A strong dipole-dipole attraction between molecules with a hydrogen atom bonded to a very electronegative atom (N, O or F). This leaves a very positive hydrogen and when you have a N O or F on another molecule its lone pair interacts with the hydrogen molecule and creates a strong intermolecular bond. You need a hydrogen bonded to N, O, or F for a positive dipole and a lone pair from a very electronegative atom (N, O, or F) for a negative dipole.
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