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J. Phys. Chem. B 2004, 108, 17992-18002
Computational Study of γ-Butyrolactone and Li+/γ-butyrolactone in Gas and Liquid Phases
Marco Masia* and Rossend Rey
Departament de Fı´sica i Enginyeria Nuclear, UniVersitat Polite´cnica de Catalunya,Campus Nord B4-B5, Barcelona 08034, Spain ReceiVed: July 8, 2004; In Final Form: September 8, 2004 A comprehensive study of structural and dynamical properties of γ-butyrolactone (GBL) and the extent towhich they are affected in the vicinity of a lithium ion, both in gas and liquid phases, is reported. The isolatedGBL molecule is found to be nonplanar, with a barrier of ≈9 kJ/mol to ring inversion. As expected, thelithium ion coordinates the carbonyl oxygen with an almost collinear configuration relative to the carbon-oxygen bond but with a slight tilting toward the lactone oxygen. This configuration holds for clusters of upto four molecules and in the liquid phase as well (where a tetrahedral first solvation shell is found). A highlevel ab initio vibrational analysis, with a new assignment of bands has been performed, which shows substantialred and blue shifts upon lithium solvation, which decrease in a nontrivial way upon increasing the clustersize. To study the solvent effect of the vibrational spectrum, an accurate intramolecular force field has beendeveloped, based on the concept of relaxed potential energy profiles. The inclusion of stretch and bendanharmonicity is shown to be essential in order to explain, not only the absolute value, but the sign of theshifts, particularly for the carbonyl stretching which is substantially downshifted. The shifts obtained for therest of the bands, together with the diffusion coefficients for bulk GBL and for lithium, are in fair agreementwith experimental results.
I. Introduction
γ-Butyrolactone (GBL, 4-hydroxybutyric acid gamma-lac- tone, Figure 1), the simplest cyclic ester, is a major chemicalcompound with extensive application in pharmaceuticals,pesticides and petrochemicals.1-6 It is also known to be abuilding block of many natural products of biological activity,like the sesquiterpene lactones, flavor components, alkaloids,antileukemics, and pheromones.8-11 Its biological relevance is Figure 1. γ-Butyrolactone with the atom labeling used in the text
attributed to its similarity with cyclic peptides.
(notice that hydrogens are grouped in classes).
Recently, GBL has become the focus of increasing techno- logical interest for its application in lithium ion batteries (LIBs).
Here we have aimed to obtain a comprehensive theoretical Its physicochemical properties make it suitable to enhance LIBs understanding at the molecular level: from the isolated molecule capabilities (reciclability, power, etc.).12 It is an aprotic polar up to the solvation of the lithum ion in the liquid phase. Both solvent of moderate viscosity with a dielectric constant of 41.7 ab initio and MD calculations have been used to that purpose.
at ambient temperature, which shows a good solubilizing power For the gas phase, the optimal structure and vibrational for lithium salts. Contrary to other good plasticizers employed frequencies have been computed for the monomer, including a in LIBs, the liquid-phase exists over a wide range of temper- complete assignment of bands. Structure and vibrational fre- atures (the melting and the boiling points are -42 and +206 quencies have also been studied for clusters of Li+, with up to °C, respectively). Takami et al.6 have recently reported that the four GBL molecules, as a function of solvation number. Finally, mixture of GBL with ethylene carbonate (EC) is a promising and still within the gas phase, an accurate anharmonic intramo- lecular force field has been developed, following a novel Despite its importance for basic and applied areas, to our procedure for parametrization based on the concept of relaxed knowledge, there are no complete ab initio studies of its structure potential energy profiles along internal coordinates. Concerning and vibrational manifold, nor any molecular dynamics (MD) the liquid state, both neat liquid GBL and Li+ dissolved in GBL simulation in the liquid phase, particularly in the vicinity of have been studied. To this end, a standard intermolecular force the lithium ion. The only theoretical studies to date concern field has been refined, checking its goodness against counter- molecular mechanics (MM) calculation of structures,13-15 and poise corrected potential energy profiles. Finally, a detailed study ab initio computations of some partial aspects16-20 (see below).
of diffusion and vibrational shifts for molecules within the first In contrast, and probably due to the aforementioned high solvation shell of lithium has been performed.
technological impact on LIBs, a substantial amount of experi- mental work has been reported for Li+-GBL1,2,18,21-28 and for computational details are described, section III contains the its mixtures with other plasticizers.6,29 results of the ab initio calculations in the gas phase, and sectionIV contains those for the liquid phase. Finally, the main aspects * To whom correspondence should be addressed. E-mail: marco.masia@ are summarized in the conclusions section.
J. Phys. Chem. B, Vol. 108, No. 46, 2004 17993
TABLE 1: Cartesian Coordinates for the Minimum Energy
TABLE 2: Intramolecular Force Field Parameters For
Structure, Lennard-Jones Parameters, and Charges for the
Stretchingsa
Intermolecular Interaction
921.20 -2225.23 3362.30 1.2028 1.211 1.239 Units: [kr ] ) kcal mol-1 Å-i, [r0] ) Å. Comparison of the equilibrium values with previous studies: molecular mechanics calcula- tions (MM14), experiment (exp.24), and ab initio (AI18).
