Interactions between dipolar molecules fixed

In general, independent dipole-dipole interactions are weaker than the ion-dipole interactions, and only very polar molecules are strong enough to bind to each other at normal temperatures in the liquid state. However, water is an exception to this due to its hydrogen-bonding properties. Hydrogen bonding may be considered to be a special type of directional dipole-dipole interaction (see also Section 2.8). 

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Interactions between dipolar molecules and induced non-polar molecules (fixed)... 

Interactions between ions and dipolar molecules (fixed and rotating)... 

The relationships between the various forms of the dipolar Hamiltonian are explained in appendix 8.2. As we see from (1.59), the dipolar interaction has various components in the molecule-fixed axis system but the most important one, and often the only one to be determined from experiment, is Tq(C). This leads us to define a constant to, the axial dipolar hyperfine component, given in SI units by,... 

Conditions are very different in condensed phases, however. As has been shown above all but the ion-ion and ion-induced dipole interactions alter their values upon randomization. The contribution that is most dramatically altered — increased — by fixing the molecules is the dipole-dipole part. We quote from Davies . . predominance of the dispersion energy is a characteristic of non-polar molecules or of the gas phase only. In liquids or solids where the molecules are at much closer distances, the random orientation which reduces the dipole-dipole term to dependence upon r in the gaseous state, is far less likely to be maintained. Dipolar molecules can then assume fixed orientations with respect to one another with greatly increased energies of interaction . Thus dipole-dipole, ion-dipole and to a lesser extent, dipole-induced dipole interactions become more important in condensed phases. (For a comparison between gaseous and solid HCl, see ° p. 166)... 

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