Finding optimal ion-solvent configurations using FTIR spectroscopy for studying
ion association and dissociation dynamics in thiocyanate salt solutions 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 1, 2008
Prepared in partial fulfillment of the requirement of the Office of Science, Department of
Energy’s Science Undergraduate Laboratory Internship under the direction of Kelly
Gaffney in the Stanford Synchrotron Radiation Laboratory division of the Stanford
Linear Accelerator Center.
Participant:
__________________________
Signature
Research Advisor:
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Signature
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Table of Contents
Page
Number
1. Abstract
2
2. Introduction
3-5
3. Methods
5
4. Results & Discussion
5-8
5. Conclusion
8-9
6. Future Work
9
7. Acknowledgements
9
8. Supplementary Information
10-11
9. References
12
1
ABSTRACT
Finding optimal 2D-IR Solution Parameters for Thiocyanate Salts using FTIR. MARIA G.
GONZALEZ (Loyola University, New Orleans, LA 70118) KELLY GAFFNEY
(Stanford Linear Accelerator Center, Palo Alto, CA 94305)
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, limiting
the understanding of very basic chemical processes. Recently, two dimensional infrared
spectroscopy (2D-IR) has been used to probe molecular dynamics under thermal
equilibrium conditions occurring on femto- and picosecond time scales. 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 as well as
their dependence on the interactions with surrounding solvents. Vibrational modes of
thiocyanate ion (NCS-), which were sensitive to ionic configurations (free ion and contact
ion pair forms), were used as a probe in studying the association and dissociation
equilibrium dynamics between the thiocyanate ion and its counter cation in solution. The
ion-solvent configurations in the thiocyanate salt solutions optimal for 2D-IR
spectroscopy were determined by a one to one area ratio of the NCS- peaks in free ion
and contact ion pair forms using Fourier transform infrared (FTIR) spectroscopy. Optimal
ion-solvent configurations for lithium thiocyanate, sodium thiocyanate and potassium
thiocyanate were respectively determined to be 0.040 ±0.002M lithium thiocyanate in
diethyl ether with 0.118mols ± 0.001 of lithium chloride, 0.060±0.003M sodium
thiocyanate in acetonitrile and 0.050±0.004M of potassium thiocyanate in methanol.
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IntroductionPredicting the equilibrium structural dynamics of molecular systems in biology
and chemistry is founded on understanding the molecular interactions with surrounding
solvents as well as their local structures, heterogeneities, and interconversion between
different configurations occurring under thermal equilibrium conditions. Understanding
very basic molecular dynamics such as conformational changes in solutes, vibrational
relaxation processes, and fluctuations in solvent configurations has been constrained by
the fast time scale of the processes. Effective experimental probes of molecular dynamics
in solution must be sensitive to structural changes occurring on subpicosecond and
picosecond time scales linking chemical understanding to technological advancement1.
Two dimensional infrared spectroscopy (2D-IR) has now been used for some time as a
sensitive tool for determining structural dynamics in solution phases giving insight into
the fundamental behaviors of molecules in solution.
Vibrations are the mechanical degrees of freedom of molecules and therefore are
sensitive to the dynamics of intermolecular attraction and structure2. When probing a
molecular system with many degrees of freedom, one-dimensional spectroscopy projects
the average response of electronic or nuclear coordinates onto one frequency axis,
showing a single set of experimental observables mutually affected by multiple molecular
coordinates. This makes the spectra featureless. Two dimensional infrared spectroscopy
(2D-IR) is a method that incorporates the structural selectivity of two-dimensional
nuclear magnetic resonance (2D-NMR) and circumvents the difficulties with traditional
optical spectroscopy. 2D-IR experiments on multiple vibrational systems are performed
3
using three successive short pulses with enough spectral bandwidth to excite all the
vibrational transitions of interest2. Spectra contain peaks with different positions, signs,
amplitudes, and line shapes characterizing the vibrational Eigen states of the transition
frequencies between the ground and excited states of the system. These frequencies
describe the anharmonic vibrational potential3. Peak amplitudes reflect the relative
magnitudes and orientations of the transition dipole moments in the molecular frame, the
electrical anharmonicity of the system and the vibrational relaxation dynamics (which are
sensitive to solvent interactions)3. The rate of chemical exchange can be determined by
observing the growth of the off-diagonal cross peaks in the 2D-IR spectrum3. In essence,
2D-IR spectra can be used to characterize time-dependent couplings both between
vibrational coordinates and vibrations and their surroundings which are necessary to
describe the dynamics of molecules in solution2.
