Bonding in ClFn (n=1-7): Further
Insights into Hypervalent Molecules
and Recoupled Pair Bonds
Lina Chen, David E. Woon and Thom H. Dunning
Department of Chemistry
University of Illinois at Urbana-Champaign
Columbus, Ohio
June 25, 2009
Motivation
• Test the robustness of the recoupled pair
bonding (RPB) model
• Obtain the optimized structures and energies
of low-lying states of ClFn and ClF+ species
• Explore the dependence of bond energies on
the choice of the hypervalent atom,
comparing S and Cl.
• Identify factors that influence the bonding
efficiency.
Methodology
• RCCSD(T) : ground states of ClFn (n=1-7) and
low-lying excited states for ClF and ClF2
• MRCI/MRCI+Q: ClF (1S+,3P), ClF+ (4S-, 2P)
• GVB
Guidelines
1S +
Cl
F
90
Cl
Guidelines
1S +
A. Recoupled pair bonding (RPB) makes
two electrons available for bonding.
1. 1st RPB
Cl
Weaker than covalent bond (CB)
3P
Antibonding character long Re
2nd RPB
nd
2. 2 RPB:
Cl
Stronger than 1st RPB
2P
Often stronger than CB
Res (1st + 2nd RPB) < Re(1st RPB)
2nd RPB favored over CB
F Cl
3. Linear or quasilinear for two RPBs
2A
with strong p character orbitals
B. Bonding will rearrange to maximize the
stability
F
90
F
2B
1
F
F
Cl
F
2A
1
Predictions
Results
RCCSD(T)/AVQZ results
Bond length in Å
Bond Energy in kcal/mol
F
4A
2
101.4
1.969
F
Cl
4.1
49.9
1.674
54.0
3P
2A
82.5
1.765
4.8
Cl
67.6
F
-2.7
13.7
1.991
F
56.7
1S +
Cl
F
66.7
2B
1
63.0
1.704
71.4
3.7
61.5
Cl
14.7
F
1.699
F
152.8
Cl 1.708
1A
1
F3
2A
1
49.3
1.599
87.1
F
F1
F2
Cl
1.699
RCCSD(T)/AVQZ results, except AVTZ for ClF7
Bond length in Å
Bond Energy in kcal/mol
Hypervalent Hypervalent w rearrangement
Covalent w antibonding e-
Covalent
ClF4(2A1)
ClF5(1A1)
F
F
F
F
ClF6(2A1g)
F
ClF7(1A1)
F
F
F
F
F
F
10.9
1.5
F5
F5
85.6
1.683Å
F4
F2
Cl
F3
F1-Cl-F3=162.7
1.597Å
F4
F1
F2
Cl
F3
1.660Å
F1
F2
1.572Å
1.676Å
F4
Cl
F3
F6
F1-Cl-F3=171.2
F
-34.9
F1
88.7
F
F
46.8
F
F
F
F
F
Cl
Cll
F
F
F
F5
F1
F3
F4
1.752Å
Cl
F2
F3
F6
Energy (kcal/mol)
Results: Potential energy surfaces
(PES) of ClF
RClF (Angstrom)
RClF (Angstrom)
Potential energy curves for low-lying states of ClF calculated at MCSCF, MRCI+Q and
RCCSD(T) levels with AV5Z basis sets. (a) 1Σ+ ground state; (b) 3П excited state.
•ClF(3P) is not bound at the MCSCF level
Results: 2D GVB orbitals of ClF (3П)
•AVTZ level
Results: 2D GVB orbitals of ClF (3П) in
comparison with SF(4S-)
GVB: 7sL
Cl
GVB: 7sR
F
Cl
F
NO: 8s
Cl
F
2.01 Å
(Re)
s=0.91
1.61 Å
s=0.92
For ClF (3P), recoupling continues after R < Re
S
F
S
s=0.91
•AVTZ level
F
S
F
1.90 Å
(Re)
Comparison: PES of SF and ClF
Energy (kcal/mol)
Hypervalent
Te (kcal/mol)
Covalent
SF(2P  4S-)
ClF(1S+  3P)
RClF (Angstrom)
47.1
56.7
Oscillating Bond Energies in SFn and ClFn
SF5-F
SF3-F
SF-F
Energy (kcal/mol)
S-F
SF2-F
Cl-F
2st RPB SF -F
4
1st RPB
ClF4-F
ClF2-F
ClF-F
ClF3-F
Conclusions
• Trends in the calculated geometries and energies agree with
predictions using recoupled pair bonding model.
• Similar oscillating trends are found in both the SFn and ClFn
series, and the differences are consistent with the difference
between S and Cl at the atomic level.
• Future work:
a. PFn series as well as various combinations of P, S, Cl and F
with other ligands such as monovalent H, Cl, and OH and
divalent O.
b. Reactions: ClF+F2  ClF3
Acknowledgment
• Funded by the Distinguished Chair for Research Excellence in
Chemistry at the University of Illinois at Urbana-Champaign.