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 center dot center dot center dot N noncovalent bond, with an interaction energy of 4 kcal/mol, nearly as strong as a NH center dot center dot center dot N H-bond. Cl or Br-substitution also leads to an attractive force between the two N atoms, although the XN center dot center dot center dot N geometry does not represent a true minimum on the potential energy surface. Trimethylation of the electron donor NH3 strengthens both the NH center dot center dot center dot N and XN center dot center dot center dot N bonds. (C) 2011 Elsevier B.V. All rights reserved.