Tatsuya Ishiyama and Akihiro Morita

Molecular Dynamics Study of Sum Frequency Generation Spectrum for NaI Aqueous Solution

Tohoku University, Sendai, Japan

Department of ChemistryGraduate 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 sizeBoundary condition

Ewald summation method is employed for the long-range Coulombic interaction

2.1M NaI aqueous solutionMD 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

Pure water NaI aqueous solution

The orientational order of surface water molecules in NaI solution is opposit

e 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 waterNaI 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 .