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?


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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