Protonated Water Clusters Revisited: Investigating the Elusive Excess Proton Vibrational Signature using Cryogenic Ion Spectroscopy Joseph Fournier, Christopher Johnson, Conrad Wolke, Arron Wolk, Gary Weddle, and Mark Johnson 69 th International Symposium on Molecular Spectroscopy June 19, 2014
“Eigen”“Zundel”“Eigen” H9O4+H9O4+ H5O2+H5O2+ H9O4+H9O4+ Mobility of the Excess Proton in Water
Vibrational Signatures of the Eigen and Zundel Motifs: Johnson, Duncan, Jordan, Science, 308, 1764 (2005)
Mass Spectrum of H + (H 2 O) n : The “Magic” H + (H 2 O) 21 Cluster Ion Intensity (arb. unit) Time (μs) * n=21
Vibrational Spectra of Larger Clusters and the “Magic” n=21 MAJ, MAD, KDJ, Science, 304, 1137 (2004) AD AAD
Photon Energy (cm -1 ) H3O+H3O+ F S A Spectroscopic Evidence of the Dodecahedral H + (H 2 O) 21 Structure: Where are the Predicted Surface-Bound Eigen Bands? NO Bands in Sight!!!
Photon Energy (cm -1 ) Vibrational Spectra Near the Magic n=21 Cluster: Water Loss vs H 2 Predissociation from Clusters formed in a Supersonic Jet Still No Proton Bands! A. Fujii, J. Phys. Chem. Lett. 2, 2130 (2011) Need to Search the Spectroscopic Badlands! and have COLDER ions AD
Flash Forward 10 Years: Can We Recover the Excess Proton Bands using Cryogenically Processed Ions? ESI Needle Temperature Controlled Ion Trap Mounted to He Cryostat Ion Optics 2m Flight Tube Wiley- McLaren TOF Reflectron-1 Coaxial TOF Reflectron-2 e-e- DC-Turning Quad Pulsed Valve Electron Gun Photoelectron Imager MCP Detector MCP Detector Nd:YAG OPO/OPA Tunable IR cm -1 Nd:YAG OPO/OPA Tunable IR cm -1 Nd:YAG Ion Optics Differential Aperture Skimmers Heated Capillary RF-Ion Guides Instrument Control Wavemeter
ESI Needle RF-Ion Guides Time of Flight ( s) 30 ms 50 ms 40 ms 20 ms 10 ms doubly-charged parent a) b) c) d) e) trap residence time: Signal Intensity (arbitrary units) hydrogen adduct formation * Cryogenic Ion Trap Temperature Controlled Helium Cryostat 4 to 300 K 10% H 2 or D 2 in He Buffer Gas Cooled close to vibrational zero point Condense lightly bound “tags” Cryogenic Ion Vibrational Predissociation Spectroscopy
Photon Energy (cm -1 ) Librations H 2 O bends ν bound AAD ν DDA ν free AAD A B C D E F G H ν a H3O+H3O+ ν umb H3O+H3O+ ν s H3O+H3O+ ν a H3O+H3O+ ν s H3O+H3O+ D 2 -Tagging in a Cryogenically Cooled Ion Trap Torrent-Sucarrat and Anglada, JCTC, 7, 467 (2011) Harmonic H 2 O Loss H 2 tag, jet D 2 tag, ion trap Librations Bend
Photon Energy (cm -1 ) A B C D E F G H ν a H3O+H3O+ ν s H3O+H3O+ D 2 -Tagging in a Cryogenically Cooled Ion Trap Harmonic H 2 O Loss H 2 tag, jet D 2 tag, ion trap Librations Bend Cs + (H 2 O) 20
Photon Energy (cm -1 ) H 2 O bend Librations ν bound AAD ν free AAD ν DDA ν Association Band OH stretches ‡ DDA AAD νsνs νaνa Å Å 108 o 112 o n = 21 n = Å Å ν a H3O+H3O+ ν umb H3O+H3O+ Comparison of n=21 with Eigen and Bulk Water Water 1M HCl Science, 344, 1009 (2014) bend
H + (H 2 O) n n= Evolution of the Excess Proton Band with Cluster Size: Connecting 2005 to Photon Energy (cm -1 ) 400
Beyond H + (H 2 O) Photon Energy (cm -1 ) D 2 Predissociation Yield H + (H 2 O) 24 H + (H 2 O) 28 H + (H 2 O) 21 Proton signature(s) remain constant, but are losing their structure H + (aq)
Prof. Mark Johnson Prof. Gary Weddle Prof. Chris Johnson Dr. Arron Wolk Dr. Andrew DeBlase Conrad Wolke Olga Gorlova Stephanie Craig Patrick Kelleher Acknowledgments Asmis Lab Nadja Heine Prof. Knut Asmis
Pred. Yield Photon Energy (cm -1 ) Calc. Anh. Int. H 3 O + asym. Librations + Eigen H 2 O bends VPT2 B3LYP/6-31+G(d) Calculations (with anharmonic intensities) Association Band? H 2 O bend H 3 O + umb.
Watching the Bands Shift in D + (D 2 O) 21 D + (D 2 O) D 2 Predissocation Yield Photon Energy (cm -1 ) H + (H 2 O) A B C D E F G H A B C D E F G H ν free AAD ν free AAD A, B: third shell DDA asym. Plausible Assignments (cam-B3LYP/6-31+G(d)): C: coupled H 2 O (H 3 O + ) asym., DDA D: coupled H 2 O (H 3 O + ) sym., DDA E: DDA OH stretches F - H: AAD bound OH stretches H 2 O bend overtone
Photon Energy / cm cm cm and 674 cm -1
Evolution of the Excess Proton Band with Cluster Size H + (H 2 O) Photon Energy (cm -1 ) H + (H 2 O) 10 H + (H 2 O) 14 H + (H 2 O) 18 H + (H 2 O) 21 AD AAD ? DDA All Eigen H 2 O DDA Increased Eigen distortion and/or solvation effect Formation of clathrate-like cage x3
D 2 Pred. Yield Ar Pred. Yield Photon Energy (cm -1 )
Photon Energy (cm -1 ) α ~ H 3 O + umbrella H 3 O + rocks ν H 3 O + umb.+ rocks rock+ rock ν H 3 O + harmonic ν Zundel ~ harmonic
D 2 Pred. Yield Ion Dip Signal Photon Energy (cm -1 ) Two-Color Enhancement Bare H + (H 2 O) 4
Two-Color Enhancement D 2 /H 2 O yield Photon Energy (cm -1 ) D 2 tag Probe