The solvation structure around the Li + ion in a mixed cyclic/linear carbonate solution, an important factor for the performance of lithium-based rechargeable batteries, is examined by measuring and analyzing the noncoincidence effect observed for the CO stretching Raman band. This technique has the advantage of perceiving relative distances and orientations of solvent molecules clustering around an ion in the first solvation shell and, hence, of developing information on the solvation structure along the wavenumber axis rather than along the intensity axis of the spectra. It is shown that, taking the solution of Li + ClO 4 − in the 1:1 mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) as an example case, the Li + ion is preferentially solvated by PC molecules [primarily as (PC) 3 (DEC) 1 Li + ] and is totally protected from direct interaction (contact ion pairing) with the ClO 4 − ion. The solvation structures in neat PC and neat DEC solvents are also discussed.I on solvation is a central, and still an open, issue in many chemical, biochemical, and electrochemical processes. One of those important processes would be the functioning of lithium-based rechargeable batteries. 1−3 Their performance depends on the electrode materials and processes on the one hand and on the charge carrier concentration and mobility in the electrolyte solution on the other hand. With regard to the latter, high charge density of the Li + ion should be sufficiently stabilized, and at the same time, the electrolyte solution should have sufficiently high fluidity. A usual practice to make these two factors compatible is to employ a mixed solvent, consisting of a highly dipolar liquid such as a cyclic carbonate stabilizing the high charge density (but highly viscous) and a liquid of lower viscosity such as a linear carbonate (being less dipolar). Quite often ethylene or propylene carbonate (with dielectric constant ε = 65−90 and viscosity η ≅ 2.5 cP, abbreviated as EC and PC) is used for the former, and dimethyl, diethyl, or ethyl methyl carbonate (with ε ≅ 3 and η = 0.6−0.9 cP, abbreviated as DMC, DEC, and EMC) is used for the latter.The solvation structure around the Li + ion, especially that of the first solvation shell, has been suggested to be important for the interphase chemistry on the electrodes. 4−6 The use of a mixed solvent introduces a complexity in this. One controversial subject in this regard is the presence/absence of the preferential solvation and (if present) its nature for the Li + ion in a mixed cyclic/linear carbonate solution. 7−19 On the basis of electrospray ionization mass spectroscopy (ESI-MS), 7,8 it has been suggested that there is a strong preferential solvation for Li + in EC/EMC, with the Li + (EC) 2 species as the main ingredient. 7 The same type of preferential solvation (i.e., with a higher population of cyclic carbonate around the ion than in the bulk) has also been suggested in some NMR studies 9−11 but with a much larger total solvation number (≥6). 9,20 It has been argued that some...
The concentration dependence of the Raman noncoincidence effect (NCE) of the C-O and O-H stretching bands of methanol is investigated in methanol/CCl 4 mixtures in the range of 1.0 g x m g 0.1, where x m is the mole fraction of methanol, by performing Raman spectroscopic measurements and molecular dynamics (MD) simulations. Band asymmetry observed for both bands is carefully taken into account. The experimental and simulation results are in satisfactory agreement with each other. For the C-O stretching band, it is observed that the magnitude of the negative NCE gets larger upon dilution in CCl 4 down to x m ∼ 0.2, contrary to the expectation of becoming smaller from simple guess that the NCE arises from intermolecular vibrational resonant interactions between methanol molecules, which, on average, get separated from each other upon dilution. For the O-H stretching band, the magnitude of the positive NCE remains almost the same upon dilution down to x m ∼ 0.3. These apparently peculiar experimental results are reasonably explained by the MD simulations on the basis of the transition dipole coupling (TDC) mechanism of intermolecular resonant vibrational interactions and the simulated hydrogen-bonded liquid structures. In the case of the C-O stretching band, the negative NCE arises mainly from positive vibrational coupling between hydrogen-bonded pairs of molecules, which is partially canceled by negative vibrational coupling between molecules in different hydrogenbonded chains. In the case of the O-H stretching band, the positive NCE arises predominantly from negative vibrational coupling within hydrogen-bonded chains. As a result, a locally anisotropic change in the liquid structure that occurs upon dilution, in which, around each molecule, intermolecular distances do not change very much along hydrogen-bond directions but do change significantly in other directions, gives rise to the apparently peculiar behavior of the NCE described above.