TABLE 3: Intramolecular Force Field Parameters for
Bendingsa

II. Computational Details
All ab initio calculations were performed with Gaussian 98.30 Vibrational analysis and geometry optimization were performed at the MP2 level with the 6-311G basis set augmented with diffuse and polarization functions.31 The same model chemistry has been employed for a relaxed potential energy surface scan.
Because of the high memory requirements, the study of the n]+ with n ranging from 1 to 4 is performed using the MP2/6-31G model chemistry.
Classical calculations were performed with an in-house MM code, together with the DL_POLY32,33 suite. The MM code was used for the scan of the potential energy surface of a single GBL molecule using a classical intramolecular force field, and for the vibrational analysis. Finally, the DL_POLY package was used to perform the liquid-phase simulations. Data analysis (FFT, curve smoothing, and curve fitting) was performed with the commercial package Microcal Origin 6.1.34 Units: [] ) kcal mol-1 rad-i, [θ equilibrium values with previous studies: molecular mechanics calcula-tions (MM14), experiment (exp.24), and ab Initio (AI18).
III. AB Initio Calculations
A. Structure 1. 1. Single Molecule. On the experimental side,
TABLE 4: Comparison of the Equilibrium Values for the
Most Representative Dihedral Angles (degrees) with

infrared,21,24 Raman,24 and microwave spectra22,23,25 of GBL Previous Studies: Molecular Mechanicsa (MM14) and ab
have been reported. On the other hand, most of the theoretical Initio (AI18) Calculations
studies correspond to MM calculations (with generic force fields) of properties such as heats of formation and minimum energy structures.13-15,17 To our knowledge, previous ab initio calculations for GBL (using lower levels of theory) were aimed to study partial aspects such as ring inversion,16 the effect of isotopic substitution on vibrational circular dichroism,18 intrinsic basicities,19 and thermal decomposition.20 As a consequence most of the structural and vibrational measures remain to be a The sign conventions have been adapted to the ones used here.
In first place, a geometry optimization of the molecule at the MP2/6-311++G(d,p) level has been performed. The Cartesian demonstrated that the barrier for inversion of the GBL ring could coordinates obtained for the minimum energy structure are given be reliably described using a one-dimensional potential function.
in Table 1. Tables 2-4 contain the equilibrium values obtained Indeed, a typical double well potential for inversion is obtained for the internal coordinates, together with those reported in from a relaxed potential energy scan of the C - previous works (obtained experimentally,24 with MM methods14 dihedral angle (Figure 2, see details in section III C). Microwave or with lower level quantum chemical calculations18). A good spectroscopy measurements25 predict a barrier height for ring agreement among all results is achieved for bond lengths and inversion of ≈8.0 kJ mol-1. Our quantum chemical calculation bending angles, whereas the values for some dihedral angles produces a slightly higher value (≈9.0 kJ mol-1), with the show somewhat larger deviations, particularly for the O - maximum located at 0° (i.e., a planar conformation). This C3 angle (to our knowledge no experimental results are conclusion agrees with the expectation of Cremer and Pople in their study on general monocyclic rings,35 according to which A basic aspect to consider is that of molecular planarity.
a planar ring should imply a more highly strained ring angle at Confirming previous works,13,14,16,25 we found that the -carbon the carbonyl atom than a twisted conformation. Regarding other lies out of the plane of the remaining four ring atoms resulting dihedrals (Table 4), our results are very similar to previous ab in C1 symmetry. With the assumption that the two ring initio calculations18 but show deviations of up to 8° if compared puckering coordinates could be treated separately, Lopez et al.25 17994 J. Phys. Chem. B, Vol. 108, No. 46, 2004
only varies by ∼5°. To convey a clearer idea of the changesinduced by the complexation, in Table 5, we report the valuesfor the most affected internal coordinates.
Experimental results obtained with Raman spectroscopy for the liquid state suggest that the lithium ion is coordinated byfour GBL molecules37 (a coordination number that has beenfound both for small molecules as water and for larger onessuch as EC). We studied the structure of all GBL complexes(from 1 to 4 molecules plus the lithium ion, Figure 3) with aMP2/6-31G model chemistry (the calculations for the singlemolecule have also been repeated at this lower level of theory, Figure 2. rPES profile along the C -
to facilitate a consistent comparison along the series). Theminimum energy geometry for the two-coordinated complexhas a linear arrangement with the lithium ion coordinated atopposite sides by the carbonyl oxygens, with the two GBLmolecules lying on perpendicular planes. The three-coordinatedcomplex shows a trigonal configuration with the GBL moleculesslightly tilted to reduce the repulsion. The four-coordinatedcomplex shows a tetrahedral like arrangement as the carbonyloxygens form a dihedral angle of ∼75°. Similar results for thestructure of these complexes where also obtained for EC. As inthat case, distortions of the molecular structure become smallerupon increasing the coordination number, most probably dueto the increasing distance between lithium and the carbonyloxygens. Again, if we compare the distortion induced in thetorsional angle in EC and GBL, we notice that the GBLmolecule is more rigid than EC. Finally, the angle betweencarbonyl axis and the vector joining the ion with the oxygendecreases from ∼157° to ∼140° as the coordination numberincreases, an aspect of interest in the analysis of liquid-phase Figure 3. γ-Butyrolactone and its complexes [Li(GBL)n]+ with 1 e
n e 4. The following colors are assigned to different atomic species: B. Vibrations. 1. Single Molecule. In Table 6, we report the
red to oxygen, gray to carbon, white to hydrogen, and violet to lithium.
