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

LoyolaUniversity, New Orleans, LA

Stanford Linear AcceleratorCenter

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 StanfordLinearAcceleratorCenter.

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Table of Contents

Page Number

  1. Abstract2
  1. Introduction3-5
  1. Methods5
  1. Results & Discussion5-8
  1. Conclusion8-9
  1. Future Work9
  1. Acknowledgements9
  1. Supplementary Information10-11
  1. References12

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 theirdependence 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.

Introduction-

Predicting 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 scaleslinking 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 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 vibrationswhich 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 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 frequencystretch (~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 ion-solvent 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.

Methods-

Optimal 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. 0.005-0.001mol 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 associatedcontact 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.

Results & Discussion-

The 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-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 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.

Conclusion-

The 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 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 Work-

With ion-solvent configurationsdetermined here for sodium thiocyanate, lithium thiocyanate, and potassium thiocyanate, two dimensional infrared spectroscopy will be performedwith 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.

Acknowledgements-

This 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.

Supplementary Information-

Thiocyanate Salts / Solvents / Dielectric Constant
Potassium Thiocyanate
Lithium Thiocyanate
Sodium Thiocyanate / 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

Table 1. Thiocyanate salts and solvents used in determining optimal IR parameters6

Figure 1. FTIR absorption spectra of lithium thiocyanate in different solvents

Figure 2. FTIR absorption spectra of sodium thiocyanate in acetonitrile, acetone, DMSO,

and one to one acetone to acetonitrile

Figure 3. FTIR spectra of potassium thiocyanate in acetone, acetonitrile, DMSO and

methanol

Figure 4. FTIR absorption spectra of solvents used

Resources-

  1. Suydam, I. T.; Boxer, S. G. Biochemistry 2003, 42, 120
  1. Park, S.; Kwan, K. Laser Phys. Lett., 2007, 705, 710
  1. Zheng, J.: Kwan, K; Fayer, M.D. Acc. Chem. Res. 2007, 76, 78
  1. Marcus, Y., Ion Solvation, New York, Wiley, 1985, in Chapter 3 “Infra-red Spectra and Solvation of Ions in Dipolar Protic Solvents”
  1. Butcher, P.N; Cotter, D. The elements of Nonlinear Optics, Cambridge University Press: Cambridge, U.K., 1990, 50-78
  1. Landolt-Börnstein, Group IV Physical Chemistry,Springer BerlinHeidelberg Press: Berlin, Germany, 2008, Volume 17, 269-270

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