Dynamic processes in the liquid state can be examined from different points of view; the rapid development of highly sophisticated time-resolved experiments and very complex computer simulation techniques have certainly improved the accuracy of some labratory results, but they are often not followed by an adequate attempt at interpretation. In any case, the extent of the subject and the enormous spread of experimental data do not help systematic analysis.This review attempts to contribute to a critical examination of a particular aspect of the dynamics of the liquid phase. Through a study of vibrational relaxation in isotropic molecular liquids it is possible to gain information about the molecular environment and the main intermolecular forces operative in this state of matter. It is therefore possible to formulate hypotheses about the force potentials and to describe the dynamic regime of the liquid system under consideration.(1) diatomic and/or highly symmetric molecules (e.g. HCl, 0, , CH,);(2) derivatives of hydrocarbons, especially methane (e.g. CH,NO, , CHCl, , CH,CI,);(3) aromatic and cyclic aliphatic molecules (e.g. benzene and its derivatives, cyclohexane, pyridine); (4) hydrogen bonded (with OH and/or HCO groups).This paper concerns the comparison between vibrational motions that have similar normal coordinate descrip tions; for this reason, we shall limit our considerations to the second and third groups in the above list. Hydrogenbonded and highly symmetrical systems are excluded because they need specific considerations.The currently used basic nomenclature of this field will be illustrated, in addition to the physical meaning of the dynamic parameters usually obtained by steady-state (continuous Raman and infrared), time-resolved spectroscopy and computer simulation techniques; these different experimental data will be compared, when available. We shall also outline and discuss current theoretical models, and attempt to emphasize their potential and limitations.At present, we may single out four groups of molecules that have been investigated by these methods: VIBRATIONAL RELAXATION IN LIQUIDS: THE PROBLEMSince the early 1960s, the study of vibrational relaxation in condensed phases has been considered a way of investigating dynamic processes,' but only with the advent of lasers as spectroscopic sources have frequency-and time-resolved experiments been performed; lineshape and coherent picosecond excitation studies allowed accurate information to be obtained on the dynamics in the solid and liquid states. Since that time, many papers2-14 have proved useful in the development of theoretical models and experimental results; for a detailed analysis of the whole question, one may refer to the above publications and references therein. For the purpose of this paper, the basic definitions of the main physical entities and variables involved in the subject must be clarified, avoiding the conflicts and ambiguities often present in the literature. DEPHASING: DEFINITIONSDephasing is important as probe of the...
The nu(C=O) Raman band frequencies of acetone have been analyzed to separate the contributions of the environmental effect and the vibrational coupling to the gas-to-liquid frequency shifts of this band and to elucidate the changes in these two contributions upon dilution in DMSO. We have measured the frequencies of the nu((12)C=O) band in acetone/DMSO binary mixtures, the nu((13)C=O) band of the acetone-(13)C=O present as a natural abundance isotopic impurity in these mixtures, and both the nu((12)C=O) and nu((13)C=O) bands in the acetone-(12)C=O/acetone-(13)C=O isotopic mixtures at infinite dilution. These frequencies are compared with those of the nu((12)C=O) band in the acetone/CCl(4) binary mixtures measured previously. We have found the following three points: (i) The negative environmental contribution for the nu((12)C=O) oscillator of acetone completely surrounded by DMSO is reduced in magnitude by +5.5 cm(-1) and +7.8 cm(-1) upon the complete substitution of DMSO with acetone and CCl(4) molecules, respectively, indicating the progressive reduction of the attractive forces exerted by the environment on the nu((12)C=O) mode of acetone. (ii) In DMSO and other solvents, the contribution of the vibrational coupling to the frequency of the isotropic Raman nu((12)C=O) band of acetone becomes progressively more negative with increasing acetone concentration up to a value of -5.5 cm(-1), while the contribution to the frequency of the anisotropic Raman band remains approximately unchanged. The only difference resides in the curvatures of the concentration dependencies of these contributions which depend on the relative solute/solvent polarity. (iii) The noncoincidence effect (separation between the anisotropic and isotropic Raman band frequencies) of the nu(C=O) mode in the acetone/DMSO mixtures exhibits a downward (concave) curvature, in contrast to that in the acetone/CCl(4) mixtures, which shows an upward (convex) curvature. This result is supported by MD simulations and by theoretical predictions and is interpreted as arising from the reduction and enhancement of the short-range orientational order of acetone in the acetone/DMSO and acetone/CCl(4) mixtures, respectively.
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