The ion association and dissociation dynamics in thiocyanate salt solutions will be
probed using 2D-IR allowing for the determination of equilibrium kinetics as well as its
dynamic interactions in solution. A stretch vibration (~2050 cm-1) of thiocyanate ion
(NCS-) was chosen due to its relatively long vibrational lifetime, allowing for measuring
the dynamics up to a few hundred picosecond time range. Also, its vibrational resonances
are particularly sensitive to ionic configurations depending on free ion and contact ion
pair forms. Thiocyanate ion is linear with C∞v symmetry and has three IR-active
vibrations which are perturbed by ion-ion interactions5. The frequencies of the v1(CS),
v2(NCS), and v3(CN) modes of thiocyanate anion increase as a result of interactions with
counter cations4. Because the cation bonds to the nitrogen atom, the nitrile stretch, v3(CN)
(~2050 cm-1), which is typically quite intense (50-1000M-1 cm-1) is useful in revealing the
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dynamics of interaction among the thiocyanate anion and the counter cations in solution4.
Free thiocyanate anion has a low frequency nitrile stretch (~2050 cm-1) while a high
frequency stretch (~2070 cm-1) is indicative of the contact ion pair (X+:NCS-). In order to
quantify the ion association and dissociation dynamics of thiocyanate salts in solutions,
free thiocyanate ion and contact ion pair must be at thermal equilibrium. Optimal ionsolvent configurations for potassium thiocyanate, sodium thiocyanate and lithium
thiocyanate can then be determined using FTIR by a one to one peak ratio of free
thiocyanate ion to the contact ion pair.
MethodsOptimal ion-solvent configurations for sodium thiocyanate, lithium thiocyanate,
and potassium thiocyanate were determined by varying the salt concentration in various
solvents, listed in Table 1 below. Optimal ion-solvent configuration is referred to as a one
to one area ratio of free thiocyanate ion and contact counter cation-thiocyanate anion pair
(contact ion pair). Salt concentration was maintained at a molar concentration range of
0.2-0.002M so as not to exceed the absorbance of ~ 0.3. Thiocyanate salts were dissolved
in different solvents to prepare tens of mM sample solutions and were diluted with the
same solvents to get the final sample solutions with the absorbance of ~0.3. 0.005-0.001
mol of KCl, LiCl, or NaCl was added depending on the solution used in order to shift
equilibrium from the dissociated free ion form to the associated contact ion pair form to
approximately obtain a one to one peak ratio between free thiocyanate ion and contact ion
pair when appropriate. FTIR spectra of all samples were measured at room temperature in
a home-made sandwich cell with two CaF2 windows and 100 μm Teflon spacer.
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Results & DiscussionThe optimal ion-solvent configuration for lithium thiocyanate was in diethyl ether
at a molar concentration of 0.040 ±0.002M with 0.118mols ± 0.001 of lithium chloride
added to the solution. Lithium cation having a half filled s-orbital is strongly attracted to
the thiocyanate anion, requiring a significantly polar solvent at high concentration to
dissociate it into free lithium cation and thiocyanate anion. Thiocyanate salt in acetone
and acetonitrile was predominantly in a contact ion pair form showing a relatively strong
peak at the higher frequency (~ 2070cm-1) as depicted in Figure 1 below. In order to shift
equilibrium to the free thiocyanate ion form, significant dilution would have to occur.
However, in this case, the absorbance would be too small to be experimentally practical.
