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

Maria GonzalezOffice of Science, Science Undergraduate Internship Program

Loyola University, New Orleans, LA

Stanford Linear Accelerator CenterMenlo Park, California

August 14, 2008

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

Nitrile Stretch ~ 2100-2240 cm-1 Free thiocyanate ion ~ 2050 cm-1 Contact Ion Pair ~ 2070 cm-1

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.

•

v = 0

v = 1

v = 2

Fundamental

First overtone

Anharmonicityv = 0

v = 1v = 2

v` = 0

v` = 1v` = 2

S0visiblehν

IRhν

S1

2DIR Experiments

ksig = -k1+k2+k3

k1

k2

k3

k1

k2

k3

Sample

Monochromator

loca

l osc

illat

or

MCT Array Detector

beam combiner

k1

k2 k3

ksig

Esig

- 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)

AA B B Chemical ExchangeChemical Exchange

A A species A - frequency A

B B species B - frequency B

A and B givediagonal peaks

0-1 region only

m

B

A

A B Consider one diagram for the 0-1 region

0

1

2

A

0

1

2

BA B

Tw

1st interaction - A

Last interaction - B

Off-diagonal

m

B

A A

B

0

1

2

B

0

1

2

AB A

Tw

B A

1st interaction - B

Last interaction - A

Off-diagonal

m

B

AA

B

AA B B Chemical ExchangeChemical Exchange

m

B

A A

B

A

B

- Combining A B and B A

0

1

2

A

0

1

2

BA B

Tw 0

1

2

B

0

1

2

AB A

Tw0

1

2

A

0

1

2

A

0

1

2

B

0

1

2

0

1

2

BA BA B

Tw 0

1

2

B

0

1

2

B

0

1

2

A

0

1

2

AB A

Tw

- Including the 1-2 pathways

0

1

2

A

0

1

2 B

A BTw 0

1

2

B

0

1

2 A

B ATw0

1

2

A

0

1

2

A

0

1

2 B

0

1

2

0

1

2 B

A BA BTw 0

1

2

B

0

1

2

B

0

1

2 A

0

1

2 A

B ATw

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

The Tw dependent growthof the off-diagonal peaksin each block gives thechemical exchange rate.

A

B

A

B B

A

m

B

AA

B

A

B

all peakspositive going

all peaksnegative going

A

B

A

B B

A

m

B

AA

B

A

B

A

B

A

B

A

B

A

B B

A

B

A

m

B

A

B

AA

B

A

B

A

B

A

B

all peakspositive going

all peaksnegative going

Two-state chemical exchange dynamics

AA BBkAB

kBA

T1,A, or,A T1,B, or,B

Decay Decay

FFCFBFFCFA

Spectral diffusionSpectral diffusion

• Finding Optimal Solution Parameters

Thiocyanate Salts Solvents Dielectric Constant

Sodium Thiocyanate

Lithium Thiocyante

Dimethylsulphoxide(26.8ºF)Acetonirile (70° F)

Acetone (77° F)

Ethyl Ether (68° F)

Ethanol (77° F)

Methanol (77° F)

47.1

37.5

20.7

4.3

24.3

32.6

Nitrile Stretch = (2100-2240 cm-1)

Free thiocyanate ion ~ 2050 cm-1

Contact Ion Pair ~ 2070 cm-1

Contact Ion Pair

Free Ion

LiNCS

0

0.2

0.4

0.6

0.8

2020 2040 2060 2080 2100

Frequency

Abso

rban

ce

LiNCS-CH3CN

LiNCS-DMSO

LiNCS-Acetone

LiNCS-Ethanol

LiNCS-Ether

LiNCS-Ether-LiCl

NaNCS

0

0.2

0.4

0.6

0.8

2040 2060 2080

Frequency

Ab

sorp

tio

nNaNCS-CH3CN

NaNCS-Acetone

NaNCS-DMSO

NaNCL- 1:1 Acetone:Acetonitrile

Absorption of Solvents

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2000 2020 2040 2060 2080 2100

Frequency (cm-1)

Abs

orba

nce

DMSO

Acetonitrile:Acetone

Acetone

Methanol

Ethanol

Ethylether

Acetonitrile

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• Department of Energy• Dr. Steven Rock, Farah Rahbar, & Susan Schultz• Dr. Kelly Gaffney• Dr. Sugnam Park• Minbiao Ji

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