The Raman and neutron scattering spectra of 46 mol% AlCl[sub 3] -54 mol% 1-ethyl-3-methyl imidazolium chloride (EMIC) and 67 mol% AlCl[sub 3] - 33 mol% EMIC melts are presented. Ab initio molecular orbital calculations have been carried out on structures of chloroaluminate anion and EMI cation and the interaction between anion and cation

Curtiss, L. A.

Gosztola, D.

Koura, N.

Loong, C. K.

Saboungi, M. L.

Takahasi, S.

UNT Digital Library

• l ilDIALKYLIMIDAZOLIUM CHLOROALUMINATES" AB INITIO CALCULATIONS,RAMAN AND NEUTRON SCATTERING MEASUREMENTSS. TakahashiNisshin Steel Company, Ltd.7-1 Koya-shinmachi, Ichikawa,Chiba 272 JapanL. A. Curtiss, D. Gosztola, and N. KouraTokyo University of Science,Tokyo, JapanC.-K. Loong and M.-L. SaboungiMaterials Science DivisionArgonne National LaboratoryArgonne, IL 60439I Thesubmittedmanuscripthasboonauthoredbya contract_'oitheIU.S. Governmentunder contract No. W-31-109-ENG-38.|Accordingly,the U.S.Governmentretainsa nonexduslve,royalty-]free licenseto publishor reproducethe publishedformof thlslcontribution,orallowotherstodoso,forU.S.Governmentpurposes.April, 1993/smVED_t';:,'! 12 i.;33DISCLAIMER :_ .....,•This report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-once herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof. Mi_ _ T __._.,Will be published in the Proceedings of the Electrochemical Society.This work is supported by the Division of Materials Sciences, Office of Basic Energy Sciences ofDOE, under contract No. W-31-109-ENG-38... DISTRIBUTION OF THIS DoCUMENT IS UNLIMrilEDtJ.Dialkylimidazolium Chloroaluminates: Ab lnitio Calculations,Raman and Neutron Scattering MeasurementsS. Takahashi*, L. A. Curtiss, D. Gosztola, N. Koura**C.-K. Loong and M.-L. SaboungiArgonne National Laboratory9700 South Cass AvenueArgonne, Illinois 60439-4814AbstractThe Raman and neutron scattering spectra of 46 tool% A1CI3 -54tool% 1-ethyl-3-methyl imidazolium chloride (EMIC) and 67 mol%A1C13- 33 mol% EMIC melts are presented. Ab initio molecularorbital calculations have been carried out on structures ofchloroaluminate anion and EMl cation and the interaction betweenanion andcation.IntroductionA new class of room-temperature molten salts, A1Cl3-1-ethyl-3-methylimidazolium (EMI) chloridel has been receiving renewed interest since their recentapplication in the electroplating of Al on steel. 2 The chemistry of these systems is quite: similar to that of the chloroaluminates and it has been shown that the ionic, speciesformed are strongly dependent on the acidity or basicity of the melts. In basic solutions,i.e. A1C13< 0.5, the major species is A1CI4-. The major equilibrium describing acid-base properties of tetrachloroaluminate ionic liquids is the dissociation of thetetrachloroaluminate anion:2AICI4- .--->AI2C17-+ CI-.At more acidic compositions, A12C17",which consists of two AIC14-tetrahedm sharinga comer, becomes the predominant species. These species have been identified fromelectrochemical techniques,3 NMR (lH, 13C, 27A1,35C1),4_6and IR.7In this paper, we present results of Raman and neutron scattering experiments onselected melts as well as ab initio molecular orbital calculations of the structure and thevibrational modes associated with A1CI4, A12C17-,and the EMI cation.* On leave from Nisshin Steel Co., Ltd, 7-1 Koya-shinmachi, Ichikawa, Chiba 272Japan**Tokyo University of Science, Tokyo, Japan6Experimental MethodsA. Raman spectroscopyThe Raman spectra were measured using a Laser Ionics 1400-15 ion laser, SPEXmodel 1877 triple monochromator, Photometrics Model CH210 liquid nitrogen cooledCCD detector, and argon 457.9 and 476.5 nm excitation. The spectra taken for 46 and67 mol% A1CI3 - EMIC melts were recorded in the range of 150-750 cm -1. Thewavelength of 476.5 nm was used to measure spectrum for 46 mol% AICI3 melt. For67 mol% A1CI3 melt, two wavelengths