harmonic frequencies obtained from ab initio calculations, those Some final remarks can be made on the structure: the obtained with the force field developed in this work (see section C1) is slightly tilted (3°) with respect III C), the experimental measures, and, finally, the band assignment. It is known that the neglect of anharmonicity is a distance among the two oxygens shorter than the O - source of disagreement with experimental results, mainly for separation. For what concerns the hydrogen atoms, differences high frequency modes. Recently, Scott et Radom38 published in their distances from the carbons (∼1.09 Å), or in the H-C-H generic scaling factors for these frequencies so that ab initio bending angle (∼109°) are negligible.
results can be brought to better agreement with experiment. For 2. [Li(GBL)n]+ (n ) 1-4) Clusters. In a recent study of MP2/6-311G(d,p) quantum chemical calculations, they proposed ethylene carbonate,36 a molecule very similar to GBL (the a scaling factor of 0.9496. Even though our model chemistry is R-methylene group is substituted by an oxygen), we found that slightly different (for the inclusion of the ++ diffuse function the interaction with lithium affects the structure causing the in the basis set), using the same factor for the highest distortion of the molecule. A high level calculation (MP2/ frequencies, the corrected ab initio frequencies agree very well 6-31++G(d,p)) of the complex [Li(GBL)]+ has been performed to look into the most important changes in the equilibriumgeometry of the molecule (Figure 3a). In the previous subsection, The most recent vibrational analysis is the one by McDer- it was observed that the carbonyl axis of the single molecule is mott,24 who used a modified Urey-Bradley force field, with slightly tilted toward the lactone oxygen; this would suggest structural assumptions based on experimental measures22,23 and that the lithium ion might be coordinated by both oxygens if previous theoretical works.13 Fourteen modes differ from our the oxygen atoms could get closer upon ion coordination. This assignment (see Table 6, bold typeface), although only a few possibility has to be discarded because both the angle among of them can be considered to be substantial. Particularly the carbonyl axis and the bisetrix of the C - stretching, whereas we find that this stretch probably corre- O2 distance remain fixed. On the other hand, our calculations clearly show that the lithium ion is only coordinated sponds to ν17 (what agrees with typical results for lactones39).
1, but still lying out of the carbonyl axis, a muted signal of 2 rocking modes had been assigned to bands for the presence of the lactone oxygen. Compared to EC, GBL which we find C-C or O-C stretching modes and vice versa, seems to be slightly more rigid: coordination affects some bond a shuffling that can probably be explained if we notice that this zone of the spectrum is particularly crowded (7 bands in ca.
and dihedral angles are almost unaffected. A representative 300 cm-1). At lower frequencies, we find important differences example is given by the change of the torsional angle C - for ν27, ν28, and ν30 which had been previously assigned O2 upon coordination: when passing from the monomer respectively to the in-plane ring-CdO torsion, the out of plane, to the dimer it diminishes by ∼13° in EC, whereas in GBL it and the in-plane bending of the carbonyl, although here they J. Phys. Chem. B, Vol. 108, No. 46, 2004 17995
TABLE 5: Values for the Most Affected Coordinates by Ion Coordination Both for High and Low Level Calculations
TABLE 6: Vibrational Analysis: High Level Ab Initio, Classical, and Experimental24 Frequencies (cm-1) and Mode
Assignmentsa

CH2 wagging
CH2 twisting
C1 O2 stretching
CH2 rocking
O2 C4 stretching
C2 C3 stretching
C3 C4 stretching
CH2 rocking
CH2 rocking
ring stretching
ring distortion
out of plane ring-C1dO1 torsion
in plane ring-C1dO1 bending
in plane ring-C1dO1 torsion
a The results for the mono-coordinated lithium complex are ordered following the assignment for the single molecule. The numbers in brackets are the high-frequency ab initio scaled values. The shifts with respect to the single molecule are given in the last column (positive sign is used forblueshifts).
