[complex] - D e X-H [isolated], showed to be linearly connected with the change in stretching frequencies, ?? = ? X-H[complex] - ? 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: ?D e X-H > 50.3 kcal mol -1 leads to blue-shifting whereas ?D 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 3C- H?C 6H 6), fluoroform-benzene (F 3C- H?C 6H 6), chloroform-fluorobenzene, as blue-shifting hydrogen-bonded complexes and cyanide acid-benzene (NC-H?C 6H 6), bromide and chloride acids-benzene ((Br)Cl-H?C 6H 6) and acetylene-benzene (C 2H 2?C 6H 6) as red-shifting complexes. © 2011 the Owner Societies.