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This is a problem from the 2016 International Chemistry Olympiad (IChO):

Which of $\ce{NF3}$, $\ce{NHF2}$, and $\ce{NH2F}$ condenses at the lowest temperature?

The answer book says that it is $\ce{NF3}$. But I don't understand why. In $\ce{NF3}$, $\ce{N}$ should have the greatest positive charge of the three molecules; therefore, it should have the highest b.p., no?

Maybe the answer lies in somewhere in the geometry of ions. Can't figure out what. Any hint?

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The interactions do not limit to only dipole-dipole, which means positive charge is not everything.

What's special about $\ce{NF3}$? It has no hydrogens. As a result, it cannot form hydrogen bond, therefore its boiling point (or condense point) is far lower than the others'.

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