are assigned to the out of plane ring-CdO torsion, the in-plane A preliminary understanding of condensed phase effects might carbonyl bending and the in-plane ring-CdO torsion, respec- be obtained from the study of n-coordinated complexes. As it has been shown in the previous subsection, the structural changes on the GBL molecule decrease with increasing coor- n]+ (n ) 1-4) Clusters. As pointed out in subsection III A the coordination of lithium bears nonnegligible dination number, an effect that can be expected as well for the structural changes, what suggests that the strong interaction vibrational shifts (an issue that was studied in detail for the EC between GBL and the cation may also induce noticeable shifts molecule36). According to experimental results,37 the four of the vibrational frequencies. With high level quantum calcula- coordinated complex is the most likely in liquid phase. A tions (MP2/6-311++G(d,p)) substantial shifts (higher than 30 detailed study of the shifts as a function of the coordination cm-1) have been found for the following modes: ν6, ν7, ν11, number (with up to four molecules) has been performed with a ν17, ν25, ν28, and ν30. Actually, these modes are associated with MP2/6-31G model chemistry. As the number n of coordinating the most affected degrees of freedom upon ion coordination molecules increases, also the number m of modes increases (see section III A). Table 6 contains the shifts for all modes of (according to m ) 3 × (12 × n + 1) - 6). The majority of the mono-coordinated complex. It should be noted that a modes are localized on single molecules so that in a n- reordering of modes takes place in some cases upon coordina- coordinated complex one can usually discern n frequencies that tion. It is the case, for instance, for ν17, which frequency is can easily be associated to a single mode (the average of these upshifted by ∼115 cm-1; since this large shift is not experienced n frequencies is taken as the mode frequency). In some cases, by ν14-16, it results in a swapping of modes.
there is a nonnegligible dispersion of frequencies (more than 17996 J. Phys. Chem. B, Vol. 108, No. 46, 2004
TABLE 7: Vibrational Analysis: ab Initio Low Level
TABLE 8: Intramolecular Force Field Parameters For
Frequencies (cm-1) for Single GBL and the Relative Shifts
Dihedralsa
with its Lithium Complexes [Li(GBL)n]+a
normal mode single GBL ∆ν n ) 1 ∆ν n ) 2 ∆ν n ) 3 ∆ν n ) 4 Units: [An] ) kcal mol-1, [φ0] ) [δ] ) degrees, [] ) kcal mol-1 we add to the view that, given the increased computational power, force fields tailored to each system can be developed (at least for molecules of the size of GBL) using as a source of reference data quantum mechanical results. This is the path followed for instance to parametrize very flexible force fields for transition metal complexes, where an accurate description Positive and negative values of ∆ν correspond to blue and red shifts, of the quantum mechanical PES far from the minimum is Recently,36 we applied an efficient methodology to develop 10 cm-1), so that the average value might not be fully a force field from first principles and applied it to the EC informative. The carbonyl stretching for the four-coordinated molecule. The starting point is the usual expansion of the complex is a relevant example, with frequencies of 1708, 1710, intramolecular potential in terms of internal coordinates (note 1720, and 1736 cm-1. As it will be shown, this behavior is that anharmonic terms are included for stretchings and bend- probably a precursor of the broadening of the absorption band found in the liquid state, both in experiments and MD simula-tions (see section IV D). Obviously, a subset of modes is V(r,θ,φ) ) ∑ [k (r - r )2 + k (r - r )3 + k (r - r )4] + associated to vibrational motion of the whole cluster, and they have a complex character; most of them fall at wavenumbers ∑ [k (θ - θ )2 + k (θ - θ )3] + ∑ A [1 + lower than 150 cm-1. An exception corresponds to some lithium-OdC modes which are found within the range of ring ∑ [k (φ - φ )2] ) V distortion vibrations; in the four-coordinated complex, there are cm-1, which will be discussed when the vibrational spectrumfor the liquid phase is addressed.
where r, θ, and φ denote respectively bond lengths, bending Table 7 illustrates how the shifts become smaller when the coordination number increases. As will be shown in section IV The method used to determine the parameters in the previous D, the results for the four coordinated complex are rather similar expansion makes use of the relaxed potential energy surface to those obtained in the liquid phase. Several other features are (rPES) concept.63 In a rPES scan, the energy is computed along worth noticing in the shifts experienced by GBL molecules for a given internal coordinate simultaneously optimizing all the clusters. One would expect a monotonic variation of the shifts unconstrained degrees of freedom, so that the minimum total with the coordination number; remarkably, this is not the case energy is obtained along the chosen internal coordinate. Such for many degrees of freedom, as the shifts for the bis coordinated a procedure can be performed both at the ab initio level and complex do not follow this trend (see for example ν9, ν14, ν15, with the classical potential embodied in eq 1. Since the ν16, ν17, ν23, ν24, ν25, ν26, ν27, ν28, and ν30 in Table 7). Finally, calculation is done for all internal coordinates, more rPES the frequency shift decreases at different rates depending on profiles are obtained than intramolecular degrees of freedom.
the mode, it is not possible to find a simple relation for the This redundant description indirectly takes into account cross magnitude of the shift as a function of the coordination number.
