Blue shifts vs red shifts in σ-hole bonding

Abstract

σ-Hole bonding is a noncovalent interaction between a region of positive electrostatic potential on the outer surface of a Group V, VI, or VII covalently-bonded atom (a σ-hole) and a region of negative potential on another molecule, e.g., a lone pair of a Lewis base. We have investigated computationally the occurrence of increased vibration frequencies (blue shifts) and bond shortening vs decreased frequencies (red shifts) and bond lengthening for the covalent bonds to the atoms having the σ-holes (the σ-hole donors). Both are possible, depending upon the properties of the donor and the acceptor. Our results are consistent with models that were developed earlier by Hermansson and by Qian and Krimm in relation to blue vs red shifting in hydrogen bond formation. These models invoke the derivatives of the permanent and the induced dipole moments of the donor molecule. Figure Computed electrostatic potential on the molecular surface of Cl-NO₂. Color ranges, in kcal mol⁻¹, are: red, greater than 25; yellow, between 10 and 25; green, between 0 and 10; blue, between −4 and 0; purple, more negative than −4. The chlorine is facing the viewer, to the right. Note the yellow region of positive potential on the outer side of the chlorine, along the extension of the N–Cl bond. The blue region shows the sides of the chlorine to have negative potentials. The calculations were at the B3PW91/6–31G(d,p) level.