Unusual acid-base properties of the P₄ 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 P₄ and the FH, ClH, and FCl molecules. P₄ has three unique interaction sites: at a vertex (designated the P₁ atom); at an edge (the P₂-P₃ bond); and at the P₂-P₃-P₄ face. The uniqueness of molecular P₄ is its ability to act as an electron donor and an electron acceptor at the same site, except for the P₂-P₃ bond, which is only an electron donor. FCl and FH form five different complexes with P₄, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P₄ and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P₄ molecule acting as the base are greater than the binding energies when P₄ is the acid. Thus, P₄ 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 P₄ complexes. Relative to ¹J(P_i-P_j) in molecular P₄, ¹J(P₁-P₂) coupling constants decrease in absolute value and ¹J(P₂-P₃) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a PF halogen bond. Absolute values of ¹J(P₁-P₂) increase and those of ¹J(P₂-P₃) decrease in hydrogen-bonded complexes and complexes with PCl halogen bonds. ¹J(P₁-P₂) and ¹J(P₂-P₃) exhibit a single linear correlation with the corresponding P_i-P_j distances.