Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and

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Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and LiClOTetrahydrofuran Solutions Studied by Low-Frequency Isotropic Raman Scattering and Neutron Diffraction with Li/Li Isotopic Substitution Methods 6

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Yasuo Kameda, Saki Ebina, Yuko Amo, Takeshi Usuki, and Toshiya Otomo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b03550 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 11, 2016

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The Journal of Physical Chemistry

Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and LiClO4-Tetrahydrofuran Solutions Studied by Low-frequency Isotropic Raman Scattering and Neutron Diffraction with 6

Li/7Li Isotopic Substitution Methods

Yasuo Kameda,*† Saki Ebina,† Yuko Amo,† Takeshi Usuki,† Toshiya Otomo‡

†

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University,

Yamagata 990-8560, Japan ‡

Institute of Material Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan

ACS Paragon Plus Environment

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ABSTRACT: Low-frequency isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for 6Li/7Li and H/D isotopically substituted *LiCl- and *LiClO4tetrahydrofuran (*THF) solutions in order to obtain microscopic insight into solvated Li+, Li+…Cl- and Li+…ClO4- contact ion pairs formed in concentrated THF solutions. Symmetrical stretching vibrational mode of solvated Li+ in LiCl- and LiClO4 solutions was observed at ν = 181~184 and 140 cm-1, respectively. The stretching vibrational mode of Li+…Cl- and Li+…ClO4solvated contact ion pairs formed in 4 mol% 6LiCl-THF-h8 and 7LiCl-THF-h8 solutions was found at ν = 469 and 435 cm-1, respectively. Detailed structural properties of solvated Li+ and the contact ion pairs were derived from the least squares fitting analyses of the first-order difference function, ∆Li(Q), obtained from neutron diffraction measurements on 6Li/7Li isotopically substituted THF-d8 solutions. It has been revealed that Li+ takes four-fold coordination in the average local structure of Li+X-(THF)3, X = Cl and ClO4. The nearest neighbor Li+…O(THF) distance was determined to be 2.21 ± 0.01 Å and 2.07 ± 0.01 Å for 4 mol% *LiCl- and 10 mol % *LiClO4-THF-d8 solutions, respectively. The Li+…anion distances for Li+…Cl- and Li+…O(ClO4-) contact ion pairs were determined to 2.4 ± 0.1 Å and 2.19 ± 0.01 Å, respectively. The nearest neighbor Li+…THF interaction is significantly modified by anion in the first solvation shell.

INTRODUCTION Structure of solvated ions plays an essential role in extensive fields of both fundamental sciences and industrial processes. Although structure and dynamics of hydrated ions in aqueous solution have long been major subjects of a number of research papers,1 relatively fewer structural works on solvation structure of ions in non-aqueous solutions have been conducted due


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to the complexity of structure of the solvent molecule itself. Since physicochemical properties of non-aqueous solutions are mostly governed by intermolecular interactions namely, ion-solvent, solvent-solvent, and ion-ion interactions, which are characteristic of the individual solvent molecule, extremely wider variation of potential properties are therefore expected for nonaqueous solutions.2 Investigation of structure and dynamics in non-aqueous solutions has currently been one of the most important subjects in the fields of fundamental sciences. For example, dynamics of LiX (X= Br, I)-acetone solutions has been investigated by means of depolarized Rayleigh and lowfrequency Raman scatterings.3 The observed slow dynamics of acetone molecules is assigned to the orientational relaxation of the molecules forming Li+ solvation shells which are significantly affected by the Li+…X- ion pairing.3 More recently, electronic properties of contact ion pair oligomer anions in LiClO4-THF solutions have been studied by picosecond pulse radiolysis measurements.4 It has been revealed that the dimer anion, (LiClO4)2-, presents higher stability toward ClO4- reduction into ClO3-.4 These investigations clearly show that the ion pair formation is important to understand physical properties of concentrated non-aqueous solutions. In recent years, much attention has been received on the solvation structure of metal ions in non-aqueous solutions closely connected with development of advanced energy storage devices such as lithium ion secondary batteries.5 Non-aqueous solutions involving organic molecules containing ether- and carbonyl oxygen atoms, which are expected to interact strongly with metal cations, have been a matter of great interest as electrolyte solutions used for high performance Li ion batteries, next generation Li-air6,7 and Mg ion batteries.8,9 Since the physical properties of non-aqueous solutions are mainly determined by the short-range order arising from


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intermolecular interactions among ion-ion and ion-solvent molecules, the insight into the microscopic structure of the solvated ion is therefore indispensable. Molecular dynamics (MD) simulation has a great advantage to investigate the structure of nonaqueous solutions in the atomic resolution.10,11 Individual partial pair correlation functions in the multi-component system can readily be obtained from calculated trajectories of constituent atoms, however, the reliability of the results of the MD simulation such as the nearest neighbor intermolecular distances and coordination numbers can completely be decided by the validity of interaction potentials and partial charges employed in the simulation.12 It is necessary to verify the consistency of the MD results and corresponding experimental ones before discussing detailed microscopic properties of the solution.12,13 Indeed, the nearest neighbor Li+…O distance for propylene carbonate (PC) and dimethyl carbonate (DMC) solutions involving Li-salts obtained from previous MD simulations has been both reported to be ca. 1.8 Å10,11, which is remarkably shorter than that determined from our recent neutron diffraction studies with 6Li/7Li isotopic substitution method, rLiO = 2.04 ± 0.01 Å for 10 mol% LiPF6-PC14 and rLiO = 2.08 ± 0.02 Å for 10 mol% LiPF6-DMC solutions,15 respectively. More recent MD investigations indicated longer Li+…O(carbonyl) distances, 1.95 Å (for Li+…ethylene carbonate (EC) interaction)16 and 1.98 Å (for Li+…EC and Li+…DMC interactions),17 however, these are still ~ 0.1 Å shorter than those determined from the experiment. It is obvious that more accurate Li+…O (carbonyl) pair potential is needed for the Li+…O(carbonyl) interaction. Non-aqueous solutions involving ether and glyme molecules have attracted much attention as promising electrolyte solutions employed for the next generation Li-O2 batteries.7 Recent MD simulations of electrolyte solutions involving Li+…oligoglyme interactions reported the nearest neighbor Li+…O(ether) distance of 2.1 Å (for Li+…tetraglyme interaction)18 and 2.0 Å (for Li+…hexaglyme


