The Short N−F Bond in N₂F⁺ and How Pauli Repulsion Influences Bond Lengths. Theoretical Study of N₂X⁺, NF₃X⁺, and NH₃X⁺ (X = F, H)

Exceptionally short N−F bond distances of only 1.217 Å (crystal) and 1.246 Å (gas phase) have been reported for N2F+, making it the shortest N−F bond ever observed. To trace the origin of this structural phenomenon, we have analyzed the model systems N2X+, NF3X+, and NH3X+ (X = F, H) using generalized gradient approximation density functional theory at BP86/TZ2P. In good agreement with experiment, the computations yield an extremely short N−F bond for N2F+:  we find N−F bond distances in N2F+, NF4+, and NH3F+ of 1.245, 1.339, and 1.375 Å, respectively. The N−X bonding mechanisms are quantitatively analyzed in the framework of Kohn−Sham MO theory. At variance with the current hypothesis, reduced steric and other Pauli repulsion (of substituents or lone pairs at N with F) rather than the extent of s−p hybridization of N (i.e., sp versus sp3) are responsible for the much shorter N−F distance in N2F+ compared to NF4+. The results for our nitrogen compounds are furthermore discussed in the more general context of how bond lengths are determined by both bonding and repulsive orbital interactions.