Infrared Spectra of Chloride-Fluorobenzene Complexes in the Gas

Phase:

Electrostatics versus Hydrogen Bonding

Holger Schneider

OSU

International Symposium on Molecular Spectroscopy

June 20th

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Questions: • binding motifs?

• competition between different binding sites?

Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects

Questions: • binding motifs?

• competition between different binding sites?

Needed: structural information

Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects

Questions: • binding motifs?

• competition between different binding sites?

Needed: structural information

Possible Tool: infrared spectroscopy

Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects

Questions: • binding motifs?

• competition between different binding sites?

Needed: structural information

Possible Tool: infrared spectroscopy

Model system in this study: Cl-·C6F

nH

6-n (n = 1-5)

Beer et al, Angew. Chem. Int. Ed., 40, 486, 2001

Motivation: Anion Recognition

Hydrogen bonds

ElectrostaticsLewis acids

Hydrophobic effects

Experimental Realization: Infrared Predissociation Spectroscopy

Step 1: Ion formation and mass selection

[A-·Bn]

Step 1: Ion formation and mass selection

Step 2: Exposure to infrared light

h[A-·Bn]

*

Experimental Realization: Infrared Predissociation Spectroscopy

[A-·Bn]

Step 1: Ion formation and mass selection

Step 2: Exposure to infrared light

[A-·Bn-m] + Bm

Step 3: Registration of fragment ions

Experimental Realization: Infrared Predissociation Spectroscopy

[A-·Bn]

h[A-·Bn]

*

Mechanism:

one-photon-dissociation only if EB[A-·Bn] < h

[A-·Bn] h

[A-·Bn]* [A-·Bn-m] + m·B

Mechanism:

[A-·Bn] h

[A-·Bn]* [A-·Bn-m] + m·B

[A-·Bn·Arm] h

[A-·Bn·Arm]* [A-·Bn] + m·Ar

one-photon-dissociation only if EB[A-·Bn] < h

Attachment of a weakly bound “messenger” atom (e.g. Ar)Alternative:

Advantages:

- one-photon-dissociation

- production of cold clusters

Disadvantage:

- difficult to produce

Power M eter

Reflectron

Entra inm entSource

Ion B eamIon D etector

M ass G ateIon O ptics

E lectron G un

IR -O P O

N d:YA G2200 - 3800 cm -1

Experimental Setup

Entrainment Source:

• W.H. Robertson, M.A. Kelley, M.A. Johnson, Rev. Sci. Inst., 71, 4431, 2000

• Weber J.M., Schneider H., J. Chem. Phys., 120, 10056, 2004

General Considerations: Cl-·C6F

nH

6-n

1. EACl: 3.61 eV(1) >> EAC6F6: 0.53 eV(2)

negative charge will be predominantly localized on chloride

fluorobenzenes will serve as ligands with planar ring structures

General Considerations: Cl-·C6F

nH

6-n

(1): Berzinsh, U. et al, Phys. Rev. A, 1995, 51, 231(2): Miller, T.M et al, Int. J. Mass Spectrom., 2004, 233, 67-73

2. Consider interaction of aromatic charge distribution with anion:

• Quinonero et al., Angew. Chem. Int. Ed., 41, 2001

• Hiraoka, K. et al, J. Phys. Chem., 91, 529,1987

• Loh et al, J. Chem. Phys., 119, 9559, 2003

General Considerations: Cl-·C6F

nH

6-n

1. EACl: 3.61 eV(1) >> EAC6F6: 0.53 eV(2)

negative charge will be predominantly localized on chloride

fluorobenzenes will serve as ligands with planar ring structures

- anion binds to hydrogen

atoms at the periphery of the ring

(Loh et al, J. Chem. Phys., 119, 9559,

2003)

- red shift of hydrogen bonding

CH oscillators

- anion binds to positive partial

charge in the ring

(Quinonero et al., Angew. Chem. Int. Ed., 41,

2001;

Hiraoka, K. et al, J. Phys. Chem., 91, 529,

1987)

- minor influence on bonds of

the aromatic ring

General Considerations: Cl-·C6F

nH

6-n

• at what degree of fluorination does the binding motif switch?

• how will different distributions of the fluorine atoms around

the ring influence the binding motif?

Questions:

General Considerations: Cl-·C6F

nH

6-n

Consider Simplest Case: Cl-·C6H6…

Two possible binding motifs:

Linear Bifurcated

Loh et al, J. Chem. Phys., 119, 9559, 2003

Two possible binding motifs:

Linear Bifurcated

Loh et al, J. Chem. Phys., 119, 9559, 2003

Consider Simplest Case: Cl-·C6H6…

2400 2600 2800 3000 3200

Fra

gm

ent I

on S

ignal [

Arb

. Units

]

Photon Energy [cm-1]

Consider Simplest Case: Cl-·C6H6…

2400 2600 2800 3000 3200

Fra

gm

ent I

on S

ignal [

Arb

. Units

]

Photon Energy [cm-1]

1

2

3

1: overlap of 20 and 7 (Wilson numbering of benzene)

2: 2mode

3: combination band (Fermi interaction)

Consider Simplest Case: Cl-·C6H6…

Consider Simplest Case: Cl-·C6H6…

2400 2600 2800 3000 3200

Fra

gmen

t Ion

Sig

nal [

Arb

. Uni

ts]

Photon Energy [cm-1]

• spectra complicated due to interaction of

several CH stretch modes (Fermi resonances)

• large intensity only due to H bonding of CH

groups to the chloride anion

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Fra

gmen

t Ion

Sig

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. Uni

ts]

Photon Energy [cm-1]

... the consequences for Cl-·C6FnH6-n:

• spectra complicated (e.g. by combination bands / Fermi resonances)

• intense features centered around H bonded groups: allows structural interpretation!