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interaction),19 however, experimental determination of the nearest neighbor Li+…O distance in the ether solutions, especially in cyclic ethers by means of neutron diffraction has not yet been reported at present. Neutron diffraction experiment with 6Li/7Li isotopic substitution method is one of the most effective experimental techniques to obtain detailed information on the environmental structure around Li+ in the solution.20 Since the statistical quality of difference function, ∆Li(Q), between observed scattering cross sections for 6Li and 7Li samples, is roughly proportional to the atomic fraction of Li in the sample solution, cLi, it has been considered to be quite difficult to obtain reliable ∆Li(Q) for dilute non-aqueous solutions in which the value of cLi become considerably small due to the larger number of atoms involved in the solvent molecule. In order to overcome these difficulties, the use of high performance neutron spectrometer installed at the next generation high intensity pulsed neutron source is necessary. In this paper, we describe results of neutron diffraction measurements on 6Li/7Li substituted concentrated LiCl- and LiClO4-THF solutions in order to obtain direct information on the structure of solvated Li+ and Li+…anion ion pair formed in the ether solution. Neutron diffraction was carried out using newly constructed high performance total scattering spectrometer installed at high intensity pulsed neutron source in the J-PARC, Japan. Furthermore, the low-frequency isotropic Raman spectral measurements have been conducted to obtain information on the intermolecular vibrational modes of solvated Li+ and Li+…anion ion pair, which may assist the structural analysis of observed neutron diffraction data. Low-frequency isotropic Raman spectra can extract intermolecular vibrational modes with higher symmetry out of a number of overlapping other intermolecular vibrational modes involved in the solution.21,22


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EXPERIMENTAL SECTION Materials. 6Li- and 7Li-enriched lithium chloride and lithium perchlorate were prepared by reacting 6Li2CO3 (95.6% 6Li) and 7LiOH.H2O (99.99% 7Li) obtained from Tomiyama Chemical Co. Ltd. Tokyo with a slightly excess amount of concentrated aqueous HCl and HClO4 solutions (Nacalai Tesque, guaranteed grade), respectively. The dehydration of the product solutions were achieved by heating at 120 °C under vacuum for more than a day. The weighted amount of enriched anhydrous *LiCl and *LiClO4 were dissolved into anhydrous THF (natural abundance, Nacalai Tesque, guaranteed grade) and THF-d8 (99.5% D, ISOTECH Inc.), dried on molecular sieves 3A (Nacalai Tesque), to prepare 4 mol% *LiCl and 10 mol% *LiClO4 solutions with different 6Li/7Li and H/D isotopic compositions. Raman Scattering Measurements. The sample solution was introduced in a Pyrex® Raman cell (10 × 10 mm, 40 mmH). Raman spectra were measured at 25°C in the frequency range of 30 ≤ ν ≤ 1000 cm-1 using Jobin-Yvon Ramanor U1000 spectrometer with 488 nm excitation line of NEC GLG-2165 Ar+ laser operated at 200 mW. Raman scattering intensity was recorded at 1 cm1

intervals in both the parallel and perpendicular polarization modes with a spectral slit width of

2.4 cm-1. Observed parallel and perpendicular spectra were converted to the reduced intensity which is proportional to the square of the polarizability change during the vibration.23-25

Icorrected (ν) = (ν0 - ν)-4ν[1 – exp(-hν/kT)]Iobs(ν),

(1)


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where, ν andν0 denote the Stokes Raman shift and wavenumber of the incident light, respectively. h and k stand for the Planck constant and the Boltzmann constant, respectively. The absolute temperature is denoted by T. The isotropic Raman intensity was obtained by

Iiso (ν) = I//(ν) – 4/3 τ I⊥ (ν).

(2)

Where, I//(ν) and I⊥(ν) are the corrected parallel and perpendicular intensities, respectively. The calibration factor, τ, was carefully determined through measurements of ν1, ν2, and ν4 bands of liquid pure CCl4. In the isotropic spectra, vibrational band with higher symmetry is selectively observed. The peak decomposition of the observed isotropic intensity was performed using the least squares fitting procedures by the SALS program.26 The Gaussian peak shape function was employed for the analyses of low-frequency intermolecular vibrational components with the background of the third order polynomial of ν, which has successfully been applied to the analysis of totally symmetric vibrational band of hydrated alkali metal ions.22,27,28 For deconvolution analysis of the ν1 band of ClO4-, overlapping with the C-O-C symmetrical stretching band of the THF molecule, the Voigt peak shape function29,30 was employed with the straight background. In the present analysis, the value of Gaussian width parameter in the Voigt function was fixed at the optical resolution of the spectrometer (2.4 cm-1). Neutron Diffraction Measurements. The sample solution was introduced into a thin-walled cylindrical V-Ni null alloy cell (6.0 mm in inner diameter and 0.1 mm in wall thickness) and sealed by an indium seal. The sample parameters are listed in Table 1.


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Neutron diffraction measurements were carried out at 25°C using the NOVA total scattering spectrometer32 installed at BL21 of the MLF pulsed neutron source in J-PARC, Tokai, Japan. Incident beam power of the proton accelerator was 200 and 300 kW in measurements for 4 mol% LiCl- and 10 mol% LiClO4 solutions, respectively. Scattered neutrons (neutron wave band of 0.1 ≤ λ ≤ 8.7 Å) were detected by ca. 900 of 20 atm 3He position sensitive proportional counters (1/2 inch φ, 800 mm in active length with 5 mm in positional resolution) installed at 20° (13.1-27.9°), 45° (33.6-54.9°), 90° (72.7-107.4°) and back scattering (136.5-169.0°) detector banks. The data accumulation time was ca. 3 h for each sample. Measurements were made in advance for 6 mm in diameter vanadium rod, empty cell and instrumental background. Observed scattering intensities for the sample were corrected for instrumental background, absorption of sample and cell33, multiple34 and incoherent scatterings. The coherent scattering lengths, scattering and absorption cross sections for the constituent nuclei were referred to those tabulated by Sears.31 The wavelength dependence of the total cross sections for H and D nuclei was estimated from the observed total cross sections for H2O and D2O, respectively.35 The inelasticity correction was adopted using the self-scattering intensity observed for the null-H2O.36 The first-order difference function, ∆Li(Q),20,37 is derived from the numerical difference between scattering cross sections observed for two solutions that are identical except for the scattering length of Li. The ∆Li(Q) normalized for stoichiometric units, (*LiCl)x(THF-d8)1-x and (*LiClO4)x(THF-d8)1-x, can be written as linear combination of partial structure factors, aLij(Q), involving correlations from the Li-j pair:

