J. Phys. Chem. B 2005, 109, 18209-18210
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COMMENTS Comments on “Vibrational Study of the Crystalline Phases of (CH3(OCH2CH2)2OCH3)2LiSbF6 and P(EO)6LiMF6 (M ) P, As, Sb)” Jean-Claude Lasse`gues* and Joseph Grondin Laboratoire de Physico-Chimie Mole´ culaire, UMR 5803 CNRS, UniVersite´ Bordeaux I, 351 Cours de la Libe´ ration, 33405 Talence, France ReceiVed: June 17, 2005; In Final Form: July 20, 2005 Frech et al. have recently published a correction1 to the Raman data they previously reported on the P(EO)6:LiAsF6 complex.2 We would like to make a few comments on the consequences of this correction. The crystal structure of the P(EO)6:LiMF6 compounds, where P(EO) is poly(ethylene oxide) (CH2CH2O)n and M ) P, As, or Sb, has been solved by P. G. Bruce’s group.3,4 Spectroscopic studies on this series of compounds were published simultaneously by Frech et al.2 and by Ducasse et al.5 They revealed discrepancies on the wavenumber values for the P(EO)6:LiAsF6 derivative and lead to quite different conclusions. After Frech et al.’s correction,1 we are left with only one conclusion: the vibrational spectra of the complexed polymer in the three P(EO)6:LiMF6 compounds (M ) P, As, Sb) are very similar despite different PEO conformational sequences and Li-O distances. The recognized agreement between the two separate spectroscopic results has another deep consequence: it shows that the spectral similarity between the three P(EO)6:LiMF6 compounds applies to samples prepared independently and in a rather different way. Indeed, the samples for spectroscopy were prepared by either the solvent-casting method2,5,6 or by grinding together the polymer and salt in an inert atmosphere at liquid nitrogen temperature.7 In the second method, devised by MacGlashan et al. for the diffraction experiments,3 the mixture was sealed in a capillary tube, heated, and annealed. Also, polymers having quite different molecular weights, from 4 × 106 to 2000, were used in the various preparations. Gadjourova et al. note that the same crystalline complexes, as seen by neutron powder diffraction, were obtained in this range of PEO molecular weights.4 Therefore, the similarity of the P(EO)6: LiMF6 spectra cannot be attributed to some effect of sample preparation, polymer mass, or degree of crystallinity that would have smoothed out the spectroscopic differences expected from the structural differences. It can still be argued that the samples for spectroscopy were all prepared with hydrogenated PEO,1,2,5-7 whereas the P(EO)6: LiPF6 and P(EO)6:LiSbF6 samples for neutron powder diffraction used fully deuterated poly(ethylene oxide) PEO-d.4 Therefore, we have recorded the Raman spectra of the P(EO-d)6: LiMF6 series using PEO-d purchased from Polymer Science (99%, Mw ) 42 800) and the same experimental conditions as * To whom correspondence should be addressed. E-mail: jc.lassegues@ lpcm.u-bordeaux1.fr
Figure 1. Raman spectra at room temperature of PEO-d (broken line) and of the complexes P(EO-d)6:LiMPF6 (M ) P, As, Sb). The spectra are shifted vertically for a more convenient comparison.
described in ref 5. The pure polymer is known to adopt a (tgt)n helical conformation,8 and vibrational assignments have already been proposed for the hydrogenated and deuterated forms.9-11 It is out of the scope of the present comment to enter into the vibrational assignment of the P(EO-d)6:LiMF6 crystalline complexes. Let us rather recall that the PEO conformational sequences in the complexes are tct-gg′t-gcg′-tct-tgt-g′cg for M ) As,3 tg′g′-tct-tcc-tgt-tcg′-gg′g′ for M ) Sb, and tct-g′g’t-tg′ctgt-tg′g′-gg′c for M ) P.4 In this notation, the torsion angles in the range 0 ( 45° are cis (c), in the range 180 ( 45° are trans (t), and the rest are either gauche (g) or gauche-minus (g′). The major conformational changes occurring between the pure polymer and the complexes easily explain the observation of large spectral differences and of a higher multiplicity for the complexes occurring, for example, in the region of the CD2 stretching vibrations (Figure 1). However, the spectral similarity between the complexes themselves is surprising, since as previously pointed out for the hydrogenated derivatives,5 the series of dihedral angles of the six OCD2CD2O units are different in the three complexes. The results of Figure 2 confirm that the three complexes have identical wavenumbers for the CD2 twisting and rocking modes occurring in the 960-850 cm-1 and 810-650 cm-1 regions, respectively, although these modes are even more sensitive than the methylene stretching vibrations to the conformational state of the PEO chain. The anion vibrations, indicated by asterisks in Figure 2, are easy to identify, inasmuch as they occur at the same wavenumbers as previously reported with PEO-h.5 The vibrations of these “spectroscopically free” anions have no reason to be affected by the deuteration of the polymer. The similarity of the PEO spectra in the P(EO-d)6:LiMF6 series is verified in all the other spectral regions. Let us just point out that variable relative intensities can be observed from one compound to another and even from one point to another for a given compound. This is due to different and uncontrolled orientations of the chains under the polarized laser beam, as previously illustrated on the P(EO-h)6:LiMF6 series.5 However,
10.1021/jp0532622 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/07/2005
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Comments (diglyme)2Li+ type.12 In addition, we hope that they can reveal the specificity of the PEO conformation in the P(EO)6:LiMF6 series and, in particular, the presence of a variable content of cis forms, the spectral signature of which has never been observed nor calculated. It is important to reconcile the structural and spectroscopic data for a better understanding of the very interesting P(EO)6:LiMF6 system, in the same way as recent molecular dynamics simulations predicting an anion transference number close to 1.0 and a strong lithium coordination have to be reconciled with NMR experiments where the transference number is found to be close to 1.0 for Li+.13 References and Notes
Figure 2. Same as Figure 1 in a different spectral region. The intense lines indicated by asterisks are due to the ν1 vibration of the SbF6-, AsF6-, and PF6- octahedral anions at 645, 679, and 741 cm-1, respectively. They are truncated for clarity.
if the peaks can have variable relative intensities because of polarization effects, their position is unchanged. In conclusion, the spectroscopic results accumulated on the P(EO)6:LiMF6 and P(EO-d)6:LiMF6 crystalline complexes raise the following question: Are, in reality, the PEO conformational sequences in the M ) P, As, and Sb series less different than deduced from the diffraction analysis, or is vibrational spectroscopy insufficiently sensitive to distinguish them? We are going to undertake ab initio calculations of the IR and Raman spectra of the three complexes to answer this question. Such calculations are essential to fully exploit the H/D isotopic effect and to improve the vibrational assignments, as they were to interpret the vibrational properties of model compounds of the
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