Bisulfate Dehydration at the Vapor/Solution Interface Probed by Vibrational Sum Frequency Generation Spectroscopy Aaron M. Jubb and Heather C. Allen 67 th meeting of the International Symposium on Molecular Spectroscopy June 19, 2012 – Columbus, OH

 Raman Change in polarizability with vibration  vis  Stokes Vibrational Spectroscopies Utilized  IR  vis  IR  sum v = 0 v = 1 Virtual excited state  vis Virtual excited state  Anti- Stokes  Infrared Change in dipole moment with vibration  Vibrational Sum Frequency Generation Lack of inversion symmetry Vibration must be both IR and Raman active

What role does cation (H +, Na +, Mg 2+ ) identity play for HSO 4 - behavior? Ion-pair formation? Hydration disruption? How does interfacial behavior compare to bulk behavior? VSFG versus Raman Target ν SS – SO 3 of HSO 4 -

Dehydration versus Ion-pairing Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), Dehydration Breaks H-bonds Shortens S-O bonds Blue shift for ν SS – SO 3 Ion-Pairing Induces polarization Electron withdrawing Lengthens S-O bonds Red shift for ν SS – SO 3

H 2 SO 4 results correspond well with literature Blue shifted from bulk frequency of 1035 cm -1 pH 1 Na 2 SO 4 solutions exhibit bimodal spectra Indicates two distinct populations of HSO 4 - molecules at interface MgSO 4 results feature only blue-shifted peak Entire HSO 4 - interfacial population disrupted VSFG: HSO 4 - Dehydration Ion-Pairing Dehydration A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: /jp302585h

Cations Induce Dehydration Within Interface Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), Dehydration Breaks H-bonds Shortens S-O bonds Blue shift for ν SS – SO 3 Ion-Pairing Induces polarization Electron withdrawing Lengthens S-O bonds Red shift for ν SS – SO 3

 Bulk solution HSO 4 - ν SS – SO 3 frequency increases with: Concentration H + < Na + < Mg 2+  Na 2 SO 4 exhibits higher ν SS – SO 3 intensity than MgSO 4 CIP formation between SO 4 2- and Mg 2+ inhibiting HSO 4 - formation – Kester et al Raman: HSO 4 - Bulk Dehydration A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: /jp302585h

 HSO 4 - ν SS – SO 3 frequency increases: At vapor/solution interfaces With concentration H + < Na + < Mg 2+  ν SS – SO 3 frequency blue-shifts indicate HSO 4 - dehydration  First time VSFG has been used to directly interrogate cation effects on HSO 4 - at vapor/solution interfaces  Implies acid catalyzed aerosol reactions may be inhibited by interfacial cations Summary

Acknowledgements