The proportional magnitude of the intermolecular forces are: ##CF_4 < OF_2 < CHF_3 < HF.##
All molecules will have London dispersion forces that get stronger as the molecule gets heavier (more electrons causes a shift in electron cloud distribution resulting in a temporary dipole).
##CHF_3## is a polar molecule. So it will have dipole-dipole interaction along with the weaker dispersion forces.
##OF_2## is a polar molecule with a bent shape just like ##H_2O##. There are two pairs of bonded electrons and two pairs of unbonded lone pair electrons. These lone pairs repel the bonded electrons resulting in a bent shape with an angle less than ## 105^0##. The dominant intermolecular forces would be dipole-dipole.
HF is a polar molecule. Hence the primary intermolecular forces would be dipole-dipole and hydrogen bond which is a special type of dipole-dipole interaction between the hydrogen atom and electronegative F atom.
##CF_4## has a tetrahedral structure. It is a non-polar molecule. The dominant intermolecular force would be the London dispersion force.
The strongest intermolecular forces in each case are:
• CHF3: dipole-dipole interaction
• OF2: London dispersion forces
• HF: hydrogen bonding
• CF4: London dispersion forces
Each of these molecules is made up of polar covalent bonds; however, in order for the molecule itself to be polar, the polarities must not cancel one another out. The polar bonds in OF2, for example, act in opposite directions and are of the same electronegativity difference [Δ(EN)], so the molecule is not polar. A similar principle applies to CF4. As a result, the strongest type of intermolecular interaction between molecules of these substances is the London dispersion force.
Due to the fact that the polar bonds do not cancel in the remaining molecules, they exhibit dipole-dipole interactions: these are stronger than London dispersion forces. However, HF exhibits hydrogen bonding – a stronger force still that is similar to the dipole-dipole interaction – whilst CHF3 does not. Thus, the strength of intermolecular forces between molecules of each of these substances can be expressed, in terms of strength, as: