The short N [bond] F bond in N(2)F(+) and how Pauli repulsion influences bond lengths. Theoretical study of N(2)X(+), NF(3)X(+), and NH(3)X(+) (X [double bond] F, H)

Exceptionally short N bond] F bond distances of only 1.217 A (crystal) and 1.246 A (gas phase) have been reported for N(2)F(+), making it the shortest N bond] F bond ever observed. To trace the origin of this structural phenomenon, we have analyzed the model systems N(2)X(+), NF(3)X(+), and NH(3)X(+) (X double bond] F, H) using generalized gradient approximation density functional theory at BP86/TZ2P. In good agreement with experiment, the computations yield an extremely short N bond] F bond for N(2)F(+): we find N bond] F bond distances in N(2)F(+), NF(4)(+), and NH(3)F(+) of 1.245, 1.339, and 1.375 A, respectively. The N bond] 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 bond] p hybridization of N (i.e., sp versus sp(3)) are responsible for the much shorter N bond] F distance in N(2)F(+) compared to NF(4)(+). 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.