Influence of Aromatic Molecules on the Structure and Spectroscopy of Water Clusters Daniel Tabor1, Patrick Walsh2, Timothy Zwier2, Edwin Sibert1 1University of Wisconsin—Madison 2Purdue University

Background (Water)n with n=3-5: cycles Bz-(H2O)n: H-bond with benzene Developed model include effects of Fermi resonance Lengthen Going to add pictures, but these are the main points Shorten

Summary of Bz-(H2O)n Interaction with phenyl ring leads to ~55 cm-1 shift in free OH frequency H-bond lengths in the cycle are systematically distorted Exp Theory Intra + Fermi 3000 3200 3400 3600 3800 Wavenumber (cm-1)

DPOE and TCP DPOE TCP Model system for studying the basic building blocks of polyethylene glycol Prototypical macrocycle Large aromatic binding pocket Both a phenoxy oxygen and phenyl ring for water to interact with J. Phys. Chem. A, 118, 8583–8596 (2014). J. Chem. Phys. 142, 154303 (2015).

DPOE and TCP Where does the water go? DPOE TCP Model system for studying the basic building blocks of polyethylene glycol Prototypical macrocycle Large aromatic binding pocket Both a phenoxy oxygen and phenyl ring for water to interact with J. Phys. Chem. A, 118, 8583–8596 (2014). J. Chem. Phys. 142, 154303 (2015).

Resonant Two-Photon Ionization Resonant Ion-Dip Infrared Spectroscopy Experimental Method R2PI Resonant Two-Photon Ionization RIDIR Resonant Ion-Dip Infrared Spectroscopy M (S0) M* (S1) M++ e- M (S0) M* (S1) M++ e- Records the IR spectrum of a single conformation free from interference from others present in the expansion I(signal)= I(HB) – I(no HB) 20 Hz 20 Hz These are of course, the real colors used in the experimental lasers  Δt=200ns 10 Hz

Model Hamiltonian Construction Normal Modes Form Local Mode Basis Scale Add Anharmonicity Spectra

Transformation from Normal to Local Modes Normal Mode Scissors Normal Mode OH Stretches Normal Mode Local Mode

Transformation from Normal to Local Modes Normal Mode Scissors Normal Mode OH Stretches Unscaled Local Stretches Unscaled Local Scissors Normal Mode Local Mode

Transformation from Normal to Local Modes Normal Mode Scissors Normal Mode OH Stretches Unscaled Local Stretches Unscaled Local Scissors Normal Mode Scaled Local Scissors Scaled Local Stretches Local Mode

Hamiltonian Matrices DPOE-H2O DPOE-W1 Exp Full Model H2O Wavenumber (cm-1) The label D is for it being a weaker donor than a full D. Analogy to a vs. A in the benzene water work. Even a little donor character shifts the intramonomer coupling down by 10 cm-1 2.64 Å 2.02 Å “d” shift: 86 cm-1

Hamiltonian Matrices DPOE-(H2O)2 Exp Full Model Intra + Fermi Wavenumber (cm-1) The pure H2O dimer H-Bond distance is 1.93 at this level of theory, so the donor pulls in the H bond as expected 1.87 Å 1.96 Å 2.64 Å “d” shift: 120 cm-1

DPOE-(H2O)3 Cycle vs. Chain Exp Theory DPOE-(H2O)3 Cycle Theory DPOE-(H2O)3 Chain Add Structures Wavenumber (cm-1)

Hamiltonian Matrices DPOE-(H2O)3 Cycle Exp Full Model Intra + Fermi Wavenumber (cm-1) 1.92 Å 1.82 Å 2.02 Å 2.22 Å 2.54 Å

Hamiltonian Matrices DPOE-(H2O)3 Cycle Exp Full Model Bz-(H2O)3 Intra + Fermi Wavenumber (cm-1) 1.92 Å Lowest Frequencies associated with waters? 1.82 Å 2.02 Å 2.22 Å 2.54 Å

DPOE-(H2O)4 Exp Full Model Intra + Fermi 1.79 Å 1.74 Å Intra Only 1.82 Å 1.73 Å 2.04 Å 2.65 Å Wavenumber (cm-1)

