Recoupled pair bonding in PF_n (n=1-5)

Following our previous studies of hypervalency in SF_n (n = 1-6) and ClF_n (n = 1-7), we have characterized the structures and energetics of PF_n (n = 1-5) species with RCCSD(T) coupled cluster calculations and triple- and quadruple-ζ quality correlation consistent basis sets. The prior studies demonstrated that hypervalent bonding occurs when it is energetically favorable to uncouple a pair of electrons to form new bonds, a process we describe as recoupled pair bonding. In contrast to S and Cl, ground state P(⁴S) has no 3p² pairs that can be recoupled, but the 3s² pair of all three elements is susceptible to recoupled pair bonding when more energetically accessible bonding pathways have been exhausted. We found that this can first occur when F is added to PF ₂(X²B₁), which yields PF₃(X ¹A₁) via normal covalent bonding but yields PF ₃(a³B₁) via recoupled pair bonding. PF ₃(a³B₁) lies 92.1 kcal/mol above PF ₃(X¹A₁) but is still bound by 42.0 kcal/mol with respect to PF₂(X²B₁) + F at the RCCSD(T)/aug-cc-pVQZ level. We characterized both of the isomers of PF ₄: the more stable and familiar one that has two covalent equatorial bonds and two axial hypervalent bonds (that use both electrons of the recoupled 3s² pair) and the less-studied one that has three covalent bonds and only one hypervalent bond. The transition state between these two minima was also located. In addition to the states that can be formed from P( ⁴S), there is another group of low-lying excited state species that can be formed from P(²D) via various combinations of covalent and recoupled pair bonding. Additions of the latter type include PF(B ³Π) formed from P(¹D) + F and PF₂(B ²B₂) formed from either PF(a¹Δ) + F or PF(B³Π) + F.