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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