DPOE-(H2O)4 Exp Full Model Intra + Fermi 1.79 Å 1.74 Å Intra Only 1.82 Å 1.73 Å 2.04 Å 2.65 Å Wavenumber (cm-1)

DPOE-(H2O)n Changes in Site Frequencies

Onto TCP-(H2O)n

Hamiltonian Matrices TCP-H2O TCP-W1 Exp Full Model H2O Wavenumber (cm-1) Shifts: 61, 44 cm-1

Hamiltonian Matrices TCP-(H2O)2 Exp Full Model Intra + Fermi Wavenumber (cm-1) The pure H2O dimer H-Bond distance is 1.93 at this level of theory. DPOE-W2 was 1.82. 1.83 Å H-bond distances (H2O)2: 1.92 Å DPOE-(H2O)2: 1.82 Å

Two candidate structures: Clockwise cycle (CW) and Counterclockwise cycle (CCW) TCP-(H2O)3 CCW Exp Full Model 1.89 Å Intra + Fermi 1.71 Å 2.02 Å Wavenumber (cm-1) Hamiltonian Matrices

Two candidate structures: Clockwise cycle (CW) and Counterclockwise cycle (CCW) TCP-(H2O)3 CCW Exp Full Model 1.89 Å Intra + Fermi 1.71 Å 2.02 Å Wavenumber (cm-1) Hamiltonian Matrices Add a picture of it sticking in the middle.

Two candidate structures: Clockwise cycle (CW) and Counterclockwise cycle (CCW) TCP-(H2O)3 CCW Exp Full Model View from top Intra + Fermi Wavenumber (cm-1) Hamiltonian Matrices Add a picture of it sticking in the middle. 1.71 Å 1.89 Å 2.02 Å

TCP-(H2O)4 Candidate Structures CW Up CW Down CCW Up CCW Down Hard to tell these apart, but the “down ones” have a peak near 3700 cm-1 that looks off. Telling the up ones apart is difficult, but they seem less incorrectly shifted up for the CCW up. Looks more clear when you zoom in. All sort of miss the 3300-3400 cm-1 region disappointingly at this level of approximation.

TCP-(H2O)4 Candidate Structures Exp CCW Up CW Up CCW Down Hard to tell these apart, but the “down ones” have a peak near 3700 cm-1 that looks off. Telling the up ones apart is difficult, but they seem less incorrectly shifted up for the CCW up. Looks more clear when you zoom in. All sort of miss the 3300-3400 cm-1 region disappointingly at this level of approximation. CW Down Wavenumber (cm-1)

TCP-(H2O)4 CCW Up Exp Full Model Intra + Fermi 1.79 Å 1.72 Å 1.76 Å 1.83 Å Stretches are quite strongly coupled. Perhaps the Fermi coupling is overestimated when I use the simple 45.3 coupling. Will work on computing it from cubic force constants. Hard to fit matrices in Intra Only Wavenumber (cm-1)

TCP-(H2O)4 CCW Up Exp Full Model Intra + Fermi 1.79 Å 1.72 Å 1.76 Å 1.83 Å Stretches are quite strongly coupled. Perhaps the Fermi coupling is overestimated when I use the simple 45.3 coupling. Will work on computing it from cubic force constants. Hard to fit matrices in Intra Only Wavenumber (cm-1)

TCP-(H2O)5 CW Exp Full Model Intra + Fermi Intra Only CW and CCW are similar but CW fits the intensity profile in the low frequency region better. Had a hard time fitting the bond lengths in on this one. Hard to fit matrices in Intra Only Wavenumber (cm-1)

TCP-(H2O)5 CW Exp Full Model Intra + Fermi Intra Only CW and CCW are similar but CW fits the intensity profile in the low frequency region better. Had a hard time fitting the bond lengths in on this one. Hard to fit matrices in Intra Only Wavenumber (cm-1)

TCP-(H2O)n Changes in Site Frequencies

Summary Binding to the phenoxy O atom in DPOE can lead to larger shifts in frequency than binding to the phenyl ring in Bz The aromatic interaction with TCP and water is about the same strength as with Bz In some clusters we see half of a piH-bond

Acknowledgements Ned Sibert Patrick Walsh Britta Johnson Funding: NSF