vibrational predissociation spectra in the shared proton region of protonated formic acid wires:...
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Vibrational Predissociation Spectra in the Shared Vibrational Predissociation Spectra in the Shared Proton Region of Protonated Formic Acid Wires:Proton Region of Protonated Formic Acid Wires:
Characterizing Proton Motion in Linear H-Bonded NetworksCharacterizing Proton Motion in Linear H-Bonded Networks
Helen K. Gerardi6/24/2010
The 65th International Symposium on Molecular Spectroscopy
Proton Transport Mechanism
What are the spectroscopic signatures of large amplitude motion along the proton conduction pathway?
Proton Exchange Membrane Fuel Cell
K. Schmidt-Rohr, Q. Chen, Nat. Mater. 7, 75-83 (2008)
Proton Transport in PEM Membrane
Biological Energy Conversion: Bacteriorhodopsin
Proton channel in gramicidin A
Grotthuss mechanism
N N
C C
C
N N
C C
C
N N
C C
C
O
O
C
Outline of Talk
Goal: Spectral characterization of mobile proton in H-bonded clusters
• Method used to obtain resolved structure of intermolecular proton
• Application to proton motion in protonated imidazole clusters (N–H·····N)
• Application to proton motion in protonated formic acid clusters (O–H·····O)
Identification of low-frequency modes
• Effect of cluster size on vibrational features in spectra of protonated formic acid complexes.
Spectroscopic Signatures of Shared Proton
• Little known about vibrational character of active protons in molecular wires
• A challenge to characterize even a single localized proton
O H+
OCH2CH3
CH2CH3
CH3CH2
CH3CH2
Stoyanov and Reed, J. Phys. Chem. A, 2006
Ab
so
rpti
on
1000 1500 2000 2500
Wavenumber (cm-1)
Argon Vibrational Predissociation Spectroscopy
mass
Generate Clusters in Supersonic
Expansion with Ar
ExciteWith Laser
h
kIVR
kevap
a) b)
c)d) mass
Separate in TOFand Isolate Mass
SecondaryMass Spec
mass
photofragments
Pred
isso
ciat
ion
Yie
ld
1000 1400 1800 2800 3200 3600
Photon Energy, cm-1
• Generating target with Ar-tag ensures vibrationally “cold” target via sequential Ar evaporations (i.e. energy of target below the binding energy of Ar)
• The action spectra recovered in this method are directly comparable to calculated IR absorption spectra
Spectroscopic Signatures of Shared Proton
Vibrational Predissociation Spectroscopy
[RH+·Ar] + h → [RH+] + Ar
O H+
OCH2CH3
CH2CH3
CH3CH2
CH3CH2
1000 1500 2000 2500 3000 3500
Wavenumber (cm-1)
Stoyanov and Reed, J. Phys. Chem. A, 2006
Ab
so
rpti
on
Ar
Pre
dis
so
cia
tio
n Y
ield
Roscioli and Johnson, Science, 2007
Formation of Protonated Imidazole Clusters, ImH+
Ar (40 psig) + Trace H2Im
1 keVelectron beam
T.O.F.
kV
Mass Spec.
To
135 °C
kV
Ar (40 psig) + Trace H2Im
1 keVelectron beam
T.O.F.
kV
Mass Spec.
To
135 °C
kV
TOF
H3+·Arm
+ Im → ImH+·Arn + H2 + (m-n)·ArImH+·Arn + Im→ Im2H+·Aro + (n-o)·Ar
1 2 3 4
3 4 5
5 6 7
6 7 8 9
90Ion Time of Flight (μs)
75 80 85 95 100
= p, Im3H+·Arp
= m, H3+·Arm
= o, Im2H+·Aro
= n, ImH+·Arn
N N
C C
CImidazole
(Im)
3000 3200 3400 3600 3800Photon Energy (cm-1)
Neutral Im N−H
Im3H+·Ar, loss Ar
N
N N
N N
N
νC-H
νN-H
N
N
N
N
Free N-H stretching modes return to neutral transition energies
Not able to directly probe shared proton vibration for ImnH+·Ar complexes
symmetric structure
B3LYP
6-311G(d,p)
1.0 1.2 1.4 1.6
Proton Transfer Coordinate RN-H (Å)
N
N
N
N
Im2H+·Ar, loss Ar
barrier to PT below ZPVE
Shared Proton in Protonated Imidazole Clusters (N–H····N)
192 cm-1
1.0 1.2 1.4 1.6RN-H (Å)
401 cm-1
Tatara et al. J.Phys. Chem. A 107 (2003) 7827-31.
