Influence of Solute-Solvent Coordination on the Orientational Relaxation of Ion
Assemblies in Polar Solvents
Minbiao Ji1,2,, Robert W. Hartsock1,3, Zheng Sung1, and Kelly J. Gaffney1,
1
PULSE Institute, SLAC National Accelerator Laboratory, Stanford University,
Stanford, California, 94305, USA
2
3
Department of Physics, Stanford University, Stanford, California, 94305, USA
Department of Chemistry, Stanford University, Stanford, California, 94305, USA
Supporting Information for Publication
Assignment of ion assembly structures
Page S2-S4
Fig. S1
LiNCS concentration dependent FTIR spectra
Page S5
Fig. S2
Orientational relaxation dynamics
Page S6

current address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge,
Massachusetts, 02138, USA

email: kgaffney@slac.stanford.edu
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Assignment of Ion Assembly Structures
In extensive experimental studies, Martial Chabanel and coworkers characterized the
vibrational spectroscopy and nuclear structure of LiNCS ionic assemblies in aprotic polar
solvents.1-5 These studies have determined the structure and spectroscopic signatures of
free NCS-, LiNCS ion pairs,3,5 (LiNCS)2 ion pair dimers,4,5 and LiNCS ion-pair
tetramers6 with multiple spectroscopic techniques including FTIR spectroscopy, Raman
spectroscopy, and NMR spectroscopy. While the solutions we have investigated possess
multiple ionic structures in equilibrium, careful choice of solvent and LiNCS
concentration leads to solutions that exhibit only one CN-stretch absorption peak in the
FTIR spectrum. Under these circumstances, the properties of the (LiNCS)N can be
characterized unambiguously. Additionally, FTIR spectra can be measured as a function
of LiNCS concentration. These measurements have the expected behavior, where the
relative amplitudes of the CN-stretch peaks associated with distinct ionic structures vary
with concentration without significant variance in the peak widths or positions. We will
now discuss the structural assignments of Chabanel and co-workers to validate the
interpretation presented in our manuscript.
1. The mid-IR CN-stretch absorption of free thiocyanate (no contact ion pairing) occurs at
2056 cm-1 in aprotic polar solvents. The absence of this peak in the FTIR spectra and the
equivalent Li+ and NCS- concentrations for the solutions investigated in our manuscript
demonstrate that all anions make direct contact with Li+ cations.
2. The ionic structure that leads to an absorption peak at 2074 cm-1 in the FTIR spectrum
was assigned a structure analogous to that shown in Figure 2(A) long before our
calculations. The chemical shift for the 15N NMR spectrum, the equivalence of the CN-
S2
stretch frequency in the Raman and FTIR spectra, and the degeneracy of the NCS - bend
have all been used to assign the CN-stretch absorption at 2074 cm-1 to a linear LiNCS ion
pair where the Li+ is coordinated by the N-end of the NCS-.
3. The ionic structure that leads to an absorption peak at 2042 cm-1 in the FTIR spectrum
was assigned to a structure analogous to that shown in Figure 2(B) long before our study.
Again, this assignment was based on NMR, FTIR, and Raman spectroscopy. NMR
measurements demonstrate that both Li+ cations are N bonded in this structure. The CNstretch has distinct frequencies in the FTIR and Raman spectra, consistent with an ion
assembly structure with a center of inversion. Additionally, the structure leading to
absorption at 2042 cm-1 in the FTIR spectrum, no longer has degenerate NCS- bending
absorptions. Taken together, the structure associated with the peak at 2042 cm-1 must be
non-linear and centro-symmetric, like the structure in Figure 2(B).
4. The assignment of the peak at 2099 cm-1 to a linear dimer proves more challenging than
the ion pair and quadrupole ion-pair dimer assignments because a simple LiNCS solution
cannot be made that only has one CN stretch at 2099 cm-1 in the FTIR spectrum. What
has been done experimentally is investigate solutions where the Li+ concentration
exceeds the NCS- concentrations. In these solutions, the peak at 2099 cm-1 grows at the
expense of the ion pair and quadrupole ion-pair dimer peaks and has been assigned to
LiNCSLi+ ion assemblies with Li+ cations associated with both the N and the S ends of
thiocyanate.2 For our solutions, the Li+ and NCS- concentrations are equal and we
observe no free thiocyanate absorption. We conclude based on these observations, that
the ion structure associated with the peak at 2099 cm-1 should be charge neutral and have
one of the two thiocyanate anions coordinated by two Li+ cations. This picture is
supported by quantum chemical calculations.
5. Chabanel and co-workers have concluded from extensive studies that the largest LiNCS
ionic assembly in polar aprotic solvents is an ion-pair tetramer that has been observed in
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ether and trialkyl amine solutions. These tetramers have a CN-stretch absorption at 1993
cm-1 in the FTIR spectrum. We see no evidence for these tetramers in the solutions
studied in our manuscript.
References
1
2
3
4
5
6
M. Chabanel, Pure and Applied Chemistry 62, 35 (1990).
P. Goralski and M. Chabanel, Inorg. Chem. 26, 2169 (1987).
D. Paoli, M. Lucon, and M. Chabanel, Spectrochim. Acta A 34, 1087 (1978).
D. Paoli, M. Lucon, and M. Chabanel, Spectrochim. Acta A 35, 593 (1979).
J. Vaes, M. Chabanel, and M. L. Martin, J. Phys. Chem. 82, 2420 (1978).
M. Chabanel, M. Lucon, and D. Paoli, J. Phys. Chem. 85, 1058 (1981).
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Figure S1: Concentration-dependent FTIR spectra of LiNCS desolved in benzonitrile.
While LiNCS appears mainly as ion pairs absorbing at 2073 cm-1 at low ionic
concentration, it forms quadrupole ion-pair dimers at higher concentration, contributing a
new absorption peak at 2042 cm-1. The weak shoulder at 2099 cm-1 that increases with
increasing LiNCS corresponds to the linear ion-pair dimer.
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Figure S2: Anisotropic pump-probe measurements of the orientational relaxation
dynamics of the CN stretch of NCS- in the ion pair and quadrupole ion-pair dimer
configurations for 1.2 M LiNCS concentrations in (A) dimethyl carbonate and (B) ethyl
acetate. The rotational relaxation time constants and amplitudes extracted from triexponential fits to the experimental data can be found in main text of the article.
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