Consider Simplest Case: Cl-·C6H6…

• spectra complicated due to interaction of

several CH stretch modes (Fermi resonances)

• large intensity only due to H bonding of CH

groups to the chloride anion

… and increase the level of fluorination!

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men

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Sig

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Arb

.Uni

ts]

Photon Energy [cm ]

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n = 0

Cl-·C6F

nH

6-n

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.Uni

ts]

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.Uni

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n = 0

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

n = 1

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n = 0

n = 1

n = 2

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

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n = 0

n = 1

n = 2

n = 3

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

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n = 0

n = 1

n = 2

n = 3

Dramatic change

in red shift!

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

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men

tIon

Sig

nal[

Arb

.Uni

ts]

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n = 0

n = 1

n = 2

n = 3

n = 4

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

Dramatic change

in red shift!

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men

tIon

Sig

nal[

Arb

.Uni

ts]

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n = 0

n = 1

n = 2

n = 3

n = 4

n = 5 Red shifted CH stretch vibration

Still hydrogen bonding!

Cl-·C6F

nH

6-n

… and increase the level of fluorination!

Dramatic change

in red shift!

IR Spectra of Cl-·C6FnH6-n

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n = 0

n = 1

n = 2

n = 3

n = 4

n = 5

Cl-·C6F

nH

6-n

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n = 0

n = 1

n = 2

n = 3

n = 4

n = 5

Absorption ~ 3000 cm-1

Absorption ~ 2600 cm-1

Cl-·C6F

nH

6-n

IR Spectra of Cl-·C6FnH6-n

only linear binding motif possible

linear and bifurcated binding motif possible

2400 2600 2800 3000 3200 3400

Ph

oto

fra

gm

en

t Sig

na

l [a

rb. u

nits

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Photon Energy [cm-1]

Cl-·C6H

6·Ar

Cl-·C6F

5H·Ar

IR Spectra of Cl-·C6H

6 (top trace) vs. Cl-·C

6F

5H (bottom trace)

2 3 4

0

1

2

3

4

5

C6F

5

-HCl

E

[eV

]

dist C-H [a.u.]

C6F

5HCl-

Calculated potential energy surface along the CH stretching

coordinate in [C6F

5···H···Cl]- (MP2/TZVP)

Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

CH = 2544 cm-1

CH, bound = 2586 cm-1

CH, free = 3091 cm-1

CH, asymm = 2999 cm-1

CH, symm = 3006 cm-1

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

intense

Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

intense

Calculations (DFT, B3-LYP, TZVP, scaled harmonic frequencies):

Comparison to Experimental Spectrum

2400 2600 2800 3000 3200 3400

0,000

Fra

gmen

t Ion

Sig

nal [

Arb

. Uni

ts]

Photon Energy [cm-1]

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

2400 2600 2800 3000 3200 3400

0,000

Fra

gmen

t Ion

Sig

nal [

Arb

. Uni

ts]

Photon Energy [cm-1]

Comparison to Experimental Spectrum

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

2400 2600 2800 3000 3200 3400

0,000

Fra

gmen

t Ion

Sig

nal [

Arb

. Uni

ts]

Photon Energy [cm-1]

Comparison to Experimental Spectrum

ν [cm-1] Erel [meV]

2982 0

3036

3088

ν [cm-1] Erel [meV]

2656 ~170

3066

3079

So How About The Ring Bound Isomer After All?

• doesn’t show up in any of the recorded infrared spectra

• but is a stable structure according to our calculations, for Cl-·C6F5H it

lies ~ 300 meV above the ground state (DFT, B3-LYP, TZVP)

Summary / Interpretation

• no change in binding motif until perfluorobenzene (Even for highest fluorination degree under study hydrogen bonding is preferred to binding to the top of the ring!)

• hydrogen atoms become more and more acidic with increasing fluorination

(red shift of CH bands increases)

• calculations support these assignments

Thanks to...

• Prof. J.M. Weber

• Kristen Vogelhuber

• Financial support by

- NSF (JILA Physics Frontier Center)

- Petroleum Research Fund

• ... all of you for your attention!!!

CH Stretch region of benzene

Benzene Fermi tetrad

Page et al., J. Chem. Phys., 88, 4621 (1988)

Page et al., J. Chem. Phys., 88, 5362 (1988)

Normal modes of benzene (Wilson‘s numbering)

Page et al., J. Chem. Phys., 88, 5362 (1988)

Quinonero et al., Angew. Chem. Int. Ed., 41, 3389 (2002)

Bryantsev et al., Org. Lett., 7, 5031 (2006)

16

543

2

7

12

10

9 11

8

16

543

2

7

12

10

9 11

8

1: -0.10 7: 0.18

2: 0.19 8: -0.17

3: 0.09 9: -0.19

4: 0.11 10: -0.19

5: 0.09 11: -0.19

6: 0.19 12: -0.17

Cl-: -0.83

1: -0.21 7: 0.18

2: 0.23 8: -0.17

3: 0.11 9: -0.16

4: 0.15 10: -0.15

5: 0.11 11: -0.16

6: 0.23 12: -0.17

Mulliken population analysis of Cl-·C6F5H and neutral C6F5H

ν [cm-1]

3103

3159

3213

ν [cm-1]

2764

3190

3204

Illustration of electrostatic potential around C6H6 (left) and C6F6 (right)

(calculated with Gaussian03W, HF/3-21G* level)

The color coding from blue to red represents positive to negative electrostatic potentials.