∆Li(Q) = A[aLiO(Q)-1] + B[aLiC(Q)-1] + C[aLiD(Q)-1] + D[aLiCl(Q)-1] + E[aLiLi(Q)-1],

(3)


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where, A = 2x(1-x) (b6Li - b7Li) bO, B = 8x(1-x) (b6Li - b7Li) bC, C = 16x(1-x) (b6Li - b7Li)bD, D = 2x2 (b6Li - b7Li) bCl, and E = x2 (b26Li - b27Li) for (*LiCl)x(THF-d8)1-x solution, and A = 2x(1+3x) (b6Li - b7Li) bO, B = 8x(1-x) (b6Li - b7Li) bC, C = 16x(1-x) (b6Li - b7Li)bD, D = 2x2 (b6Li - b7Li) bCl, and E = x2 (b26Li - b27Li) for (*LiClO4)x(THF-d8)1-x solution, respectively. Numerical values for coefficients, A-E in Eq 3 are summarized in Table 2. Since the observed ∆Li(Q) from forward angle detector pixels located at 13.1 ≤ 2θ ≤ 54.9° agrees well within the statistical uncertainties, they were combined at the Q-interval of 0.1 Å-1 and used for subsequent analyses. The distribution function around Li+, GLi(r), is deduced from the Fourier transform of ∆Li(Q):

Qmax

∫

GLi(r) = 1 + (A + B + C + D + E)-1(2π2ρ0r)-1 Q∆Li(Q) sin(Qr) dQ 0

= [AgLiO(r) + BgLiC(r) + CgLiD(r) + DgLiCl(r) + EgLiLi(r)] (A + B + C + D + E)-1.

(4)

The upper limit of the integral was set to 20 Å-1 in the present study. Structural parameters concerning the solvation shell of Li+ were obtained through the leastsquares fitting procedure of the following model function:38-40


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∆Limodel(Q) = Σ2cLinLijbj(b6Li – b7Li)exp(-lLij2Q2/2)sin(QrLij)/(QrLij) + 4πρ0(A + B + C + D + E)exp(-l02Q2/2)[Qr0cos(Qr0) – sin(Qr0)]Q-3,

(5)

where, cLi and nLij are the number of Li+ in the stoichiometric unit and the coordination number of the j atom around Li+, respectively. Parameters, lLij and rLij denote the root mean square amplitude and internuclear distance of Li+…j pair, respectively. The long-range parameter, r0, is the distance beyond which continuous distribution of atoms around Li+ can be assumed. The parameter, l0, describes the sharpness of the boundary at r0. Structural parameters, nLij, lLij, rLij, r0, l0, were determined from the least squares fit to the observed ∆Li(Q). The fitting procedure was performed in the range of 0.1 ≤ Q ≤ 20 Å-1 with the SALS program,26 assuming that statistical uncertainties are distributed uniformly. In the least squares fitting calculation, the Marquardt method was employed. The dynamical bi-weight procedure was used for weight adjustment of the observed data.26 Prior to the fitting analysis, correction for the low-frequency systematic error involved in the observed ∆Li(Q) was adopted.41 The model function, ∆Limodel(Q), in Eq 5 was evaluated on the basis of the following assumptions. (a) For the nearest neighbor interaction between the Li+ and THF-d8 molecule, structural parameters, rLiO, lLiO, nLiO, the bond angle, α, between Li+…O(THF) axis and the bisector of ∠C-O-C in the THF molecule, and the dihedral angle β between the plane involving atoms, Li+…O and the bisector of ∠C-O-C in the THF molecule, and the plane involving C-O-C atoms, are treated as independent parameters. The molecular geometry of THF was fixed to that


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determined from a gas phase electron diffraction study.42 The root mean square displacements for non-bonding interaction between Li+…THF molecule except for the first nearest Li+…O pair, lLij, were approximated through the following equation:38

lLij = (lLiO + l*) × (rLij/rLiO)1/2

(6)

In the present analysis, the modification factor for the r.m.s displacement, l*,was introduced to take into account for the geometrical fluctuation of the Li+…THF interaction within the first solvation shell of Li+. (b) Structural parameters for the nearest neighbor Li+…Cl- and Li+…ClO4interactions were treated as independent parameters. Parameters, rLiCl, lLiCl and nLiCl, were determined independently in the analysis of ∆Li(Q) for the LiCl-THF-d8 solution. For the LiClO4-THF-d8 solution, the structure parameters for the nearest neighbor Li+…ClO4- interaction, rLiO(ClO4-), lLiO(ClO4-), nLiO(ClO4-), the bond angle, γ (= ∠Li+…O-Cl), and the dihedral angle δ between the plane involving Li+…O-Cl atoms and the plane involving O-Cl-O atoms within ClO4-, were refined independently. The geometry of ClO4- was fixed to that determined from single crystal X-ray43,44 and neutron diffraction45 studies. (c) Structural parameters for long-range random distribution of atoms, r0 and l0, were treated as independent parameters.

RESULTS AND DISCUSSION Raman Spectra. Polarized Raman spectra observed for 4 mol% XLiCl-THF-Y8 (X = 6, 7, Y = h, d) solutions are compared with that for pure THF-h8 in Figure. 1. Depolarized peaks at ~290


