Tatsuya Ishiyama and Akihiro Morita Molecular Dynamics Study of Sum Frequency Generation Spectrum for NaI Aqueous Solution Tohoku University, Sendai, Japan Department of Chemistry Graduate School of Science

Experimental SFG studies Raymond and Richmond, JPCB, (2004) Sum Frequency Generation (SFG) spectroscopy of NaCl and NaI solutions Liu, Ma, Levering, and Allen, JPCB., (2004) How to interpret the spectra? Interfacial structure SFG spectrum ?

The present study We examine the interfacial structure of NaI aqueous solution by means of a direct calculation of its SFG spectrum in MD simulation. Resonant susceptibility SFG intensity Intensity Susceptibility Resonant part Non-resonant part The resonant susceptibility is calculated by the Fourier-Laplace transformation of the time correlation function of A and M. System polarizability System dipole Morita and Hynes, JPCB, (2002) Morita, JPCB, (2006) The system polarizability A(t) and dipole moment M(t) can be calculated in MD simulation, and thereby we can evaluate.

Molecular dynamics (MD) simulation Number of molecules 1000 (water) +40 (sodium) +40 (iodide) [2.1M NaI] Time step Set tempetature 3D-periodic B.C. System size Boundary condition Ewald summation method is employed for the long-range Coulombic interaction 2.1M NaI aqueous solution MD simulation with flexible and polarizable model developed by us T. Ishiyama and A. Morita, J. Phys. Chem. C, 111, 721 (2007)

Density profile Peak position of density Position of Gibbs dividing surface Peak position of density The strong peaks of and densities exist at the interface. The electric double layer is formed at the interface.

Orientational order of water molecules NaI aqueous solution Pure water NaI aqueous solution The orientational order of surface water molecules in NaI solution is opposite to that in pure water, due to the electric double layer structure. Peak position of density Position of Gibbs dividing surface Peak position of density

ssp polarized SFG spectrum Pure water NaI aqueous solution Why ? Raymond and Richmond, JPCB., (2004) Present MD simulation ssp spectrum The spectral enhancement at about can be seen in NaI solution. It seems that the spectral enhancement is modest in spite of the strong order of water molecules due to the double layer structure. Dangling OH Hydrogen bonding OH

Frequently used assumption: Self term Cross term If the cross term (intermolecular correlation) can be neglected, then Is the intermolecular correlation term small compared to the self-correlation term?

Real and imaginary part of and Pure water NaI solution Intermolecular correlation is not large in the qualitative sense. Intermolecular correlation is relatively large.

Intermolecular correlation of water and ion water(pol.)-water(dipole) water(pol.)-ion(dipole) ion(pol.)-water(dipole) ion(pol.)-ion(dipole) Water-Ion correlation ( ) is not negligible! Dipolar correlation in surface normal direction T. Ishiyama and A. Morita, Chem. Phys. Lett., 431, 78 (2006)

sps polarized SFG spectra 2.1M NaI Pure water The spectrum has a broad enhanced signal from to. The double layer structure is more strongly reflected in sps spectrum than in ssp spectrum, due to the net in-phase intermolecular correlation effect. Dipolar correlation in surface tangential direction T. Ishiyama and A. Morita, J. Phys. Chem. C, 111, 738 (2007)

Summary Using MD simulation with flexible and polarizable model, we conclude that 1. The calculated ssp and sps spectra for NaI solution well reproduce the experimental spectra, and are consistent with the surface structure that iodide anion is stable at the interface. 2. The intermolecular correlation effect is not negligible in SFG spectra. For ssp spectrum, the effect suppress the signal originating from the self correlation, due to its ‘canceling mechanism’ of phase of. For sps spectrum, the effect enhances the signal originating from the self correlation, due to its ‘enhanced mechanism’ of phase of.