Experimental Measurement of the Induced Dipole Moment of an Isolated Molecule in Its Ground and Electronically Excited States. Indole and Indole-H2O.* CHEOLHWA KANG and DAVID PRATT, University of Pittsburgh,and TIMOTHY KORTER, Syracuse University *Work supported by NSF (CHE-0315584).

Solvent Reorganization in Indole-H2O

High Resolution Spectrometer

Indole-Water Complex Experimental B subband A subband 35097.5 35101.4 cm-1 Indole-Water Complex

Fit

Internal Rotation S1 B A S0 O a H ρ ↔ q q θ ρ ~ cos θ ΔA (or ΔB, ΔC) = F •ωg(2) • ρ2g (g = a,b,c) ρ ↔ q complex a axis attached rotor axis θ ρ ~ cos θ a H Internal Rotation O

Inertial Parameters Indole A, MHz 3877.8 (2) 2064.2 (2) 2062.5 (1) Indole-H2O A, MHz 3877.8 (2) 2064.2 (2) 2062.5 (1) B, MHz 1636.0 (1) 945.0 (3) 945.1 (1) C, MHz 1150.9(1) 649.2 (2) 649.3 (1) ΔI, amu Å -0.111 -1.142 -1.412 A subband B subband A, MHz 3743.1 (2) 1989.0 (2) 1987.6 (2) B, MHz 1618.1 (1) 964.1 (3) 963.5 (1) C, MHz 1130.2 (1) 650.4 (2) 650.4 (1) ΔI, amu Å -0.020 -1.249 -1.745

Solvent Reorganization θ θ = 0 ±15 º Solvent Reorganization θ = 55 ± 15º h ν The axis about which the water is rotating is different in the S1 state.

Stark Experiments on Indole and Indole-H2O Determine the permanent electric dipole moment of the isolated molecule in both electronic states. water complex in both electronic states. Determine the induced electric dipole moment of the

Stark Electrodes and Collection Optics

Stark Effect Spectra of Indole 0 V/cm 348 V/cm 696 V/cm 1303 V/cm 1740 V/cm 2606 V/cm 2.24 cm-1

Stark Fits 509 V/cm 1018 V/cm 2036 V/cm 0.08 cm-1 35231.57

Dipole Moments of Indole a b μ (S0) = 1.963 D μ (S1) = 1.856 D θD θD = 33.0 (6) θD = 45.5 (4) Dipole Moments of Indole

Stark Effect Spectra of Indole-H2O 509 V/cm 1018 V/cm 2036 V/cm 35099.90 0.10 cm-1 35100.00 cm-1

Dipole Moments of Indole-Water h ν a H θD μ″ μ′ θD = + 13 º θD = - 16 º μ″ = 4.4 D μ′ = 4.0 D Rcom = 4.666 Å Rcom = 4.602Å

μtotal = μ indole + μwater + μ induced Dipole Moments μa μb μtot Indole S0 S1 1.376 (8) 1.40 (1) 1.963 (13) 1.556 (8) 1.01 (1) 1.856 (13) + Water 1.85 Indole - Water 4.20 (6) 1.2 (3) 4.4 (3) 3.90 (8) 0.9 (3) 4.0 (3) ≠ Not simple vector sum μtotal = μ indole + μwater + μ induced - Need induced dipole moment

Induced Dipole Moments ≈ Induced Dipole Moments + α μ,Θ α,Θ α = polarizability Θ = quadrupole moment

Dipole Moments Calculated Experimental 4.11 1.15 4.27 3.54 0.66 3.60 without Θ 4.24 1.18 4.40 3.81 0.77 3.89 with Θ μa 4.20 (6) S0 μb 1.2 (3) μtot 4.4 (1) μa 3.90 (8) S1 μb 0.9 (3) μtot 4.0 (2) μI (S0) = 0.7 D, μI (S1) = 0.5 D

Vector Model N N Water dipole Indole dipole Induced dipole h ν a a H O

Summary Acknowledgements High resolution electronic spectroscopy in the gas phase has been used to determine the magnitude and orientation of the induced dipole moment that is produced when an indole molecule in its ground S0 and electronically excited S1 states is polarized by the attachment of a water molecule in the hydrogen bonded complex indole-H2O. Acknowledgements We thank Dr. John Kuehne and Mr. Val Kagan for technical assistance. This work has been supported by NSF (CHE-0315584)