# For polar molecules, when does hydrogen bonding occur?

And three molecules may be used as exemplars: ${H}_{2} O$; $N {H}_{3}$; and $H F$....and a special type of bond polarity, of charge separation operates......
For each molecule we could represent the dipoles, the charge separation this way: $\stackrel{\delta +}{H} - \stackrel{\delta -}{O} - \stackrel{\delta +}{H}$; $\stackrel{\delta -}{N} {\stackrel{\delta +}{H}}_{3}$; $\stackrel{\delta +}{H} - \stackrel{\delta -}{F}$. In the bulk solvent, the dipoles line up in solution, and in aggregate this constitutes a potent intermolecular force.
If you interrogate the boiling points of these molecules (and you should, because as a physical scientist always must consider the actual data), certainly you will find the boiling points anomalously high. Certainly they are high compared to homologous, ${H}_{2} S$, $P {H}_{3}$, and $H C l$ for which hydrogen bonding does not operate so strongly, and dispersion forces (as might be expected for larger, many electron, molecules) are not large enuff to compensate.