of 476.5 and 457.9 nm were used because ofthe noise of background. When the 476.5 nm wavelength was used, the plasma wasseen on the spectra at -500 cm -1.i[ The samples were prepared by mixing highly anhydrous A1CI3 with EMIC at60°C. The weighing and handling of the samples were done in a glove box, where the. H20 and 0 2 levels were maintained at less than 2 ppm., B. Neutron spectroscopy, Inelastic-scattering measurements were performed using the Low-Resolution ," Medium-Energy Chopper Spectrometer (LRMECS) at the Intense Pulsed Neutron: _Source (IPNS) of Argonne National Laboratory. Two compositions, 46 and 67 mol%of A1CI3 - EMIC melts, were studied at 25, 243 and 297 K with neutron incident:,,, energies of 60, 160 and 400 meV. (1 meV is equivalent to 8.066 cm-I, and these two-.- units of energy will be used interch.angeab!y in this-paper.) The measurements:'_, provided a characterization of the vibr_ii0nal S_tra of this system from 5 (40 cm -1) to::: 350 meV (2823 cm "1)for both the liquid and solid state.C. TheoryThe ab initio molecular orbital calculations 8 were carried out to determinei equilibrium structure and harmonic vibrationalfrequencies. The polarized.split valence',6-31G* basis was used. Ali of the ab initio molecular orbital calculations were done atthe Hartree Fock (I-IF]level of theory. Computations were also carried out using theMNDo 9 (modified neglect of differential overlap) semiempirical molecular orbitalmethod. The I_/6-31G* basis set is expected to give vibrational frequencies that areabout 5 - 10% too high on the basis of previous studies of a large number of molecules.MNDO has done reasonably well in calculations of vibrational frequencies ofchloroaluminates and organic molecules.ResultsA. Raman spectroscopyThe Raman spectra of AIC13-EMIC melts are presented in Fig. 1. The numericalvalues for the bands are given in Table 1. For the A1CI3-EMIC melts the investigationwas carried out at 25°C. Seven absorption bands were observed for the 46 mol% AIC13i- 54 mol% EMIC melt and 11 bands were observed for the 67 mol% A1CI3 - 33 mol%EMIC melt in the measurement range of 150 - 750 cm -1. Polarization measurementswere also carded out and the results are listed in Table 1.B. Neutron spectroscopyUnlike Raman scattering, neutron scattering by atomic nuclei is not restricted byselection rules. The neutron spectra, after being averaged over a wide range of detectorangles, represent the vibrational density-of-states (DOS) weighted by the nuclearscattering cross sections of the atomic species in the system. Since H has a scatteringcross section 5-10 times larger than those of the others atoms in A1C13-EMICmelts, theobserved spectrum reflects by and large the atomic motions that involve H atoms. Fig.2 displays the observed neutron-weighted vibrational DOS obtained from the 60-meVruns for the 46 mol% AIC13- 54 mol% EMIC and the 67 tool% A1C13- 33 mol% EMICat 25 K. The intensities are normalized to the total scattering cross section of the melts.The two spectra bear strong resemblance because the prominent features arise frommotions involving the 11 H atoms in the EMIC cations. A close examination of the twospectra, however, reveals some subtle differences which are likely due to the differentatomic vibrations associated with the AICI4- and the A12C17-anions. The neutronspectra at higher energies obtained from the 160 and 400 meV runs show even smallerdifferences between the two compositions, indicating that the vibrational DOS above 60meV are mainly due to motion of the catioos.C. Ab initio calculationsAICI4": Tetrachloroaluminate anion has been studied previously at the HF/6-31G*and MNDO levels of theory: 10,11 At the HF/6-31G* level, the anion has a Td structurewith a bondlength of 2.17A. The HF/6-31G* ,vibrational frequencies were in goodagreement with experimental measurements on the anion in various melts. 