effects that are apparently neglected with the functional form C. Intramolecular Force Field. There are indeed many
used for the potential. The constants in eq 1 are obtained in an intramolecular force fields available in the literature, like UFF,40 iterative way: after a first guess, the parameter set is refined AMBER,41-43 MM3,44-51 CHARMM,52,53 OPLS,54-59 and until the classical rPES profiles reach a good convergence with COMPASS.60 They can be roughly divided into three classes: the ab initio ones. Although for the stretching degrees of freedom (i) generic ones with a large coverage (UFF), (ii) improved few iterations are required to get a 100% convergence, for models restricted to some area of applications (e.g., biochem- bending and torsional coordinates, the fitting procedure is istry, AMBER, and CHARMM), and (iii) optimized parametri- slower. The resulting force field is summarized in Tables 2, 3, zations for condensed matter simulations. In the present work, and 8. Figure 4 displays some examples of rPES profiles J. Phys. Chem. B, Vol. 108, No. 46, 2004 17997
Figure 5. Potential energy for the GBL-Li+ dimer; ab initio results
(solid line), and classical results with the new (dashed line) and old
(dotted line) set of Lennard-Jones parameters (see text).
scheme,64,65 constraining them to reproduce the total moleculardipole moment. The latter is slightly overestimated (4.708 Dversus the experimental value of 4.270 D22), which is a desirablefeature in order to balance the absence of polarization effectswith fixed charge models.65 Lennard-Jones parameters for GBLare taken from Carlson et al.66 (with geometric averagecombination rules: σ ) with this parameter set, the diffusion coefficient is lower thanthe experimental one. The origin of this discrepancy lies in theradius taken for H. The value used (σ ) 2.5 Å) is the one typicalfor hydrocarbons, whereas in GBL (and EC) the hydrogens are Figure 4. rPES profiles along selected internal coordinates: (a) O -
connected to carbon atoms that are near to electron-withdrawing C1 C2 angle, and (c) C3 C4 O2 C1 dihedral. Filled groups (carbonate oxygens). This suggests that the electronic circles, solid line, and dotted line are used respectively for ab initio,our force field, and AMBER results.
cloud for the hydrogen should be smaller. Indeed Sun et al.67have proposed that in the simulation of polycarbonates a valueof σ ) 1.8 Å should be used for hydrogen atoms which are obtained with ab initio (black circles) and classical calculations hydrogen bonded to oxygens (it will be shown in the analysis using both our (solid line) and AMBER (dotted line) force fields.
of liquid structure that the carbonate oxygen tends to bind to Here AMBER is used as a benchmark since it probably is the hydrogens). We found that with this smaller hydrogen radius most popular force field used in atomistic simulations (neverthe- the diffusion coefficient is very near to the experimental one.
less we have obtained similar results with other force fields such Along the same line of reasoning it has been found that an as CHARMM, MM3, and OPLS). Our parametrization produces optimal value for the lithium ion parameters is σ ) 1.3 Å and profiles in excellent agreement with the ab initio ones (the same ) 0.191 kcal mol-1. After this parameter fine-tuning, it is degree of accord is obtained for all intramolecular degrees of important to check that the modified force field is consistent freedom, not shown). As mentioned before, our functional form with ab initio calculations. Figure 5 displays the potential curves includes the anharmonic terms along stretching and bending obtained with quantum chemical, and with the modified classical coordinates. The quantitative importance of an anharmonic force field just described, for the Li+-GBL dimer. The ab initio description to better address solvent induced shifts is discussed result is obtained with a counterpoise68 correction using an MP2/ in section IV D. Panels a and b display the qualitative differences 6-311G(d,p) model chemistry. As can be seen, the refined in the potential curves when anharmonicity is considered (our parametrization performs substantially better in reproducing the force field) and when not (AMBER): the ab initio profile is interaction between GBL and the lithium ion. Obviously, the clearly anharmonic. Even for dihedral angles, which are dimer potential is an approximation to the interaction in the obviously anharmonic in all force fields, there are noticeable liquid phase, where many body effects will be present, but we differences. Panel c shows how AMBER fails to faithfully do not expect them to be important given the low degree of association of the neat liquid (see next section).
angle. The vibrational frequencies obtained with the model All simulations were done in the NVE ensemble with a time developed here are reported in Table 6, which also contains step of 0.2 fs. The reference temperature and density were set the quantum chemical results. The maximum discrepancy with to 298.15 K and 1.1290 g cm-3 (as reported in the Sigma- ab initio results is ≈8% (≈15% with AMBER).
Aldrich catalog for the pure product). After an equilibration runof 50 ps, three productions runs of 100 ps each were completed IV. Molecular Dynamics
to calculate structural and dynamical properties of the system.
