Simulation of hydrogen bonding in biological systems: Ab initio calculations for NH3NH3 and NH3NH4+

Ab initio calculations by Pople's 4–31G method are reported for (NH3)2 and for NH3(NH4−) in order to study the effects of geometric distortions upon NHN hydrogen bonds. The optimum nitrogen-nitrogen separations are found to be 3.37 and 2.79 Å for linear hydrogen bonds in the ammonia dimer and ammonia ammonium systems, respectively. The variation in energy with distance is well fit by Morse functions in both cases. Energy losses due to displacements of the electron donating NH3 and due to nonlinearity in the N–H–N bond are also discussed.