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Module 2 : Molecular Structure

Lecture 10 : Non-covalent interactions 1: Intermolecular Forces




Fig 10.2 Ion-induced-dipole and dipole-dipole interactions.
The interaction between two dipoles with magnitudes _i and _j separated by _ij is given by

U_dd = _i _j / r_ij³ [ - cos _i cos _j + sin _i sin _j cos ( _i- _j)]	(10.13)
The angle between the dipole vector and _ijis measured by . The angles _i and _j measure the angles of the projections of the dipoles in a plane perpendicular to the plane of the paper. We see from the formula that if the dipoles oppose one another, the interaction energy is positive and the interaction is negative or attractive if they are aligned in the same direction.

A dipole polarizes a neighbouring charge distribution. If the dipole is at an angle _i with respect to r_ij separating the dipole and another entity (that is polarized) and _j is the polarizability of that entity, then the dipole induced dipole (d-id) interaction is given by

U_d-id (r_ij) = - (3cos²_i +1) /2r_ij⁶	(10.14)
All the above interactions diminish with increasing distance. A sketch of the magnitude of these interactions as a function of distance is given in fig. 10.4



Figure 10.3 Magnitudes of various intermolecular interactions as a function of the intermolecular separation.

As we have already noted, the dispersion interaction is always present. The other induced electrostatic interactions that we discussed in this section depend on the presence of a charge or a dipole moment on one of the species.