Two more calculations of 250 ps each were done to compute A. Simulation Details. Molecular dynamics simulations of
vibrational spectra. For the intramolecular interactions, we used the pure liquid and of one lithium ion dissolved in liquid GBL the intramolecular force field developed in section III C and have been performed. Table 1 contains the parameters used for the AMBER force field for comparison. The Ewald sum was the intermolecular potential. Partial charges on the atoms were employed for electrostatic interactions.
obtained by fitting the electrostatic potential energy surface B. Structural Properties. 1. Pure GBL. The radial distribu-
(obtained by ab initio MP2/6-311G++(d,p) calculations) at tion function (RDF) corresponding to the molecular center of points selected according to the Merz-Singh-Kollman mass is displayed in panel a of Figure 6. Its overall structure is 17998 J. Phys. Chem. B, Vol. 108, No. 46, 2004
Figure 7. Structural properties of liquid GBL around lithium ion. (a)
Figure 6. Molecular dynamics simulation results: (a) radial distribution
Li+-GBL center of mass radial distribution function, and solvation function for GBL molecules’ center of mass, (b) and (c) O number (inset). (b) Probability distribution for the distance Li+-O (solid line), Li+-O2 (dashed line), and Li+-C2 (dotted line). (c)Probability distribution for R (see inset for definition). (d) Probability very similar to that of dense simple liquids, what can be further distribution for the dihedral angle formed by the four carbonyl oxygensnearest to lithium.
confirmed by analysis of the solvation number Ns, defined as carbonyl oxygen of one molecule would be located midway N ) 4πF ∫ minr2g(r) dr between both hydrogens of the C2 group (as a simple geometriccalculation confirms). Such configuration is consistent with the where g(r) denotes the RDF, F is the number density, and r lower height of the O1 C2 RDF as compared with those for is the first minimum of the RDF (7.1 Å). A solvation number C3 or O1 C4: when coordinating the C2 methylene group of 12 is found, which is typical of nonassociated liquids.
of one molecule, the carbonyl oxygen of the coordinating Although this is a signal of a low degree of order, some further molecule tends to attach preferentially to both hydrogens rather insight can be obtained from the analysis of partial RDFs.
than directly to the carbon. The peaks located at a shorter Panel b of Figure 6 displays the O1-oxygen and O1-carbon distance for the hydrogens belonging to C3 and (to a lesser radial distribution functions for representative oxygen and extent) C4 are indicative of a collinear C-H‚‚‚O configuration.
It is also interesting to note the double peak that appears at ≈4 Å, the contact oxygen-oxygen distance, and the same result is Å in both cases, which is consistent with the distances O2 (not shown). The corresponding RDFs are corresponding to the case in which the carbonyl oxygen is flat and start at larger separations. These features indicate that coordinated by both C2 hydrogens. In conclusion, this analysis the oxygens in different molecules tend to stay away from each points to a substantial amount of hydrogen bonding between other, what can be explained by the strong electrostatic the carbonyl oxygen and the methylene hydrogens.
repulsion. Concerning the carbons, the result for O - 2. GBL + Li+. The structural properties of the liquid around almost identical to those just discussed for the oxygen-oxygen lithium are collected in Figure 7. The radial distribution function RDFs, so that the configuration in which the carbonyl oxygen for the lithium ion is shown in panel a (the inset contains the would point to C1 of a neighboring molecule is not found. The solvation number for the first two solvation shells). We find behavior for the other carbons differs markedly, with a first peak that the solvation number is exactly four (in accord with the at the contact oxygen-carbon distance (≈3.2 Å). The slight experimental estimation37) and that the radius of the first differences in peak position among different carbons correspond solvation shell is 4.0 Å. The structure of the complex can be to their different radius (see Table 1). The results for C4 are compared to the one obtained with quantum chemical calcula- not shown for clarity; however, they are very similar to those tions for clusters (section III A). In panel b, the probability for C3 but slightly shifted to shorter distances due to the distribution functions for the distances Li+-O1, Li+-C2, and somewhat smaller carbon radius. The picture that results is one Li+-O2 are shown. The most probable distances to O1, O2, and for which the carbonyl oxygen preferentially solvates the C2 are respectively 1.73, 3.74, and 4.1 Å: as in the ab initio methylene groups. This is supported by the analysis of the O - calculations, the lithium ion is coordinated by the carbonyl RDFs, with the representative examples displayed in panel c oxygen and the molecule is tilted allowing the ester oxygen to of Figure 6. Two rather different behaviors are found: for the lie nearer to the ion than the R-carbon. To more clearly ascertain 3 and C4, there is a (small) first peak located the distortion from a linear arrangement of the Li+-O1 C1 at ≈2.4 Å, which corresponds to the contact O-H distance, atoms, we computed the probability distribution for the angle (R) formed between the Li+-O1 and the O1 C1 axis (see inset peak located at a somewhat larger distance (≈2.8 Å). The latter in panel c for a graphical definition). A maximum exists at is consistent with a bifurcated configuration in which the ∼160°, well above the result found in the gas phase for the J. Phys. Chem. B, Vol. 108, No. 46, 2004 17999
Figure 8. Molecular dynamics simulation results for the mean square
displacement of GBL molecules’ center of mass (solid line) and of
lithium ion (dashed line).
