Department of Chemistry, University of Georgia, Athens, GA National Science Foundation Infrared Spectroscopy of Protonated Benzene-Water Nanoclusters: Hydronium, Zundel and Eigen at a Hydrophobic Interface Tim C. Cheng Biswajit Bandyopadhyay Michael A. Duncan

Are clusters relevant for interfaces?

5 μs Production of cold ions with pulsed discharge in a supersonic expansion. IR photodissociation spectroscopy via argon tagging. Laser Vision OPO/OPA cm -1 Mass Selected IR photodissociation Spectroscopy Mass gate selects one complex Full mass spectrum [C,H 3,O] + -Ar m/z=71 Loss of Ar

Hydronium bends free OH sym. & asym st. OH-Ar free H 3 O +

“Zundel” proton stretch bend O-H stretches (split by Ar) Collaboration with Mark Johnson (Yale) and Ken Jordan (Pittsburgh). Science 308, 1765 (2005) no hydronium H + (H 2 O) 2

“Eigen” Protonated water clusters: The role of tagging J. Phys. Chem. A 114, 4570 (2010) free OH hydrogen bond stretches

Protonated benzene, C 6 H 7 + Structure was characterized with 1 H and 13 C NMR in superacid solutions - Rapidly equilibrating with ~8 kcal/mol barrier to H + migration - 7 equivalent H’s and 6 equivalent C’s above ~150 K C-NMR data consistent with allylic  electron density JACS, 1978, 100, 6299.

w/out Ar with Ar B3LYP/6-311+g (d,p) IRPD with Ar tagging; argon binding energy is ~200 cm -1 Protonated Benzene, C 6 H 7 + J. Phys. Chem. A 112, 4869 (2008)

(bz)H + (H 2 O) PA (kJ/mol) H 2 O benzene Shared structure has proton closer to water, even though benzene has the higher PA value. structure is hydronium-benzene and not benzenium-water

(bz)H + (H 2 O) 2 Spectrum indicates protonated water dimer structure is solvated by benzene. Polarization shifts proton toward benzene. Shift breaks symmetry and drives proton frequency higher. Theory does not get the proton stretch frequency.

OH-Ar band at 3577 shifts to become π-hydrogen bond At 3192 cm -1. H + (H 2 O) 3 (bz) Measured with loss of benzene π hydrogen bond

(bz)H + (H 2 O) 4 “Eigen at an Interface” 0.0

Solvation of Hydronium by Benzene

Solvation of Zundel Ion by Benzene

Conclusions Protonated water-benzene mixtures have proton on water Contact point between water and benzene is π-hydrogen bond Binding to benzene shifts frequencies of water clusters both at and remote from contact point Clusters with more benzene distribute it around water ions rather than forming segregated domains

OPO OPA 1 crystal angle tuned 4 crystals angle tuned signal (not used) idler 532 nm 1064 nm KTP oscillator KTA diff. gen idler AgGaSe 2 diff. gen. Tunable  m 1 crystal angle tuned Tunable mid-IR  m LaserVision Tunable Infrared Laser System Optical Parametric Oscillator (OPO) Tuning range: cm -1 Linewidth: ~1.0 cm -1 Pumped by pulsed, seeded YAG e.g., Spectra Physics PRO cm cm -1 Tunable IR Photodissociation Spectroscopy made possible by new laser: Combined tuning range: cm -1