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and ~600 cm-1 observed for the pure THF-h8 are mainly arising from the ring-packering and ring-bending vibrational modes of THF molecule, respectively.46,47 These vibrational modes can respectively be observed at almost the same frequency in the parallel and perpendicular spectra for 6LiCl and 7LiCl in THF-h8 solutions, indicating that the molecular geometry of the THF remains unchanged in the concentrated LiCl solutions. These peaks shift to lower frequencies at ~230 and ~470 cm-1 in the *LiCl solutions in THF-d8, respectively. A well resolved polarized peak on a broader component is found at ~180 cm-1 in the isotropic spectra for both *LiCl-THFh8 and *LiCl-THF-d8 solutions. Since this peak is not present in the isotropic spectrum of pure THF-h8, this peak can be attributable to intermolecular vibrational mode arising from THF and constituent ions. The position of this peak seems almost unchanged for 6Li/7Li substitution, suggesting that the lithium ion does not move during the vibration. The interaction between Li+ and oxygen atom of the THF molecule is known to be sufficiently strong.48 In isotropic Raman spectra, the vibrational mode with higher symmetry can be observed. Therefore, the observed polarized peak at ~ 180 cm-1 is attributable to the symmetrical stretching vibrational mode of solvated Li+, Li+(THF)n. The solvation number n can be determined from neutron diffraction data and will be discussed later. Careful investigation of the isotropic spectra indicates that another broader peak is present at ~435 and ~470 cm-1 in the isotropic spectra for 7LiCl and 6LiCl solutions in THF-h8, respectively. Corresponding peaks in the THF-d8 solutions cannot clearly be observed probably due to statistical uncertainties associated with overlapping of the depolarized ring-bending mode (~470 cm-1) of THF-d8 molecule. A large isotopic shift found in the 435 cm-1 peak observed for 7LiCl-THF-h8 solution indicates that the lithium ion does move during the vibration. In highly concentrated aqueous LiCl solutions, intermolecular vibrational peak at 370~380 cm-1 has been reported in the isotropic Raman spectra,22,49 which has been assigned to


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the stretching vibrational mode of solvated Li+…Cl- contact ion pair. The present peaks at ~435 and ~470 cm-1 can tentatively be attributed to the vibrational mode of solvated Li+…Cl- ion pair, Li+Cl-(THF)m, which may coexist with the solvated Li+. Peak parameters for vibrational bands observed in the present low-frequency isotropic spectra were determined by the least squares fitting analysis. Results of the least squares fit are indicated in Figure 2 and Table 3. The ratio of the vibrational frequency of the Li+Cl-(THF)m peaks (peak 3 in Table 3), ν(7LiCl in THF-h8)/ν(6LiCl in THF-h8), is calculated to be 0.93 ± 0.01 which agrees well with the ratio of the mass of 6Li and 7Li, [m(6Li)/m(7Li)]1/2 = 0.926, suggesting that Li+ is moving during the vibration. This confirms that the peaks observed at ~435 cm-1 (7LiCl solution in THF-h8 ) and ~470 cm-1 (6LiCl solution in THF-h8 ) are attributed to the stretching vibrational mode of solvated Li+…Cl- ion pair. On the other hand, the vibrational frequency of intermolecular vibrational mode of solvated Li+, Li+(THF)n (peak 2 in Table 3), for the H/D substituted solutions does not exhibit any pronounced change beyond the experimental uncertainties. The intermolecular vibrational mode at ν = 38750 and 388 cm-1 51 attributable to the vibration of lithium ion in concentrated

nat

LiCl-THF solutions has been reported from far-

infrared absorption spectroscopic studies. The position of this vibrational mode exhibited higher frequency shift at 415 cm-1 when natLi was isotopically substituted to 6Li.51 The ratio of observed frequencies, 388/415 = 0.935, again agrees well with the value calculated by assuming lithium moves during the vibration. The vibrational frequency of the lithium moving mode observed in the present isotropic Raman spectra is ~ 50 cm-1 higher than that reported from the far-infrared studies. Considering the selection rules adopted for the isotropic Raman and infrared spectra, we concluded that the intermolecular vibrational mode observed in the present isotropic Raman spectra is different one from that found in the far-infrared spectra.


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Parallel, perpendicular and isotropic Raman spectra observed for 10 mol% natLiClO4 solution (natLi: natural abundance) in THF-h8 are shown in Figure 3. A polarized low-frequency peak can be identified at ν ~ 140 cm-1, overlapping with broader component centered at ν ~ 90 cm-1. Considering the higher symmetry of the present intermolecular vibrational mode and similarity in the vibrational frequency for Li+(THF)n unit observed in the 4 mol% LiCl-THF solutions as mentioned above, the peak appearing at ν ~ 140 cm-1 may be ascribed to the symmetrical stretching vibrational mode of solvated Li+. The position of this peak is ca. 40 cm-1 lower than that observed for 4 mol% LiCl-THF-h8 solution, which may reflect the difference in the environmental structure of Li+ between these solutions. In order to examine the possibility of Li+…ClO4- ion pair formation in the present 10 mol% LiClO4-THF solution, the ν1 (totally symmetrical stretching mode) of ClO4- in the isotropic spectrum was then investigated, which is represented in Figure 4. The ν1 peak of ClO4- is seriously overlapped with the COC symmetrical stretching mode within the THF molecule which is centered at 913 cm-1.52,53 The parameters for the present isotropic components were determined through the least squares fitting procedure employing the Voigt peak shape function which well reproduces intramolecular vibrational peak with narrow width.29 In the fitting procedure, the Gaussian width parameter in the Voight function was fixed at the value of instrumental optical resolution for all peak components. Results of the least squares fit are presented in Table 4 and Figure 4. The present ν1 band of ClO4- was found to be well reproduced by the sum of contributions from two Voigt components. The ν1 frequency of ClO4- is known to be sensitive to the ion pair formation with cations.52-55 Since the ν1 frequency of the ClO4- is known to be sensitive to the symmetrical perturbation from the Li+, which is stronger in the sequence of free ion < solvent shared ion pair < contact ion pair, the observed ν1 frequency shifts toward higher-energy side in that order. In acetone solutions, ν1


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frequency has been reported to be 933.8 cm-1 for solvated free ClO4-, 939.3 cm-1 for solventseparated ion pair, Li+(acetone)ClO4-, and 947.7 cm-1 for contact ion pair, Li+ClO4-.55 The ν1 frequency of ClO4- in LiClO4-dimethyl formamide (DMF) solutions has been reported to be 932 cm-1 for solvated free ion, 938.5 cm-1 for solvent separated ion pair, and 945 cm-1 for contact ion pair, respectively.53 The present ν1 peak of ClO4- has revealed to consist of two components centered at 933.0 cm-1 and 937.9 cm-1, which implies that ClO4- is present as both the solvated free- and solvent separated ion pair forms in the 10 mol% LiClO4-THF solution. Detailed environmental structure around Li+ is determined from the neutron diffraction data. Neutron Diffraction. Before discussing the neutron first-order difference function, the reliability of magnitude of the observed scattering cross section for sample solutions was checked by comparing amplitude of the observed total interference term with calculated intramolecular interference term. The total interference term, i(Q), for samples was obtained from scattering intensities for forward-angle detector pixels located at 13.1 ≤ 2θ ≤ 54.9° by applying inelasticity correction using the observed scattering cross sections for the null-H2O.36 The corrected scattering intensities from each detector pixels were combined in the Q-interval of 0.1 Å-1 to obtain the total i(Q) in the range of 0.1 ≤ Q ≤ 40 Å-1. The i(Q)’s for 4 mol% *LiClTHF-d8 and 10 mol% *LiClO4-THF-d8 solutions are indicated in Figures 5 and 6, respectively. Since the contribution from the intermolecular interference term involved in the total i(Q) is negligibly small in the sufficiently high-Q region (typically Q > 10 Å-1), the normalization constant, χ, can be determined by comparing the oscillational amplitude of the calculated intramolecular interference term, iintra(Q), and that of observed i(Q) in the high-Q region. The intramolecular interference term for (*LiCl)x(THF-d8)1-x solutions was evaluated by the following equation,