four-coordinated complex (∼140°) and near to the valueobtained for the mono-coordinated one (∼158°). Similar resultswere obtained for the EC case36 and are explained by theattractive interaction with the carbonyl oxygen of second shellmolecules, which tends to draw the methylene groups of firstshell molecules away from the lithium ion, resulting in an angle Figure 9. Middle panel: whole vibrational spectrum of pure GBL.
closer to 180°. Finally, the dihedral angle formed by the carbonyl Smaller panels: details of zones (a), (b), (c), and (d).
oxygens coordinating the cation (last panel) is typical of atetrahedral structure, where the distribution is peaked at ∼71°, autocorrelation function computed during the simulation (in- just 4° less than the ab initio result.
cluding all GBL molecules or just those within the first shell C. Diffusion. Diffusion coefficients are calculated both from
of the ion, see below). According to Berens et al.72-74 the 〈|RB(t) - RB(0)|2〉 S(ω) ) (2π) ∫ dt exp(-iωt)〈M B denotes the total dipole moment. The shortness of the and from the velocity autocorrelation function (VACF) time series available results in a no negligible degree of noise, so that a filter is required. We used an FFT filter with 20 points ) 1∫ 〈VB(0)‚VB(t)〉 dt for a correlation function of 100 000 points (we checked in a previous study36 that this smoothing allows a clearer representa- where 〈‚‚‚〉 denotes the average for all time origins and all tion of the spectrum to be obtained without losing important molecules’ positions (velocities) of the center of mass. The actual cutoffs used in these formulas are 25 ps (for the MSD, 1. Pure GBL. The whole spectrum of liquid GBL is shown see Figure 8) and 5 ps (for the VACF integration). The in the middle panel of Figure 9. Contrary to EC, where a number experimental value of the GBL diffusion coefficient has been of bands did not appear in the simulated spectrum,36 here almost recently measured by means of pulsed gradient spin-echo 1H all vibrational frequencies are visible. To ease the comparison NMR,26 and at ambient temperature, it is ≈0.90 × 10-9 m2 with the vibrational analysis done in subsection III B, the s-1, with which our results agree satisfactorily (D spectrum is divided into four zones (ν29 and ν30 modes are not considered because they have a very low intensity). Panel a s-1). For what concerns lithium diffusion, we found very good contains all ring modes (ν28 to ν24); of particular intensity is agreement with experiment as well: Kikuko et al.26 measured the band for the out of plane ring-CdO torsion (ν27). In panel a value of ≈0.25 × 10-9 m2 s-1, and we obtain D b, we show all of the stretching modes for the ring bonds and the CH2 rocking modes (ν23 to ν16). For what concerns the s-1. According to Du¨nweg et al.,70 due to the finite size of the remaining CH2 modes (ν15 to ν8), we can see in panel c that simulation box, the diffusion coefficient arising from the the scissoring and the highest frequency wagging modes (ν11 simulation usually underestimates the value for infinite size to ν8) form a broad band of low intensity where the peaks cannot systems. They proposed that this could be corrected by adding be easily distinguished. ν15 to ν12 modes have higher intensity a constant term ( ) that depends on the simulation box and two wagging modes coalesce in a single band with a dimension (L), temperature (T), and viscosity (η) shoulder due to the ν13 mode. The important carbonyl stretchingmode is depicted in panel d: it shows an asymmetric band which width at half-height is ∼18 cm-1. Very small shifts (maximum ∼10 cm-1) of the frequencies are noticed if we compare the condensed phase and the harmonic analysis for the isolated In our case, considering η ) 1.727 cP,71 we have molecule. They can be observed mainly in the CH2 twisting 10-9 m2 s-1. Taking into account this correction, the diffusion and rocking modes and, of minor entity, in ring modes.
coefficients for the pure liquid are even in better agreement with 2. GBL + Li+. To discern the effect of the lithium ion on the liquid phase spectrum, the dipole moment autocorrelation D. Vibrational Spectrum. Vibrational spectra were obtained
function was computed during the simulation only for the by fast Fourier transform (FFT) of the total dipole moment molecules belonging to the first solvation shell. Figure 10 shows 18000 J. Phys. Chem. B, Vol. 108, No. 46, 2004
blueshifts of respectively ∼27 cm-1 (experimental ∼30 cm-1)and ∼6 cm-1 (experimental ∼10 cm-1). The low intensityobtained for these bands is consistent with experiment as well,as they are only observed at high ionic concentrations. Theremaining modes on which we focus are shown in panel b ofFigure 10. This part of the spectrum seems to be rather sensitiveto coordination. The ν28 mode shows a blueshift of ∼20 cm-1.
Experimentally, the presence of a new band whose intensitygrows with salt concentration is observed (with a blueshift of ∼5 cm-1). The ν27 mode is upshifted by ∼15 cm-1, in linewith the experimental blueshift of ∼8 cm-1. In addition, thisband shows a shoulder that might be interpreted as thecontribution of lithium-GBL intermolecular modes: althoughin quantum chemical calculations the majority of intermolecularmodes are found below 150 cm-1, three of them are found inthis zone of the spectrum (see subsection III B), which mightexplain the broad profile of the ν27 mode.
