Finding optimal ion-solvent configurations using FTIR For Studying ion association dynamics of thioc...
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Finding optimal ion-solvent configurations using FTIR For Studying ion association dynamics of thiocyanate salt by 2DIR spectroscopy Maria Gonzalez Office of Science, Science Undergraduate Internship Program Loyola University, New Orleans, LA Stanford Linear Accelerator Center

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Abstract 4522272

Finding optimal ion-solvent configurations using FTIR For Studying ion association dynamics of thiocyanate salt by 2DIR spectroscopy

Maria Gonzalez

Office of Science, Science Undergraduate Internship Program

Loyola University, New Orleans, LA

Stanford Linear Accelerator Center

Menlo Park, California

August 14, 2008


Abstract

Abstract

  • The ion association and dissociation dynamics in thiocyanate salt solutions will be probed using 2D-IR spectroscopy allowing for the determination of equilibrium association (or dissociation) rate constants of the thyocyanate anion and its counter cation. Thiocyanate’s nitrile stretch, which is sensitive to ionic interactions, was used in revealing the interaction among the thiocyanate ion and the cation in solution. The optimal solution parameters for the thiocyanate salts was determined by a one to one area ratio of the nitrile vibrational frequency of free thiocyanate to the contact ion pair using one dimensional infrared spectroscopy.


Infrared thiocyante

Infrared &Thiocyante

Nitrile Stretch ~2100-2240 cm-1

Free thiocyanate ion ~ 2050 cm-1

Contact Ion Pair ~ 2070 cm-1


Ultrafast vibrational spectroscopy

S1

v` = 2

v` = 1

v` = 0

S0

v = 2

v = 1

Anharmonicity

v = 2

v = 0

First overtone

v = 1

Fundamental

v = 0

Ultrafast Vibrational Spectroscopy

  • Understanding of the ultrafast structural dynamics of complex molecular systems in solution has been restricted by the fast time scale such processes take place on

  • Vibrational excitations, in contrast to electronic excitations,

  •  produce a negligible perturbation with less energetic

  • IR photons.

  •  don’t change chemical properties of molecules

  • under study.

  • Ultrafast Vibrational spectroscopy

  •  allows study molecular systems under thermal equilibrium

  • conditions.

  •  measures dynamics occurring on fs and ps time scales.


2dir experiments

beam combiner

ksig= -k1+k2+k3

Esig

k1

k3

k2

Monochromator

k2

ksig

k3

k1

Sample

local oscillator

k2

k3

MCT

Array Detector

k1

2DIR Experiments

- 2DIR experiment is performed with multiple pulse sequences.

- Time-delayed three IR pulses are focused onto the sample in a noncollinear geometry.

- Emitted signal is overlapped with a local oscillator for heterodyne detection.

- Heterodyned signal is dispersed through a monochromator and is frequency-resolved.

- Dual scan method with two different pulse sequences is used to measure purely absorptive part of signal.

Laser Phys. Lett., 4, 704 (2007)


Abstract 4522272

AB Chemical Exchange

0-1 region only

AwAspecies A - frequencywA

A

BwBspecies B - frequencywB

wm

A and B givediagonal peaks

B

wt

Consider one diagram for the 0-1 region

AB

BA

AB

A

1st interaction - wB

1st interaction - wA

wm

A

B

Last interaction - wA

Last interaction - wB

B

wt

Off-diagonal

Off-diagonal

wA

wB

t

t

Tw

A

A

B

wm

B

wt

BA

wB

wA

t

t

Tw


Abstract 4522272

AB Chemical Exchange

- Combining

ABand

BA

A

B

A

A

B

wm

B

wt

- Including the 1-2 pathways

Off-diagonal peaks in each blockgrow in as Tw is increased.

The Tw dependent growthof the off-diagonal peaks

in each block gives thechemical exchange rate.


Two state chemical exchange dynamics

kAB

T1,A, or,A

FFCFA

T1,B, or,B

FFCFB

AB

Decay

Spectral diffusion

Decay

Spectral diffusion

kBA

Two-state chemical exchange dynamics


Abstract 4522272

  • Finding Optimal Solution Parameters


Abstract 4522272

Contact Ion Pair

Free Ion

Nitrile Stretch = (2100-2240 cm-1)

Free thiocyanate ion ~ 2050 cm-1

Contact Ion Pair ~ 2070 cm-1


Conclusion

Conclusion

  • Optimal solution parameters:

    • Lithium Thiocyante:

      • 0.040 ±0.002M lithium thiocyanate in diethyl ether with 0.118mols ± 0.001 of lithium chloride

    • Sodium Thiocyante:

      • 0.060±0.003M sodium thiocyanate in acetonitrile


Special thanks

Special Thanks

  • Department of Energy

  • Dr. Steven Rock, Farah Rahbar, & Susan Schultz

  • Dr. Kelly Gaffney

  • Dr. Sugnam Park

  • Minbiao Ji


References

References

  • Suydam, I. T.; Boxer, S. G. Biochemistry 2003, 42, 120

  • Park, S.; Kwan, K. Laser Phys. Lett., 2007, 705, 710

  • Zheng, J.: Kwan, K; Fayer, M.D. Acc. Chem. Res. 2007, 76, 78

  • Marcus, Y., Ion Solvation, New York, Wiley, 1985, in Chapter 3 “Infra-red Spectra and Solvation of Ions in Dipolar Protic Solvents”

  • Butcher, P.N; Cotter, D. The elements of Nonlinear Optics, Cambridge University Press: Cambridge, U.K., 1990, 50-78

  • Landolt-Börnstein, Group IV Physical Chemistry, Springer Berlin Heidelberg Press: Berlin, Germany, 2008, Volume 17, 269-270


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