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iintra(Q) = (1 - x)iintra(Q) (for THF-d8).

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(7)

For (*LiClO4)x(THF-d8)1-x solutions, the intramolecular contribution from the ClO4- was added.

iintra(Q) = (1 - x)iintra(Q) (for THF-d8) + xiintra(Q) (for ClO4-).

(8)

The iintra(Q) for THF-d8 and ClO4- was evaluated by

iintra(Q) (for THF-d8 or ClO4-) = ΣΣbibj exp(-lij2Q2/2)sin(Qrij)/(Qrij).

(9)

i≠j

Intramolecular parameters, lij and rij, were taken from literature determined by gas phase electron diffraction study for THF molecule42 and by single crystal X-ray43,44 and neutron45 diffraction works for ClO4-. The normalization factor, χ, defined below, was determined through the least squares fitting procedure in the range of 12 ≤ Q ≤ 40 Å-1 where contribution from the intermolecular interference term is expected to be negligibly small.


16

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i(Q) = χ × iintra(Q).

(10)

Prior to the fitting analysis, the correction for the low-frequency systematic errors41 was adopted to the observed i(Q). The values of χ obtained were 1.04 ± 0.03 and 1.04 ± 0.03 for 4 mol% 6

LiCl-THF-d8 and 7LiCl-THF-d8 solutions, respectively. The values 0.96 ± 0.03 and 0.96 ± 0.02

were obtained for 10 mol% 6LiClO4-THF-d8 and 7LiClO4-THF-d8 solutions, respectively. The fact that the value of χ is close to the unity indicates that the present data correction and normalization procedures have been adequately carried out. The overall normalization error was estimated to be ca. 4% for all sample solutions. The first-order difference function, ∆Li(Q), was deduced from the numerical difference between total scattering cross sections observed for sample solutions involving 6Li and 7Li, which is shown in Figure 7. In the present ∆Li(Q), the inelasticity effect mainly arising from the self-scattering term of D atoms is expected to be cancelled out by taking the difference between two samples in which identical inelasticity distortion should be involved.16 Although data points in ∆Li(Q) observed for 4 mol% *LiCl-THF-d8 solutions are somewhat scattered due to relatively small contribution of Li-related partial structure factors in the total i(Q), interference feature can be obviously identified. The ∆Li(Q) of the 10 mol% *LiClO4-THF-d8 solutions exhibits significant oscillatory feature. The distribution function, GLi(r), around Li+ was obtained by the Fourier transform of the observed ∆Li(Q) (Figure 8). The first peak is located at r = 2.23 and 2.02 Å for 4 mol% LiCl-THF and 10 mol% LiClO4-THF solutions, respectively. Since oxygen atom of the THF molecule is considered to interact strongly with positive charged Li+,48 the first peak observed in the present GLi(r) certainly involves the nearest neighbor interaction between Li+ and


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oxygen atom of the THF molecule. Preliminary analysis has indicated that the area under the first peak roughly corresponds to ca. 4 oxygen atoms. The position of the first peak appeared in the present solutions is close to the sum of ionic radius of four-fold Li+ (0.59 Å)56 and van der Waals radius of oxygen atom (1.52 Å).57 The first peak of GLi(r) is mainly attributable to the nearest neighbor Li+…O(THF) interaction. On the other hand, according to the results from the isotropic Raman spectra, the contact ion pair, Li+…Cl-, is practically present in the 4 mol% LiCl-THF solution. The first peak of GLi(r) for 4 mol% LiCl-THF solution should involve the nearest neighbor Li+…Cl- interaction, which is expected to fall at the sum of ionic radii of Li+ and Cl(0.59 + 1.81 = 2.40 Å).56 Therefore, both contributions from the nearest neighbor Li+…O(THF) and Li+…Cl- interactions were taken into account for in the calculation of the model function, ∆Limodel(Q), used in the least squares fitting analysis for the 4 mol% LiCl-THF solution. Although the result of the isotropic Raman spectra suggests existence of solvent separated ion pair, Li+(THF)ClO4-, the first peak in GLi(r) for 10mol% LiClO4-THF solution might contain contribution from the nearest neighbor Li+…O(ClO4-) interaction, both contributions from the first neighbor Li+…O(THF) and Li+…ClO4- interactions were involved in evaluation of the model difference function. The position of the second peak in GLi(r) is observed at r = 3.24 and 2.96 Å for the 4 mol% LiCl- and 10 mol% LiClO4-THF solutions, respectively. Contributions from nonbonding interactions between Li+ and carbon atoms of the THF molecule in the first solvation shell can be involved in the second peak. The non-bonding contribution from Li+…Cl(ClO4-) interaction can possibly be contained in the second peak of GLi(r) for the 10 mol% LiClO4-THF solution. The broadened third peak located at r = 5.7 and 6.7 Å observed in the GLi(r) for the 4 mol% LiCl- and 10 mol% LiClO4-THF solutions may indicate that weak second solvation shell of Li+ is present in these solutions.


