infrared spectra of chloride- fluorobenzene complexes in the gas phase: electrostatics versus...
TRANSCRIPT
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…
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Fra
gm
ent I
on S
ignal [
Arb
. Units
]
Photon Energy [cm-1]
Consider Simplest Case: Cl-·C6H6…
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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…
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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
nal [
Arb
. 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!
Frag
men
tIon
Sig
nal[
Arb
.Uni
ts]
Photon Energy [cm ]
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men
tIon
Sig
nal[
Arb
.Uni
ts]
PhotonEnergy[cm ]
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ntIo
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igna
l[A
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]
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men
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Sig
nal[
Arb
.Uni
ts]
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nal[
Arb
.Uni
ts]
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al[A
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]
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n = 0
Cl-·C6F
nH
6-n
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tIon
Sig
nal[
Arb
.Uni
ts]
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.Uni
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Frag
men
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Sig
nal[
Arb
.Uni
ts]
Photon Energy[cm ]
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men
tIon
Sig
nal[
Arb
.Uni
ts]
Photon Energy[cm ]
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men
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nal[
Arb
.Uni
ts]
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n = 0
Cl-·C6F
nH
6-n
… and increase the level of fluorination!
n = 1
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.Uni
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igna
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nal[
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.Uni
ts]
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men
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nal[
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.Uni
ts]
Photon Energy[cm ]
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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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tIon
Sig
nal[
Arb
.Uni
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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
]
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.