Infrared Spectra of Anionic Coinage Metal-Water Complexes J. Mathias Weber JILA and Department of Chemistry and Biochemistry University of Colorado at Boulder

Dramatis Personae €€€ DFG (Emmy-Noether-Program), Universität Karlsruhe Experiment: Holger Schneider (now CU Boulder) Calculations: A. Daniel Boese (Institute for Nanotechnology, Forschungszentrum Karlsruhe, Germany)

Motivation Metal atoms and clusters deposited on surfaces with anionic defect sites have interesting catalytic properties. Example: Au n (-) + ½ O 2 + CO  CO 2 A. Cho, Science 299, 1684 (2003)

Motivation Metal atoms and clusters deposited on surfaces with anionic defect sites have interesting catalytic properties. The presence of water has been seen to strongly influence the catalytic process.  How do water molecules and noble metal anions interact?

Possible Approach: Vibrational Spectroscopy OH groups equivalent symmetric and antisymmetric stretch vibrations in free H 2 O: 3657 cm cm -1

Possible Approach: Vibrational Spectroscopy OH groups equivalent symmetric and antisymmetric stretch vibrations in free H 2 O: 3657 cm cm -1

in clusters: H bonds with ion and other ligands  stretching of the H bonding OH groups  breaking of symmetry  red shift of H bonded oscillators Possible Approach: Vibrational Spectroscopy OH groups equivalent symmetric and antisymmetric stretch vibrations in free H 2 O: 3657 cm cm -1

Experimental Method: IR Photodissociation [A·B] -  [A] - + B h M - ·H 2 O·Ar n + h  [M - ·H 2 O·Ar n ]*  M - ·H 2 O·Ar m + (n-m) Ar

Experimental Setup: Reflectron TOF-MS 2100 – 3800 cm mJ / 5 ns IR-OPO/OPA Nd:YAG J. M. Weber, Rev. Sci. Instrum. 76 (2005)

Possible structural motifs: single or double ionic H bond Single Ionic H Bond Double Ionic H Bond How will anions interact with H 2 O? Intuitive approach

free OH oscillator (F band) F band: between s and as of H 2 O  ca cm -1 Spectra of SIHB complexes: How will anions interact with H 2 O? Intuitive approach

free OH oscillator (F band) OH oscillator in H bond (IHB band) Spectra of SIHB complexes: How will anions interact with H 2 O? Intuitive approach F band: between s and as of H 2 O  ca cm -1 IHB band: red shifted against F band

Example: Cl - ·H 2 O·Ar n How will anions interact with H 2 O? Intuitive approach Kelley et al., Chem. Phys. Lett. 327 (2000) 1

Example: Cl - ·H 2 O·Ar n F band How will anions interact with H 2 O? Intuitive approach Kelley et al., Chem. Phys. Lett. 327 (2000) 1

Example: Cl - ·H 2 O·Ar n F band IHB band How will anions interact with H 2 O? Intuitive approach Kelley et al., Chem. Phys. Lett. 327 (2000) 1

Example: Cl - ·H 2 O·Ar n F band IHB band Fermi resonance of IHB with bend overtone, combination band with ion-molecule stretch vibration Kelley et al., Chem. Phys. Lett. 327 (2000) 1 How will anions interact with H 2 O? Intuitive approach

s and as of H 2 O red shifted against positions in free H 2 O Example: SO 2 - ·H 2 O Spectra of DIHB complexes Woronowicz et al., J. Phys. Chem. A 2002, 106, 7086 How will anions interact with H 2 O? Intuitive approach

So far: SIHB motif for all complexes with atomic anions  Expectation: SIHB motif How will anions interact with H 2 O? Intuitive approach

Red shift and anion proton affinity (SIHB motif)

Expectation for IHB bands of M - ·H 2 O in SIHB configuration

IR spectra of M - ·H 2 O M - ·H 2 O·Ar 2 + h  M - ·H 2 O + 2 Ar

Expectation for IHB:  Cu >  Ag >  Au IR spectra of M - ·H 2 O

Result:  Au >  Cu >  Ag Moreover: F band red shifted !!! IR spectra of M - ·H 2 O Expectation for IHB:  Cu >  Ag >  Au

Comparing SIHB / DIHB data SIHB DIHB

Potential of the water rocking motion Calculated barriers (CCSD(T)/aug-pc-2;ECP-MCDF-aug-pVTZ): Au - ·H 2 O: 42 meV Ag - ·H 2 O: 16 meV Cu - ·H 2 O: 17 meV Very low barriers! Cl - ·H 2 O: 80 meV  E [meV]

Potential of the water rocking motion Double harmonic oscillator (Schrödinger Applet) high barrier

Potential of the water rocking motion Double harmonic oscillator (Schrödinger Applet) high barrier low barrier

Asymmetric DIHB Structure C s equilibrium structure, but complex explores geometries near C 2v transition state due to zero point motion  „free“ OH group contributes to binding  red shift of F band  red shift of IHB band reduced  New H-bonding behavior: dynamic asymmetric DIHB due to ground state zero point motion H. Schneider, A. D. Boese, J. M. Weber, J. Chem. Phys. 123, (2005)

The End

Argon Effects

Asymmetric DIHB Structure