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The structural parameters for the first solvation shell of Li+ were determined by the least squares fitting procedure of the observed ∆Li(Q). The results are indicated in Figure 7 and numerical values of structural parameters for the 4 mol% LiCl- and 10 mol% LiClO4-THF solutions are summarized in Tables 5 and 6, respectively. In the fitting analysis, it has been found that the present ∆Li(Q) for the 4 mol% LiCl-THF solution cannot be reproduced with single conformation of the nearest neighbor Li+…THF interaction. Then, the second conformer (Li+…THF(b), see Table 5) with different bond angle, α, and dihedral angle , β, was introduced in which the structural parameters rij and lij for nearest neighbor Li+…O(THF) interaction being fixed to those for the first conformer (Li+…THF(a)). The nearest neighbor Li+…O(THF) distance in the 4 mol% LiCl-THF solution was determined to be 2.21 ± 0.01 Å. The value is clearly longer than those reported for Li+…water (rLiO = 1.94-2.00 Å)58-70 and for Li+…methanol (rLiO = 1.93-1.97 Å)71,72 interactions, suggesting that interaction between the Li+ and solvent molecule is weaker in the THF solution than those for aqueous and methanolic solutions. The nearest neighbor Li+…Cl- is determined to be 2.4 ± 0.1 Å. The value agrees well with the sum of ionic radii of Li+ and Cl-, indicating that Li+Cl- contact ion pair forms in the 4 mol% LiCl-THF solution. Since the ionic radius of Cl- (1.81 Å)56 is ca. 0.3 Å larger than the van der Waals radius of oxygen atom (1.52 Å)57, steric hindrance between THF molecules in the first solvation shell of Li+ becomes more pronounced. This may be a possible reason of two different conformers observed for the nearest neighbor Li+…THF interaction. In the 4 mol% LiCl-THF solution, Li+ is surrounded by, on the average, ca. 3 THF molecules and ca. 1 Cl-. The present value of Li+…O(THF) distance is considerably longer than that reported for Li+(THF)4 structural unit found in the crystalline state (1.88 - 2.05 Å).73 The present value is also much longer than the average Li+…O distance (1.962 Å) in lithium-ether complexes with 4-fold coordination in the


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crystalline state.74 This implies that the nearest neighbor Li+…O(THF) interaction is significantly modified in the 4 mol% LiCl-THF solution due to the Li+…Cl- contact ion pair formation. In the least squares fitting analysis of the ∆Li(Q) observed for 10 mol% LiClO4-THF solution, structural parameters of interatomic distances for Li+…O(THF) and Li+…O(ClO4-) interactions were both allowed to vary independently in order to consider all possibilities concerning the structure of the first solvation shell of Li+. Contribution from the second solvation shell was involved in the model function to reproduce the interference feature of the observed ∆Li(Q) in the low-Q region. The nearest neighbor Li+…O distance for Li+…THF and Li+…ClO4- interactions were obtained to be 2.07 ± 0.01 Å and 2.19 ± 0.01 Å, respectively, indicating that the interaction between Li+ and THF molecule is stronger than that between Li+ and ClO4-. It is revealed that, on the average, 0.9 ± 0.1 ClO4- is involved in the first solvation shell of Li+ to form the solvated contact ion pair. The present value of Li+…O(THF) distance is slightly longer than the average value of tetrahedrally coordinated Li+…O(ether) distance (1.962 Å) in the crystalline state.74 On the other hand, the value determined for the present 10 mol% LiClO4-THF solution (2.07 ± 0.01 Å) is significantly shorter than that determined for the 4 mol% LiCl-THF solution. The fact indicates that the nearest neighbor Li+…O(THF) interaction is strongly affected by Li+…anion interaction involved in the first solvation shell. The present nearest neighbor Li+…O(ClO4-) distance (2.19 ± 0.01 Å) is consistent with that reported for anhydrous LiClO4 crystal (1.9932.391 Å)75 in which all ClO4- are in contact with Li+, implying that ClO4- forms the contact ion pair with Li+ in the present 10 mol% LiClO4-THF solution. According to the present neutron structural information, higher-frequency component observed at ν = 937.9 cm-1 in the present isotropic Raman spectra should be attributed to the ν1 mode of ClO4- which forms the contact ion pair with Li+. This indicates that careful reconsideration is needed for the assignment of the


20

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higher-frequency components observed for ν1 mode of ClO4- in concentrated non-aqueous solutions. CONCLUSIONS Information on the microscopic structure of the first solvation shell of Li+ in concentrated THF solutions was successfully obtained from isotropic Raman spectra and neutron diffraction measurements. It has been revealed that Li+ forms, on the average, solvated contact ion pair, Li+X-(THF)3, X = Cl and ClO4, in 4 mol% LiCl- and 10 mol% LiClO4–THF solutions, respectively. The nearest neighbor Li+…O(THF) distance was obtained to 2.21 ± 0.01 and 2.07 ± 0.01 Å for 4 mol% LiCl- and 10 mol% LiClO4-THF solutions, respectively. The nearest neighbor Li+…Cl- distance in the 4 mol% LiCl-THF solution was determined to 2.4 ± 0.1 Å. The nearest neighbor Li+…O(ClO4-) distance in the 10 mol% LiClO4-THF solution was obtained to be 2.19 ± 0.01 Å. The present Li+…O(THF) distances are longer than that observed for concentrated aqueous and methanolic solutions, indicating weaker Li+…O interaction in the THF solution. The nearest neighbor Li+…O(THF) interaction is strongly affected by anions involved in the first solvation shell of Li+.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. FAX.: +81 23 628 4591

ACKNOWLEDGMENTS


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The authors thank Prof. Kazutaka Ikeda (KEK) and Dr. Fumika Fujisaki (The Graduate University for Advanced Studies, SOKENDAI) for their help during neutron diffraction measurements. Neutron scattering experiments were approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S09). All calculations were carried out at Yamagata University Networking and Computing Service Center. This work was partially supported by Grant-in-Aid for Scientific Research (C) (No. 26390110), from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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between Unlike Ions in Incompletely Hydrated Solutions. J. Phys. C: Solid State Phys. 1984, 17, L9725-L729. (60)

Ichikawa, K.; Kotani, S.; Izumi, M.; Yamanaka, T. Direct Correlation between Lithium

Cation and Carboxyl Anion in Highly Concentrated Aqueous Solutions. Mol. Phys. 1992, 77, 677-688. (61)

Kameda, Y.; Uemura, O. Neutron Diffraction Study of the Structure of Highly

Concentrated Aqueous LiBr Solutions. Bull. Chem. Soc. Jpn. 1993, 66, 384-389. (62)

Yamagami, M.; Yamaguchi, T.; Wakita, H.; Misawa, M. Pulsed Neutron Diffraction

Study on Lithium (I) Hydration in Supercooled Aqueous Chloride Solutions. J. Chem. Phys.


