1. New Perspective on PF n (n=1-5) from the Recoupled Pair Bonding Model: A Quantum Chemical Study David E. Woon & Thom H. Dunning, Jr. TI12

2. Outline  Overview of recoupled pair bonding and previous studies  I. PF n species that arise from P( 4 S) (3s 2 )  PF – PF 5  II. PF n species that arise from P( 2 D) (3p 2 )  PF – PF 3

3. Overview The PF n family completes our series of studies of bonding in neutral fluorides of P, S, and Cl, where recoupled pair bonding (RPB) has been found to account for the origin of hypervalence in species such as PF 5, SF 4, SF 6, ClF 3, and ClF 5. F(2p)S(3p) Covalent Bond S(3p 2 )F(2p)

4. Overview The PF n family completes our series of studies of bonding in neutral fluorides of P, S, and Cl, where recoupled pair bonding (RPB) has been found to account for the origin of hypervalence in species such as PF 5, SF 4, SF 6, ClF 3, and ClF 5. F(2p)S(3p)S(3p 2 ) Covalent Bond

5. Overview The PF n family completes our series of studies of bonding in neutral fluorides of P, S, and Cl, where recoupled pair bonding (RPB) has been found to account for the origin of hypervalence in species such as PF 5, SF 4, SF 6, ClF 3, and ClF 5. F(2p)S(3p) F(2p) S(3p 2 ) Covalent BondRecoupled Pair Bond 2e – in  2 3e – in  2 +  * GVB overlap: ~0.9 1 st RPB is weaker and longer due to  * occupation 2 nd RPB is stronger and shorter due to using  * (can also occur with 3s 2 pairs)

6. Summary of Recoupled Pair Bonding Studies HCNOF PSCl SeBr SF n WoonJPCA {O,S,Se} X{F,Cl,Br} WoonMP Species1 st AuthorWhere ClF n ChenJPCA LeidingTI07SF n Cl m T.B.S. WoonTI12PF n in press XuTI13H n CF m ChenRE09ClF n +/– TakeshitaTI14{H,F} n SO

7. PF n, SF n, and ClF n Species P S Cl 1 3p 2 – recouples 1 st  SF 3 & SF 4 ground states  SF & SF 2 excited states 3s 2 – recouples 2 nd  SF 5 & SF 6 g.s. 2 3p 2 pair 1  ClF 2 & ClF 3 g.s., ClF, ClF 2 e.s. pair 2  ClF 4 & ClF 5 g.s. 3s 2 pair – gives rise to ClF 6 and ClF 7, but the space around Cl is too crowded to give good bond energies no 3p 2 pair  PF, PF 2, PF 3 g.s. all covalent 3s 2 pair  PF 4 & PF 5 g.s., PF 3 e.s. States from ground state atoms

8. I. PF n Species from P( 4 S) FP P(3p z ) (  7 ) L F(2p z ) (  7 ) R P(3p x ) (  x ) 3 P F D 0 = 106.0 kcal/mol RCCSD(T)/AVQZ PF(X 3  – ) P( 4 S)+F 1.593 Covalent Bond F1

9. I. PF n Species from P( 4 S) D 0 = 118.3 kcal/mol RCCSD(T)/AVQZ PF(X 3  – )+F 1.582 Covalent Bond PF 2 (X 2 B 1 ) 98.2° F P F The third F can add form a covalent bond with the remaining singly occupied P 3p orbital to form ground state PF 3. The third F can also recouple the pair derived from the P 3s 2 pair. F2

10. I. PF n Species from P( 4 S) F P F F D 0 = 131.8 kcal/mol RCCSD(T)/AVQZ PF 2 (X 2 B 1 )+F Covalent Bond PF 3 (X 3 A 1 ) 1.567 97.5° P F F F D e = 42.0 kcal/mol RCCSD(T)/AVQZ PF 2 (X 2 B 1 )+F Recoupled Pair Bond PF 3 (a 3 B 1 ) 1.614 98.5° 1.560 F3

11. I. PF n Species from P( 4 S) F P F F D 0 = 131.8 kcal/mol RCCSD(T)/AVQZ PF 2 (X 2 B 1 )+F Covalent Bond PF 3 (X 3 A 1 ) 1.567 97.5° P F F F F3 Not observed experimentally. Only prior calculations on PF 3 (a 3 B 1 ) were reported by Grant et al., JPCA 112, 3145 (2008).

12. I. PF n Species from P( 4 S) (16a) L (16a) R 17a R PF = 2.60 Å s: 0.81 R PF = 2.10 Å s: 0.87 R PF = 1.70 Å s: 0.87 F3

13. I. PF n Species from P( 4 S) To form PF 4 from PF 3 (X 1 A 1 ), the orbital derived from the 3s 2 pair of P must be recoupled. R  A scan was performed to locate the TS between the two minima and to examine dissociation. PF 4 has two minima: the global minimum where both electrons from the pair are used, and a local minimum where only one of the electrons of the pair is used. PF 4 (X 2 A 1 ) (C 2v ) PF 4 ( 2 A 1 ) (C 3v ) F4

14. I. PF n Species from P( 4 S) R   R PF (Å) D e (kcal/mol) min 1 min 2 TS F4 D 0 = 55.4 kcal/mol RCCSD(T)/AVQZ Recoupled Pair Bond +7.3 kcal/mol

15. I. PF n Species from P( 4 S) PF 3 (17a’) L PF 3 (17a’) R F(2p) PF 3 +F orbitals at large R PF F4

16. I. PF n Species from P( 4 S) PF 3 (17a’) L PF 3 (17a’) R F(2p) As the bond forms, recoupling and rearrangement occur. F4

17. I. PF n Species from P( 4 S) D 0 = 133.3 kcal/mol RCCSD(T)/AVQZ PF 4 (X 1 A 1 )+F Second Recoupled Pair Bond PF 5 (X 1 A 1 ) 1.537 1.577 F5 In the P( 4 S)+nF series, recoupling occurs in PF 3 or PF 4, but there is a second series: P( 2 D)+nF.

18. II. PF n Species from P( 2 D) FP( 2 D) F1 PF(a 1  ) 1.598 Covalent Bond PF(B 3  ) 1.759 Recoupled Pair Bond F (experimentally observed!) +64.6 kcal/mol

19. II. PF n Species from P( 2 D) F2 P( 2 D) FF F F PF(B 3  )+F Second Recoupled Pair Bond PF 2 (A 2 A 1 ) 1.639 PF(B 3  )+F Covalent Bond PF 2 (B 2 B 2 ) 85.0° 1.615 +15.0 kcal/mol

20. II. PF n Species from P( 2 D) F3 P( 2 D) F F F PF 3 ( 1 A 1 ) TS 1.630 1.561 88.0° This is the TS for PF 3 (X 1 A 1 ) inversion.

21. Conclusions As in SF n and ClF n, recoupled pair bonding accounts for the hypervalent species PF 4 and PF 5. There is also an excited state of PF 3 (a 3 B 1 ) with a recoupled pair bond. Recoupling of the 3p 2 pair in P( 2 D) leads to several excited states: PF(B 3  ), PF 2 (A 2 A 1 ), PF 2 (B 3 B 2 ), and the PF 3 ( 1 A 1 ) TS through which PF 3 inverts. Acknowledgment Support for this work was provided by funding from the Distinguished Chair for Research Excellence in Chemistry at the University of Illinois at Urbana-Champaign.