Lithium thiocyanate in dimethyl sulphoxide (DMSO) was completely dissociated
showing only one peak at approximately 2050cm-1. Dimethyl sulphoxide (DMSO) having
a larger dielectric constant than acetone and acetonitrile and a predominantly negative
outer charge is better able to shield the cation in solution. However, it also strongly
absorbs from 2000-2100cm-1 interfering with the absorption peak of thiocyanate ion.
Thiocyanate in ethanol has an approximately equal amount of free ion and contact ion
pair forms, however, due to hydrogen bonding it has broad band widths and a third
smaller peak at approximately 2090cm-1. Lithium thiocyanate in ether had a large peak
separation between free ion and contact ion pair forms with a predominant concentration
of free ion form. Adding lithium chloride shifted equilibrium to the contact ion pair form
yielding an approximate one to one peak area between free ion and contact ion pair forms.
Lithium thiocyanate in ether with added lithium chloride proved to be the best ion-
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solvent configuration for lithium thiocyanate when utilizing 2D-IR due to the large peak
separation and approximate one to one peak area ratio between the free thiocyanate ion
and contact lithium-thiocyanate ion pair.
The optimal ion-solvent configuration for sodium thiocyanate was determined to
be in acetonitrile at a molar concentration of 0.060±0.003M. Sodium thiocyanate in
dimethyl sulphoxide was completely dissociated. In acetone, it was primarily in a contact
ion pair form as shown in Figure 2 below. Sodium thiocyanate in acetonitrile showed
relative amounts of free ion and contact ion pair forms with a larger concentration of free
thiocyanate ion. As acetone was added to the solution of sodium thiocyanate in
acetonitrile, the peak area difference between free thiocyanate ion and the contact ion pair
decreased as a result of shifting equilibrium to the contact ion pair form. The peak
separation also decreased, making it harder to differentiate peaks when utilizing 2D-IR.
Increasing the concentration of the contact ion pair form in acetonitrile would require
adding NaCl salt. However, a relatively low polarity of acetonitrile makes it difficult to
dissolve NaCl salt in the sodium thiocyanate solution.
Potassium being a large cation posed a challenge in finding a solvent with
adequate polarity to yield a one to one concentration ratio between free thiocyanate and
the contact ion pair. A molar concentration of 0.050±0.004M of potassium thiocyanate in
methanol showed the most promise due to an approximate one to one peak area ratio
between the free ion and contact ion pair as shown in Figure 3. Thiocyanate salt in
acetone and acetonitrile was predominantly in a contact ion pair form with barely any
free thiocyanate ion. In order to increase the concentration of free thiocyanate ion,
significant dilution would have to occur where the absorbance would be too small to be
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experimentally useful. Potassium thiocyanate in DMSO was entirely dissociated. DMSO
poses a problem as a solvent, first because it absorbs strongly in the frequency region of
interest and secondly because adding strong salts, such as potassium chloride, does not
result in a homogenous system. The added salt would be used to shift equilibrium to the
contact ion pair form. However, it was found that due to the low solubility of ionic salt,
DMSO could not dissolve significant amount of salt to have one to one peak ratio of free
ion and contact ion pair. Using methanol as a solvent gave approximately a one to one
peak area ratio between free ion and contact ion pair. Due to hydrogen bonding, a broad
band width, with small peak separation occurs, and there is a third smaller peak at
approximately 2090m-1. A solvent less polar than DMSO but more polar than acetonitrile,
preferably without hydrogen bonding, would be the optimal solvent for potassium
thiocyanate.
ConclusionThe optimal ion-solvent configurations for lithium thiocyanate, determined by a
one to one ratio of the stretch vibrations of thiocyanate in free ion form to the contact ion
pair form using FTIR spectroscopy, was resolved to be in diethyl ether at a molar
concentration of 0.040 ±0.002M with 0.118mols ± 0.001 of lithium chloride. The most
favorable solution parameter for sodium thiocyanate was determined to be in acetonitrile
at a molar concentration of 0.060±0.003M. For potassium thiocyanate, a molar
concentration of 0.050±0.004M of potassium thiocyanate in methanol showed the most
promise. However, methanol absorbs strongly at the frequency range of interest
(~2050cm-1). Also, due to hydrogen bonding there is broad band width, with small peak
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separation, and a third smaller peak at approximately 2090m-1. A solvent less polar than
DMSO but more so than acetonitrile preferably without a hydrogen bond would be the
optimal solvent to use for potassium thiocyanate.