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1994, 100, 3122-3126. (63)

Howell. I.; Neilson. G. W. Li+ Hydration in Concentrated Aqueous Solution. J. Phys.:

Condens. Matter 1996, 8, 4455-4463. (64)

Kameda, Y.; Suzuki, S.; Ebata K.; Usuki, T.; Uemura, O. The Short-range Structure

around Li+ in Highly Concentrated Aqueous LiBr Solutions. Bull. Chem. Soc. Jpn. 1997, 70, 47-53. (65)

Ansell, S.; Dupuy-Philon, J.; Jal, J.-F.; Neilson, G. W. Ionic Structure in the Aqueous

Electrolyte Glass LiCl. 4D2O. J. Phys.: Condens. Matter 1997, 9, 8835-8847. (66)

Kameda, Y.; Mochiduki, K.; Imano, M.; Naganuma, H.; Sasaki, M.; Amo, Y.; Usuki, T.

Neutron Diffraction Study of Concentrated Aqueous Lithium Benzoate Solutions. J. Mol. Liq. 2005, 119, 159-166. (67)

Kameda, Y.; Sasaki, M.; Amo, Y.; Usuki, T. Structure of Highly Concentrated Aqueous

Lithium Alaninate Solutions Studied by Neutron Diffraction with

14

N/15N, 6Li/7Li, and H/D

Isotopic Substitution Method. Bull. Chem. Soc. Jpn. 2006, 79, 228-236. (68)

Yamaguchi, T.; Ohzono, H.; Yamagami, M.; Yamanaka, K.; Yoshida, K.; Wakita, H. Ion

Hydration in Aqueous Solutions of Lithium Chloride, Nickel Chloride, and Cesium Chloride in Ambient to Supercritical Water. J. Mol. Liq. 2010, 153, 2-8. (69)

Kameda, Y.; Miyazaki, T.; Otomo, T.; Amo, Y.; Usuki, T. Neutron Diffraction Study on

the Structure of Aqueous LiNO3 Solutions. J. Solution Chem. 2014, 43, 1588-1600. (70)

Mason, P. E.; Ansell, S.; Neilson, G. W.; Rempe, S. B. Neutron Scattering Studies on the

Hydration Structure of Li+. J. Phys. Chem. B. 2015, 119, 2003-2009. (71)

Kameda, Y.; Usuki, T.; Uemura, O. The Structure Analysis on Highly Concentrated Li-

salt Solutions. High Temperature Materials and Processes, 1999, 18, 27-40.


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(72)

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Kameda, Y.; Ebata, H.; Usuki, T.; Uemura, O. The Coordination Structure of Li+ in

Highly Concentrated Methanolic LiBr and LiI Solutions. Physica B 1995, 213&214, 477479. (73)

Fronczek, F. R.; Halstead, G. W.; Raymond, K. N. The Synthesis, Crystal Structure of an

Actinide Metallocarborane Complex, Bi(η5-(3)-1,2-dibarbollyl) Dichloro Uranium (IV) Dianion, [U(C2B9H11)2Cl2]2-. J. Am. Chem. Soc. 1979, 99, 1769-1775. (74)

Olsher, U. Coordination Chemistry of Lithium Ion: A Crystal and Molecular Structure

Review. Chem. Rev. 1991, 91, 137-164. (75)

Henderson, W. A.; Brooks, N. R. Crystal from Concentrated Glyme Mixtures. The Single

Crystal Structure of LiClO4. Inorg. Chem. 2003, 42, 4522-4524.

FIGURE CAPTIONS


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Figure 1. Polarized Raman spectra observed for isotopically substituted 4 mol% XLiCl-THF-Y8 (X = 6, 7 and Y = h, d) solutions and for pure THF-h8 (dots).

Figure 2. Isotropic Raman spectra observed for isotopically substituted 4 mol% XLiCl-THF-Y8 (X = 6, 7 and Y = h, d) solutions (dots). The best fit of the calculated intensities are denoted by solid lines. Peak components are indicated by broken lines.

Figure 3. Low-frequency polarized Raman spectra observed for 10 mol% natLiClO4-THF-h8 solution (dots). The best fit of calculated isotropic spectrum is denoted by a solid line. Peak components are indicated by broken lines.

Figure 4. Isotropic Raman spectrum observed for 10 mol% natLiClO4-THF-h8 solution (dots). The best fit of calculated isotropic spectrum is denoted by a solid line. Peak components are indicated by broken lines. The residual functions are given below (dots).

Figure 5. Neutron total interference term observed for 4 mol% 6LiCl- and 7LiCl-THF-d8 solutions (dots). The best fit of intramolecular interference term is denoted by solid line. The residual functions are given below (dots).


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Page 32 of 47

Figure 6. The same notation as Figure 5 but for 10 mol% 6LiClO4- and 7LiClO4-THF-d8 solutions.

Figure 7. First-order difference function, ∆Li(Q), observed for 4 mol% *LiCl-THF-d8 and 10 mol% *LiClO4-THF-d8 solutions (dots). The best fit of calculated ∆Li(Q) is denoted by solid line. The residual functions are given below (dots).

Figure 8. Distribution function, GLi(r), around Li+ observed for 4 mol% *LiCl-THF-d8 and 10 mol% *LiClO4-THF-d8 solutions (dots). Fourier transform of the best fit of calculated ∆Li(Q) is denoted by solid line. Short- and long-range interactions are indicated by broken and dotted lines, respectively.


32

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Table 1. Isotopic Composition, Average Neutron Scattering Length of Li, Total Cross Section and Number Density of Sample Solutions Scaled in the Stoichiometric Units, (*LiCl)0.04(THFd8)0.96 and (*LiClO4)0.10(THF-d8)0.90, σt and ρ0, Respectively

Sample

6

(6LiCl)0.04(THF-d8)0.96

95.6

(7LiCl)0.04(THF-d8)0.96

0

Li/%

(6LiClO4)0.10(THF-d8)0.90 95.6 7

( LiClO4)0.10(THF-d8)0.90

0

7Li/% 4.4

bLi/10-12 cma

σt/barnsb ρ0/Å-3

0.182

79.440

100.0

-0.222

59.302

4.4

0.182

109.456

100.0

-0.222

59.676

a

Taken from Ref. 31.

b

For incident neutron wavelength of 1.0 Å.

