Vibrational Spectroscopy of Benzene-(Water)n with n=6,7

Daniel Tabor1, Ryoji Kusaka2, Patrick Walsh2, Edwin Sibert1, Timothy Zwier2

1University of Wisconsin—Madison2Purdue University

2

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 S4 or D2d

• Benzene: Can alter structure of 3D water networks

Cite Pate

3

Experimental Method

RIDIRResonant Ion-Dip

Infrared Spectroscopy

M (S0)

M* (S1)

M++ e-

M (S0)

M* (S1)

M++ e-

R2PIResonant Two-Photon

Ionization

20 Hz

10 Hz

Δt=200ns

20 Hz I(signal)= I(HB) – I(no HB)

Records the IR spectrum of a single conformation free from interference from

others present in the expansion

4

Local Mode Model HamiltonianNormal Modes

Local Modes

Spectra

Add Anharmonicity

For Bz-(H2O)3, see full story in the next talk

5

Obtaining Local Modes From Normal Modes

Normal Mode One Localized Stretch Mode

Normal Mode One Localized Bend Mode

6

Augmenting the Local Mode Hamiltonian

M06-2X/6-311++G(2d,p) Harmonic Frequency (cm-1)

Expe

rimen

tal F

unda

men

tal (

cm-1

)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

7

Bz-(H2O)6: Experimental Spectrum

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

8

Bz-(H2O)6: Insights from Harmonic Calculations

3000 3200 3400 3600 3800Wavenumber (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

9

Intra + FR

Intra

Harmonic

Full Model

DAA DADDA πDA F

Wavenumber (cm-1)

3690 3730

A

B

C

D

Hamiltonian Matrices

Breaking Down Bz-(H2O)6

πDA

DDA DAA

DA

DA

DA

Inverted Book

10

Bz-(H2O)6

Wavenumber (cm-1)

Full Model

Intra + FR

Intra

πDA

DDA

DAA

DA

DADADAA DA

DDAπDA F

Hamiltonian Matrices

RIDIR of Bz-(H2O)7

Intra + FR

Intra

Harmonic

DAaDAA

DDA +πAAF

Full Model

A

B

C

D

Wavenumber (cm-1)

DDA

DDA

DDA

DAA

DAa

DAa

πAA

Hamiltonian Matrices

S4-symmetry inserted cube

Bz-(H2O)7

DDA

DDA

DDA

DAA

DAa

DAa

πAA

Hamiltonian Matrices

S4-symmetry inserted cube

Full Model

Intra + FR

Intra

Intra = -23 to -31 Intra = -46 to -50 Intra = -59 to -62

πDA

DDA

DDA

DDA

DDA

DAA

DAA

DA

DA

DAa

DAa

DA

πAA

Summary of Monomer Hamiltonians

S4-symmetry inserted cube

Inverted book

14

Summary

• A local mode Hamiltonian assigns the structure of Bz-(H2O)6 to the inverted book and one isomer of Bz-(H2O)7 to an S4-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)

15

Acknowledgements

• Ned Sibert• Patrick Walsh• Ryoji Kusaka• Tim Zwier• Britta Johnson• Amber Jain• Funding: NSF

Further Information:J. Phys. Chem. Lett., 6, 1989-1995 (2015).