Future WorkWith ion-solvent configurations determined here for sodium thiocyanate, lithium
thiocyanate, and potassium thiocyanate, two dimensional infrared spectroscopy will be
performed with femtosecond mid-IR laser system. Utilizing 2D-IR spectroscopy, the ion
association and dissociation dynamics in thiocyanate salt solutions will be studied. This
will give valuable knowledge not only to the time scales of ion association and
dissociation dynamics but to further potential application of this knowledge to understand
a variety of ionic systems found in chemistry, biology, and physics.
AcknowledgementsThis work was supported by the Department of Energy at the Stanford Linear
Accelerator Center. I would like to thank my mentor Dr. Kelly Gaffney and Dr. Sungnam
Park as well as the program director Dr. Steven Rock for their guidance. I would also like
to thank Minbiao Ji and Robert Hartsock for their assistance.
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Supplementary InformationThiocyanate Salts
Potassium Thiocyanate
Lithium Thiocyanate
Sodium Thiocyanate
Solvents
Dimethylsulphoxide(26.8ºF)
Acetonirile (70° F)
Acetone (77° F)
Ethyl Ether (68° F)
Ethanol (77° F)
Methanol (77° F)
Dielectric
Constant
47.1
37.5
20.7
4.3
24.3
32.6
Table 1. Thiocyanate salts and solvents used in determining optimal IR parameters6
LiNCS
LiNCS-CH3CN
0.8
LiNCS-DMSO
Absorbance
LiNCS-Acetone
0.6
LiNCS-Ethanol
LiNCS-Ether
0.4
LiNCS-Ether-LiCl
0.2
0
2020
2040
2060
2080
2100
Frequency
Figure 1. FTIR absorption spectra of lithium thiocyanate in different solvents
NaNCS
0.8
NaNCS-CH3CN
NaNCS-A cetone
NaNCS-DMSO
Absorption
0.6
NaNCL- 1:1 A cetone:
A cetonitrile
0.4
0.2
0
2040
2060
2080
Fr e que ncy
Figure 2. FTIR absorption spectra of sodium thiocyanate in acetonitrile, acetone, DMSO,
and one to one acetone to acetonitrile
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KNCS
Absorbance
0.8
KNCS-Acetone
0.6
KNCS-CH3CN
0.4
KNCS-DMSO
KNCS-Methanol
0.2
0
2030
2050
2070
2090
2110
Frequency
Figure 3. FTIR spectra of potassium thiocyanate in acetone, acetonitrile, DMSO and
methanol
Absorption of Solvents
Absorbance
0.8
0.7
DMSO
0.6
Acetonitrile:Acetone
0.5
Acetone
0.4
Methanol
0.3
Ethanol
0.2
Ethylether
0.1
Acetonitrile
0
2000
2020
2040
2060
2080
2100
Frequency (cm-1)
Figure 4. FTIR absorption spectra of solvents used
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2. Park, S.; Kwan, K. Laser Phys. Lett., 2007, 705, 710
3. Zheng, J.: Kwan, K; Fayer, M.D. Acc. Chem. Res. 2007, 76, 78
4. Marcus, Y., Ion Solvation, New York, Wiley, 1985, in Chapter 3 “Infra-red Spectra
and Solvation of Ions in Dipolar Protic Solvents”
5. Butcher, P.N; Cotter, D. The elements of Nonlinear Optics, Cambridge University
Press: Cambridge, U.K., 1990, 50-78
6. Landolt-Börnstein, Group IV Physical Chemistry, Springer Berlin Heidelberg Press:
Berlin, Germany, 2008, Volume 17, 269-270
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