Finally, for what concerns ν24, there is a broadening of the band whose peak is upshifted by ∼11 cm-1; a blueshift of ∼22cm-1 is found experimentally. Wang et al.27 observed that thecontour becomes more asymmetric as the lithium salt concentra-tion is raised, followed by the splitting of the band at highconcentrations. We should mention that in this zone of the Figure 10. Comparison between the simulated vibrational spectra of
bulk GBL (solid line) and first shell molecules (dashed line) for the
spectrum we also observe the change in intensity of ν26 and two most representative regions, corresponding respectively to zones ν25: the former lowers substantially and the latter increases in (d) and (a) of Figure 9. In panel a, the result with an harmonic force intensity, while both are slightly upshifted (the entity of these shifts is within the order of experimental precision). Small shifts(less than 5 cm-1) are also found in all low lying vibrations.
the details of what we called zones a and d for the pure solvent Even if they are not observed experimentally, this result is spectrum (previous subsection); both the pure solvent and the consistent with our ab initio calculation on complexes as coordinating GBL frequencies are shown. These zones contain explained in section III B. We can conclude that, as a result of most of the vibrational frequencies that can be compared with the strong interaction between lithium and GBL, the most experimental results: Wang et al.27 found, with IR and Raman affected vibrational modes are the ring distortions, the methylene spectroscopy, that the most important shifts correspond to ν7, rocking and twisting modes, and, obviously, the carbonyl ν16, ν22, ν24, ν27, and ν28, which will be addressed in turn. It should be noted that it is not straightforward to compare withexperiment because, in contrast to simulated ones, experimental V. Conclusions
spectra contain contributions of bulk and ion-coordinatingmolecules. Besides, experimentally there are substantial con- Concerning structural properties, it has been found in first tributions from overtone and combination bands (the carbonyl place that the GBL monomer is nonplanar with a barrier of ≈9 stretching being a prominent example), which in contrast are kJ/mol for ring inversion, with the carbonyl bond axis slightly tilted toward the lactone oxygen. This structure is somewhat The carbonyl stretching normal mode (ν7) is depicted in panel deformed in the presence of the lithium ion but to a lesser extent a of Figure 10: upon cation coordination, we notice a redshift than what is found for instance in the ethylene carbonate case.
of ∼14 cm-1 and a broadening of the spectral band of ∼6 cm-1.
As the number of molecules solvating the ion increases, the Experimentally, Wang et al. found a bigger broadening and a distance between the carbonyl oxygen and the ion increases as shoulder at lower frequencies (which might be indicative of a well, reducing the molecular distortion. For the important case shift of ∼24 cm-1). On the other hand, there is a study of Deepa of the four coordinated cluster, the structure is tetrahedral. In et al.,29 who found a redshift of 10 cm-1. Our result thus falls addition, the carbonyl axis is not collinear with the lithium ion, midway between both experimental estimations. The same panel but the lactone oxygen is closer to lithium than the R-carbon.
dramatically illustrates the effect of neglecting anharmonicity.
Such a configuration is maintained in the liquid phase, but with If only the harmonic terms of the force field are considered, an increased tendency to a collinear configuration due to the instead of a red shift, a blueshift is obtained. To discard that attractive effect of second shell molecules. This attraction is this is not a particular feature of the force field employed, similar explained by the analysis of radial distribution functions for simulations have been run using the AMBER force field (which neat liquid GBL: the carbonyl oxygen tends to solvate the we recall does not contain any anharmonicity for stretchings or methylene groups. Particularly, the solvation of the R-carbon bendings). Again the same result is obtained: differs from the two other methylene groups in that the oxygen coordination, the carbonyl stretching mode is upshifted to higher tends to sit midway between both hydrogens.
wavenumbers. In short, a fully harmonic force field is not able Given that the main probes of GBL are spectroscopic, a to reproduce the correct sign of the shift, what should be special emphasis has been put on vibrational properties, starting regarded as an important limitation of most force fields if they with a full new assignment of bands. Substantial shifts have are to be used to interpret spectroscopic measures of solvated been found upon lithium coordination. The cases of the C-O stretches are particularly remarkable for the Li+-GBL dimer: For what concerns ν16 (CH2 twisting mode) and ν22 (CH2 the carbonyl stretch frequency is downshifted by ≈77 cm-1, rocking mode), we observed, consistently with experiment, J. Phys. Chem. B, Vol. 108, No. 46, 2004 18001
linked to the ion) is upshifted by a larger value (≈100 cm-1).
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Paleo Solution – Episode 123 Hey folks, Robb Wolf here. Greg Everett in the house and what is this, More exciting than episodes 1 through 69. Got back from Columbus, Ohio late Sunday night. I had a couple very interesting plan rides and interesting people on the plane to put it that way. So that was the national championships and the Olympic trials for women, weight lifting. Congratulation

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