How does intramolecular hydrogen bonding cause the molecules to be separated from each other?
Molecules with inter-molecular hydrogen bonds tend to associate with one another, while molecules with intra-molecular hydrogen bonds tend to associate with themselves.

Ortho- and para-nitrophenol are often used as examples of molecules with intra- and inter-molecular hydrogen bonds respectively (see picture).



Because of their molecular structures the ortho isomer can form an intramolecular hydrogen bond with itself, whereas the para isomer can only hydrogen bond with another molecule of p-nitophenol.  One would say that the para isomer associates with other p-nitrophenol molecules through hydrogen bonding, whereas the ortho isomer associates with itself through hydrogen bonding.  Because the para isomer associates with other molecules of p-nitrophenol through hydrogen bonding, it takes more energy to break up this association and cause the molecule to melt or boil.  On the other hand, the ortho isomer melts and boils at lower temperatures because there are fewer intermolecular forces (such as the hydrogen bonding) holding the ortho molecules together.