Exploring (NH2F)2, H2FP:NFH2, and (PH2F)2 Potential Surfaces: Hydrogen Bonds or Pnicogen Bonds?

An ab initio MP2/aug'-cc-pVTZ study has been carried out to identify local minima on the (NH2F)(2), H2FP:NFH2, and (PH2F)(2) potential surfaces, to characterize the types of interactions which stabilize the complexes found at these minima, and to evaluate their binding energies. With one exception, (NH2F)(2) complexes are stabilized by N-H center dot center dot center dot N or N-H center dot center dot center dot F hydrogen bonds. Only one complex, that with the smallest binding energy, has a pnicogen N center dot center dot center dot N bond. In contrast, (PH2F)(2) complexes are stabilized by P center dot center dot center dot P or P center dot center dot center dot F pnicogen bonds or by an antiparallel alignment of the dipole moment vectors of the two monomers, but not by hydrogen bonds. The most stable complex has an F-P center dot center dot center dot P-F alignment which approaches linearity. Both hydrogen-bonded and pnicogen-bonded complexes exist on the H2FP:NFH2 surface, with the most stable being the pnicogen-bonded complex with F-P center dot center dot center dot N-F approaching a linear arrangement. Charge transfer transitions from a lone pair on a P, N, or F atom in one molecule to an antibonding sigma* orbital of the other stabilize these complexes. These transitions are most important for complexes with pnicogen bonds. Although net charge transfer occurs in complexes in which the two monomers are inequivalent, charges on N and P do not correlate with N and P absolute chemical shieldings. Rather, these shieldings also reflect charge distributions and overall bonding patterns. EOM-CCSD two-bond spin spin coupling constants (2h)J(X-Y) across X-H center dot center dot center dot Y hydrogen bonds tend to be small, due in part to the nonlinearity of many of the hydrogen bonds. (1p)J values across a particular kind of pnicogen bond are relatively large and vary significantly but do not correlate with corresponding distances.