0.007508

0.007790


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Page 34 of 47

Table 2. Values of Coefficients in Eq. 3

Sample

A/barns

B/barns

C/barns

D/barns

E/barns

(*LiCl)0.04(THF-d8)0.96

0.01792

0.08212

0.16460

0.00012

-0.00003

(*LiClO4)0.10(THF-d8)0.90

0.06068

0.19246

0.38578

0.00770

-0.00017


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Table 3. Results of the Least Squares Fitting Analyses of Low-frequency Isotropic Raman Spectra Observed for 4 mol% *LiCl-THF-d8, *LiCl-THF-h8 and 10 mol% LiClO4-THF-h8 Solutionsa

sample

peak 1

peak 2

ν/cm-1

fwhm/cm

-1

h

ν/cm-1

117(20)

82(12)

peak 3 -1

h

ν/cm-1

fwhm/cm

h

181.3(8) 20(2)

171(14)

-

-

-

fwhm/cm

-1

6

Li-THF-d8

137(7)

7

Li-THF-d8

121(14) 145(33)

117(29)

183(2)

28(5)

113(16)

-

-

-

6

Li-THF-h8

163(2)

103(4)

318(15)

184.3(4) 20(1)

633(25)

469.3(7)

60(2)

607(14)

7

Li-THF-h8

160(5)

131(11)

257(23)

183.9(7) 20(2)

580(45)

434.7(8)

40(2)

766(33)

139(10)

262(31)

139.7(4) 36(1)

466(17)

-

-

-

nat

LiClO4-THF-h8 89(4) a

Estimated standard deviations are given in parentheses.


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Page 36 of 47

Table 4. Results of the Least Squares Fitting Analyses of COC Symmetrical Stretching Mode of THF Molecule and Totally Symmetric Stretching Mode (ν1) of ClO4- Observed for 10 mol% nat LiClO4-THF-h8 Solutiona

Assignment

ν/cm-1

wL/cm

hc

THF(COC str.) I

902.4(1)

13.6(1)

2.30(9)×106

THF(COC str.) II

915.0(1)

13.7(1)

1.323(1) ×107

ClO4-(ν1) I

933.0(6)

8.1(1)

1.4(1) ×106

ClO4-(ν1) II

937.9(1)

7.9(2)

6(2) ×105

a

Estimated standard deviations are given in parentheses.

b c

-1 b

Lorentzian width parameter.

Peak height parameter.


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Table 5. Structural Parameters for 4 mol% *LiCl-THF d8 Solutions Determined from Least Squares Fitting Analysis of Observed ∆Li(Q)a

Interaction

i-j

rij/Å

lij/Å

lij*/Å

nij

Li+…THF(a)

Li+…O

2.21(1)

0.11(4)

0.35(1)

1.6(3)

α = 7(3)° b

β = -1(10)° c

[2.21]d

[0.11]d

[0.35] d

1.1(3)

α = 29(6)°

β = 34(3)°

2.4(1)

0.1(1)

-

0.6(4)

r0/Å

l0/Å

4.52(1)

0.76(3)

Li+…THF(b)

Li+…Cl-

Long-range

Li+…O

Li+…Cl-

Li+…Xe

a

Estimated standard deviations are given in parentheses. bBond angle between Li+…O(THF) axis and the bisector of ∠C-O-C in the THF molecule. cDihedral angle between the plane involving atoms, Li+…O and the bisector of ∠C-O-C in the THF molecule, and the plane involving C-O-C atoms. dFixed at values for Li+…THF(a) interaction. eX: Li, Cl, C, O, and D.


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Page 38 of 47

Table 6. Structural Parameters for 10 mol% *LiClO4-THF-d8 Solutions Determined from Least Squares Fitting Analysis of Observed ∆Li(Q)a

Interaction

i-j

rij/Å

lij/Å

lij*/Å

nij

Li+…THF(I)

Li+…O

2.07(1)

0.12(1)

0.34(1)

3.2(1)

α = 16(4)° b

β = -23(10)° c

2.19(1)

0.17(2)

0.22(4)

0.9(1)

γ = 155(2)°

δ = -60(22)°

6.61(2)

0.51(2)

-

0.97(4)

r0/Å

l0/Å

5.16(2)

1.04(2)

Li+…ClO4-

Li+…THF(II)

Long-range

Li+…O

Li+…THF

Li+…Xd

a

Estimated standard deviations are given in parentheses. bBond angle between Li+…O(THF) axis and the bisector of ∠C-O-C in the THF molecule. cDihedral angle between the plane involving atoms, Li+…O and the bisector of ∠C-O-C in the THF molecule, and the plane involving C-O-C atoms. dX: Li, Cl, C, O, and D.


38

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I//

(6LiCl)0.04(THF-d8)0.96

I⊥ Iiso

I//

(7LiCl)0.04(THF-d8)0.96

I⊥ Iiso

I//

(6LiCl)0.04(THF-h8)0.96

⊥

I

Iiso

(7LiCl)0.04(THF-h8)0.96 I// I⊥ Iiso I// I⊥

pure THF

Iiso

Figure 1. Kameda et al. ACS Paragon Plus Environment

39


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Page 40 of 47

(6LiCl)0.04(THF-d8)0.96

(7LiCl)0.04(THF-d8)0.96

(6LiCl)0.04(THF-h8)0.96

(7LiCl)0.04(THF-h8)0.96

Figure 2. Kameda et al.


40

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I//

(natLiClO4)0.10(THF-h8)0.90

I⊥

Iiso



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Page 42 of 47

THF II COC str.

ClO4ν1

I I II



42

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(6LiCl)0.04(THF-d8)0.96 (+4)

δ(Q) (+2)

(7LiCl)0.04(THF-d8)0.96

δ(Q) (-2)



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Page 44 of 47

(6LiClO4)0.10(THF-d8)0.90 (+4)

δ(Q) (+2) (7LiClO4)0.10(THF-d8)0.90

δ(Q) (-2)



44

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(*LiCl)0.04(THF-d8)0.96 (+0.3)

δ(Q) (+0.15)

(*LiClO4)0.10(THF-d8)0.90

δ(Q) (-0.2)



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Page 46 of 47

(*LiCl)0.04(THF-d8)0.96 (+3)

(*LiClO4)0.10(THF-d8)0.90



46

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Table of Contents


47

Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and

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