Unusual Acid-base Properties of the P Molecule in Hydrogen-, Halogen-, and Pnicogen-bonded Complexes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate hydrogen bonding, halogen bonding, and pnicogen bonding involving tetrahedral P and the FH, ClH, and FCl molecules. P has three unique interaction sites: at a vertex (designated the P atom); at an edge (the P-P bond); and at the P-P-P face. The uniqueness of molecular P is its ability to act as an electron donor and an electron acceptor at the same site, except for the P-P bond, which is only an electron donor. FCl and FH form five different complexes with P, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P molecule acting as the base are greater than the binding energies when P is the acid. Thus, P is a better electron donor than an electron acceptor. Charge-transfer interactions and EOM-CCSD spin-spin coupling constants across hydrogen, halogen, and pnicogen bonds are reported for all of the P complexes. Relative to J(P-P) in molecular P, J(P-P) coupling constants decrease in absolute value and J(P-P) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a PF halogen bond. Absolute values of J(P-P) increase and those of J(P-P) decrease in hydrogen-bonded complexes and complexes with PCl halogen bonds. J(P-P) and J(P-P) exhibit a single linear correlation with the corresponding P-P distances.