Vibrational Predissociation Spectra in the Shared Proton Region of Protonated Formic Acid Wires: Characterizing Proton Motion in Linear H-Bonded Networks Helen K. Gerardi 6/24/2010 The 65 th 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, (2008) Proton Transport in PEM Membrane Biological Energy Conversion: Bacteriorhodopsin Proton channel in gramicidin A Grotthuss mechanism

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 Stoyanov and Reed, J. Phys. Chem. A, 2006 Absorption Wavenumber (cm -1 )

Argon Vibrational Predissociation Spectroscopy mass Generate Clusters in Supersonic Expansion with Ar Excite With Laser h k IVR k evap a)b) c)d) mass Separate in TOF and Isolate Mass Secondary Mass Spec mass photofragments Predissociation Yield 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 Wavenumber (cm -1 ) Stoyanov and Reed, J. Phys. Chem. A, 2006 Absorption Ar Predissociation Yield Roscioli and Johnson, Science, 2007

Formation of Protonated Imidazole Clusters, ImH + TOF H 3 + ·Ar m + Im → ImH + ·Ar n + H 2 + (m-n)·Ar ImH + ·Ar n + Im→ Im 2 H + ·Ar o + (n-o)·Ar Ion Time of Flight ( μs ) = p, Im 3 H + ·Ar p = m, H 3 + ·Ar m = o, Im 2 H + ·Ar o = n, ImH + ·Ar n N N C C C Imidazole (Im)

Photon Energy (cm -1 ) Neutral Im N−H Im 3 H + ·Ar, loss Ar N N N N N N ν C-H ν N-H N N N N Free N-H stretching modes return to neutral transition energies Not able to directly probe shared proton vibration for Im n H + ·Ar complexes symmetric structure B3LYP 6-311G(d,p) Proton Transfer Coordinate R N-H (Å) N N N N Im 2 H + ·Ar, loss Ar barrier to PT below ZPVE Shared Proton in Protonated Imidazole Clusters (N–H····N) 192 cm R N-H (Å) 401 cm -1 Tatara et al. J.Phys. Chem. A 107 (2003)

Vibrational Spectra of Protonated Formic Acid Clusters Wavenumber (cm -1 )

Formation of Protonated Formic Acid Clusters, H + (HCOOH) n H 3 O + ·Ar n + HCOOH → H + (HCOOH)·Ar m + H 2 O + (n-m)·Ar H + (HCOOH)·Ar n + HCOOH → H + (HCOOH) 2 · Ar m + (n-m)Ar Ar (~40 psi) H + (HCOOH) n · Ar m TOF m/q HCOOH / H 2 O

Protonated Formic Acid H + (HCOOH) Ar predissociation Spectrum Photon Energy (cm -1 ) Extra features due to different Ar binding sites as determined by isomer selective MS 3 IR 2 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)

One Shared Proton: Protonated Formic Acid Dimer Photon Energy (cm -1 ) Wavenumber (cm -1 ) 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 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 Dimer Theory/Basis Set: B3LYP/aug-cc-pVDZ D D Experimental Spectrum of mono-deuterated H + (HCOOH) Photon Energy (cm -1 ) Calculated spectrum Isomer I Calculated spectrum Isomer II

Ar Predissociation Yield Photon Energy (cm -1 ) Effects of Increasing Chain Length: Localization of Excess Charge return to neutral ν C=O Photon Energy (cm -1 ) similar to what we observe in H + (H 2 O) n networks n = 9 n = 10 return to neutral ν O-H Headrick, Science, 308, 2005 return to neutral ν C-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 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 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 HCOOH Prediss. Yield Photon Energy, cm -1 Ion Dip Signal Probe 3540 cm -1 Probe 3463 cm -1 Probe 3320 cm -1 * * *