Vibrational Spectroscopy of Benzene-(Water) n with n=6,7 Daniel Tabor 1, Ryoji Kusaka 2, Patrick Walsh 2, Edwin Sibert 1, Timothy Zwier 2 1 University of Wisconsin—Madison 2 Purdue University

Background (Water) n with n=3-5: cycles Hexamer: 3 structures seen in rotational studies: cage, prism and book Water heptamer: expanded prisms (just saw) Water octamer: cube, either S 4 or D 2d Benzene: Can alter structure of 3D water networks 2 Cite Pate

Experimental Method RIDIR Resonant Ion-Dip Infrared Spectroscopy M (S 0 ) M* (S 1 ) M + + e - M (S 0 ) M* (S 1 ) M + + e - R2PI Resonant Two-Photon Ionization 20 Hz 10 Hz Δt=200ns 20 Hz 3 I(signal)= I(HB) – I(no HB) Records the IR spectrum of a single conformation free from interference from others present in the expansion

Local Mode Model Hamiltonian Normal Modes Local Modes Spectra Add Anharmonicity 4 For Bz-(H 2 O) 3, see full story in the next talk

Obtaining Local Modes From Normal Modes Normal Mode One Localized Stretch Mode 5 Normal Mode One Localized Bend Mode

Augmenting the Local Mode Hamiltonian M06-2X/ G(2d,p) Harmonic Frequency (cm -1 ) Experimental Fundamental (cm -1 ) 6 Relationship between normal modes and experiment. Use map on local site frequencies Add in Bend Overtones and Fermi Coupling Cite Bowman Can remove parts of the model Hamiltonian to see their effects

Bz-(H 2 O) 6 : Experimental Spectrum 7 4 Free OH Stretches Assuming benzene binds to one free OH, looking for 5 free OH groups Book=5Prism=3 Characteristic of DD Waters DAA Cage=4

Bz-(H 2 O) 6 : Insights from Harmonic Calculations Wavenumber (cm -1 ) Best Fit Five possible binding sites on the book water hexamer Harmonic frequency calculations give a strong hint to which site benzene binds to 8

Intra + FR Intra Harmonic Full Model DAA DA DDA  DA F Wavenumber (cm -1 ) A B C D Hamiltonian Matrices Breaking Down Bz-(H 2 O) 6  DA DDA DAA DA Inverted Book 9

Bz-(H 2 O) 6 10 Wavenumber (cm -1 ) Full Model Intra + FR Intra  DA DDA DAA DA DAA DA DDA  DA F Hamiltonian Matrices

RIDIR of Bz-(H 2 O) 7 Intra + FR Intra Harmonic DAa DAA DDA +  AA F Full Model A B C D Wavenumber (cm -1 ) DDA DAA DAa  AA Hamiltonian Matrices S 4 -symmetry inserted cube

Bz-(H 2 O) 7 DDA DAA DAa  AA Hamiltonian Matrices S 4 -symmetry inserted cube Full Model Intra + FR Intra

Intra = -23 to -31Intra = -46 to -50Intra = -59 to -62  DA DDA DAA DA DAa DA  AA Summary of Monomer Hamiltonians S 4 -symmetry inserted cube Inverted book

Summary A local mode Hamiltonian assigns the structure of Bz-(H 2 O) 6 to the inverted book and one isomer of Bz-(H 2 O) 7 to an S 4 - symmetry inserted cube OH stretch frequencies correlate with hydrogen bond distances Inter-monomer coupling between stretches depend on coordination environment (high for AA/Aa and low for DD/πD) 14

Acknowledgements Ned Sibert Patrick Walsh Ryoji Kusaka Tim Zwier Britta Johnson Amber Jain Funding: NSF 15 Further Information: J. Phys. Chem. Lett., 6, (2015).

Records the IR spectrum of a single conformation free from interference from others present in the expansion Resonant Ion-Dip Infrared Spectroscopy  t=100 nsec Cold initial population distribution IR Hole-burn laser Probe laser R2PI S0S0 S1S1 A + +e - A * (S 1 ) I(no HB)  Popn before I(signal)= I(HB) – I(no HB) I(HB)  Popn after