Tatsuya Ishiyama and Akihiro Morita Molecular Dynamics Study of Sum Frequency Generation Spectrum...
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Transcript of Tatsuya Ishiyama and Akihiro Morita Molecular Dynamics Study of Sum Frequency Generation Spectrum...
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 .