Infrared spectroscopy of the hydrated sulfate dianion Columbus2006

Improvements in ion beam methodology, laser development have resulted in major strides in IR spectroscopy of hydrated ions, electrons Johnson, Duncan, Jordan 2005 IR spectra of H + (H 2 O) n shows evolution from “Zundel” to “Eigen” form of hydrated proton Johnson, 2006: (H 2 O) n -

IR spectra of hydrated multiply- charged anions IR work by Johnson and Okumura on X - (H 2 O) n clusters show single hydrogen bonds with solvent molecules IR work by Johnson and Okumura on X - (H 2 O) n clusters show single hydrogen bonds with solvent molecules What about doubly- charged anions? What about doubly- charged anions? Model system: SO 4 2- (H 2 O) n Model system: SO 4 2- (H 2 O) n

Raman spectra of SO 4 2- in condensed phase:

SO 4 2- (H 2 O) n clusters Wang and Nicholas, 2000 structures PE spectra Only stable for n  3

Solvation energetics and kinetics: Kebarle, Williams, Wang Wang Williams: BIRD dissociation rates Low rates for n=6, 12  shell closing?

How to measure IR spectra? Need to perform “action spectroscopy” in which photon absorption coupled to ion depletion or production Need to perform “action spectroscopy” in which photon absorption coupled to ion depletion or production Binding energy (BE) of H 2 O ligands vary from eV (Wang PE spectra) Binding energy (BE) of H 2 O ligands vary from eV (Wang PE spectra) Expect BE>h for most vibrational transitions Expect BE>h for most vibrational transitions Method: Resonant multiphoton absorption Method: Resonant multiphoton absorption

FELIX: Free Electron Laser for Infrared eXperiments cm mJ/macropulse 10 Hz, 2% bandwidth

Guided Ion Beam Tandem Mass Spectrometer ← ion sources - laser ablation - ion spray - crossed e‾-beam guiding decapole (Ar buffer gas) quadrupole mass filter 1 (parent ions) cooled RF ion trap (He buffer gas, 16 K) quadrupole mass filter 2 (product ions) Ion detection 90° deflector high vacuum mbar FELIX

IRMPD Two major fragmentation channels observed Two major fragmentation channels observed Parent depletion monitored Parent depletion monitored i = 0 i = 2 i = 1 (For n≤5)

P=SO 4 2- (H 2 O) 5 CS1=HSO 4 - (H 2 O) 2, OH - (H 2 O) 2 CS2=HSO 4 - (H 2 O), OH - (H 2 O) 3 FELIX off FELIX on (1100 cm -1 )

IRMPD Spectra of SO 4 2- (H 2 O) n A D CB * * * * * * *

Assignments Sulfate Sulfate 1 = 981 cm -1, 2 = 451 cm -1, 3 = 1104 cm -1, 4 = 613 cm -1 Water Water 1 = 3652 cm -1, 2 = 1595 cm -1, 3 = 3756 cm -1 libration: 683 cm -1 Sulfate as bend Libration Sulfate as stretch Water bend SO 4 2- (H 2 O) 6

Observed Modes Sulfate bend ( 4 ) Sulfate stretch ( 3 ) Solvent libration Water bend ( 2 )

Size-dependent Trends

IRMPDMP2 wavenumber (cm -1 ) parent ion depletion n=5 n=6 n=4 n=3 oscillator strength wavenumber (cm -1 ) n = 3-6 Clusters

IRMPD MP2 wavenumber (cm -1 ) oscillator strength n=7n=6 C3C3 TdTd “rearranged” “6+1” IRMPD MP2 parent ion depletion wavenumber (cm -1 ) Low Energy Isomers

IRMPDB3LYP wavenumber (cm -1 ) parent ion depletion n=12 n=13 oscillator strength wavenumber (cm -1 ) n=13 n=12 Solvation Shell Closure

Comparison to ions with “strong” hydrogen bonding: BrHI - Johnson, 2005 No evidence for low-frequency stretches and bends associated with very strong H-bonds

Hydrated sulfate summary: First IR spectra of multply-charged anion in gas phase First IR spectra of multply-charged anion in gas phase Triply-degenerate 3 mode of sulfate and librational modes of water are very sensitive to solvation structure Triply-degenerate 3 mode of sulfate and librational modes of water are very sensitive to solvation structure 1 st six water molecules bridged between two O atoms on sulfate 1 st six water molecules bridged between two O atoms on sulfate Subsequent waters: water-water H-bondsSubsequent waters: water-water H-bonds Direct spectroscopic evidence for 2 nd solvent shell starting at n=13 Direct spectroscopic evidence for 2 nd solvent shell starting at n=13

Many Thanks! Mark Abel Paul Crider Etienne Garand Scott Goncher Graham Griffin Aster Kammrath Beth Kautzman Jeong Hyun Kim David Moore Brad Parsons Thomas Pfeifer Sean Sheehan Niels Sveum Dave Szpunar Jan Verlet Chia Wang Terry Yen Jia Zhou Knut Asmis (FHI/Berlin)

Resonant IRMPD can be very efficient! BLISS mode: (Blast the LIving S___ Spectroscopy) Linear regime (n=3)