Vibrational Spectra of Protonated Formic Acid Clusters
Wavenumber (cm-1)
Formation of Protonated Formic Acid Clusters, H+
(HCOOH)nH3O+·Arn + HCOOH → H+(HCOOH)·Arm + H2O + (n-m)·Ar
H+(HCOOH)·Arn + HCOOH → H+(HCOOH)2 · Arm + (n-m)Ar
Ar (40 psig) + Trace H2Im
1 keVelectron beam
T.O.F.
kV
Mass Spec.
To
135 °C
kV
Ar (40 psig) + Trace H2Im
1 keVelectron beam
T.O.F.
kV
Mass Spec.
To
135 °C
kV
Ar (~40 psi)
100 125 150 175 200 225 250 275 300 325 350
H+(HCOOH)n · Arm
TOF
m/q
HCOOH/ H2O
Protonated Formic Acid
H+(HCOOH) Ar predissociation Spectrum
800 1200 1600 2000 2400 2800 3200 3600
Photon Energy (cm-1)
Extra features due to different Ar
binding sites as determined by
isomer selective MS3IR2
C–O 1105 C=O 1776
C–H 2943
O–H 3570
Neutral HCOOH vibrational energies (Blagoi and co-workers, Spectrochimica Acta, Vol. 50A, No. 6, 1994)
800 1200 1600 2000 2400 2800 3200 3600
One Shared Proton: Protonated Formic Acid Dimer
Photon Energy (cm-1)
1000 1500 2000 Wavenumber (cm-1)
O H+
OCH2CH3
CH2CH3
CH3CH2
CH3CH2
Roscioli, Science, 2007
H+(HCOOH) ·Ar Loss Ar
H+(HCOOH)2·Ar Loss Ar
Protonated Formic Acid Dimer: Isomer Complications
Experimental Spectrum
Ar- bound OH stretch
free OH stretch
Ar- bound OH stretch
free OH stretch
(O-H-O) stretch
3429
3503
3583
905
998
1371
1235
800 1000 1200 1400 1600 1800 3600800 1000 1200 1400 1600 1800 3400 3600 3800
Photon Energy (cm-1)
IR spectra from DFT
calculations on lowest energy
isomers
(O-H-O) stretch
Isomer I
Isomer II
Isotope Study: Mono-Deuterated Protonated Formic Acid DimerTheory/Basis Set: B3LYP/aug-cc-pVDZ
D
D
Experimental Spectrum of
mono-deuterated H+(HCOOH)2
500 1000 1500 2000 2500 3000 3500 4000500 1000 1500 2000 2500 3000 3500 4000
Photon Energy (cm-1)
Calculated spectrum Isomer I
Calculated spectrum Isomer II
800 1200 1600 2000 2400 2800 3200 3600
Ar
Pre
diss
ocia
tion
Yie
ld
Photon Energy (cm-1)
Effects of Increasing Chain Length: Localization of Excess Charge
return to neutral νC=O
Photon Energy (cm-1)
similar to what we observe in
H+(H2O)n networks
n = 9
n = 10
return to neutral νO-H
Headrick, Science, 308, 2005
return to neutral νC-O
Conclusions and Future Work
From protonated imidazole wires:
• Protonated imidazole dimer acts as a symmetric complex even though equilibrium structure is a double-minimum
• Systematic blue-shift of N-H stretch to higher energies towards that of neutral imidazole
• Make another attempt to obtain low-frequency spectra for these complexes
From protonated formic acid wires:
• Many isomers in play even for monomer, H+(HCOOH)
• Sharp spectral features recovered in 800-1000 cm-1 range for the dimer complex attributed to parallel stretching mode of shared proton.
• Increasing chain length of the formic acid chains results in trend from n=3 -5 toward neutral formic acid spectrum, with broad features in 2600-3200 cm-1 region observed previously for large water networks isolated in gas phase
Acknowledgments
The Johnson Group
• Usha Viswanathan• Scott Auerbach
Collaborators at UMass:
• Chris Leavitt• George Gardenier• Mark Johnson
IR-IR Depletion Data for Monomer HCOOH2
3200 3250 3300 3350 3400 3450 3500 3550 3600
Pre
dis
s. Y
ield
Photon Energy, cm-1
Ion D
ip S
ignal
Probe 3540 cm-1
Probe 3463 cm-1
Probe 3320 cm-1
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