While the two N atoms of a pair of NH3 molecules do not interact attractively, the replacement of one H with a F atom results in a direct N⋯N noncovalent bond, with an interaction energy of 4 kcal/mol, nearly as strong as a NH⋯N H-bond. Cl or Br-substitution also leads to an attractive force between the two N atoms, although the XN⋯N geometry does not represent a true minimum on the potential energy surface. Trimethylation of the electron donor NH3 strengthens both the NH⋯N and XN⋯N bonds.