1! The HF/6-31G* Raman activityis shown in Fig. 3.A12C!7": The AI2CI 7- anion has been the subject of several previous theoreticalstudies. Davis et al 12reported a MNDO study of this anion including structure andvibrational frequencies. Curtiss 13reported an ab initio molecular orbited study usingthe 3-21G basis set. Both studies found A12CI7- to have a C2 structure with a bentAICIAI angle (MNDO: 124.9°; 3-21G: 132.4°). The HF/6-31G* calcultations of thiswork also give a C2 structure (AICIA1 angle = 123.4°). The HF/6-31G* Ramanactivity is shown in Fig. 4.1-ethyl-3-methyl imidazolium cation (EMl+): At the HF/6-31G* and MNDOlevels of theory the fully optimized cation has a structure with C 1 symmetry. The EMI+cation has 51 vibrational frequencies. The HF/6-31G* Raman active frequencies in therange 0 - 800 cm 1 are shown in Fig. 5. The HF/6-31G* structure is shown in Fig. 6.AICi 4- and EMl+: The H1 hydrogen of EMl cation (see Fig. 6) has a large positivecharge suggesting that it is the most favorable position for interaction of the A1CI4anion. The comer-, edge-, and face- interactions of AICI4- with EMl + were consideredin full optimizations at the MNDO level. The most stable structure was the face-bridgedstructure. The geometries of EMI+ and A1CI4- showed only small changes in thecomplex. The major shifts in the MNDO vibrational frequencies are listed in Table 2.AI2C! 7"and EMl+: Four possible structures for the complex between AI2CI7- andEMI+ were considered. Full optimizations were carried out at the MNDO level. Themost stable structure was the face-bridged structure. The geometries of EMl + andAI2C17-also showed only small changes in the complex.DiscussionThe Raman spectra in Fig. 1 and frequencies in Table 1 are characterized bybands which are dependent on the AICI3 content of the melt as well as ones that do notchange significantly with composition. The Raman active vibrational frequencies fromthe HF/6-31G* calculations are shown in Figs. 3 - 5. The two absorption bands at 181and 350 cm -1 for 46 mol% A1CI3 melt disappeared for 67 mol% A1C13. They areassigned to AICI4-, because it is well known that A1C14-is the dominant species in thebasic region. They are also in good agreement with the HF/6-31G* predictions forA1C14-of 188 and 353 cm -1. The neutron spectrum of the 67 mol% A1C13melt alsoshows a small drop in intensity at 186 cm -1 (23 meV) and 347 cm -1 (43 meV) ascompared to the spectrum of the 46 mod% A1C13melt (see Fig. 2). In addition, thespectrum of the 67 mol% A1CI3melt shows extra intensities near 153,216 and 380 cm-l (19, 27 and 47 meV). The HF/6-31G* calculations also predict weaker bands at 121and 511 cm -1 which are not observed in both the Raman and neutron data. FourRaman bands were found for 67 mol% A1C13 melt (see Fig. 1). These bands areassigned to A12C17-because this anion is known to be dominant in acidic melts. This isconfirmed by the HF/6-31G* results which predict absorptions at 162, 311,444, and558 cm -1. These are in good agreement with the experimental absorptions. Theabsorption bands of EMI+ cation should be present in all of the melts. The bands at241, 383, 596, and 700 cm -1 in Fig. 1 are assigned to EMl +. The HF/6-31G*calculations predict vibrational frequencies at 249, 403, 632, and 700 cm -1 which areclose to the observed bands. The neutron results indicate that the broad bands centeredat 100 cm-1 (12.4 meV), 240 cm -1 (30 meV) and 298 cm -1 (37 meV) originate from Hvibrations of the EMIC anions. The first band in the 40 - 130 cm -1 region arises fromcooperative motions of the complex structure. Some of these modes are given as newfrequencies in the A1CI4-....EMI . calculations in Table 2. Overall, the agreementbetween the predicted and observed frequencies is good and supports the existence ofAICI4- and A12CI7 anions in the A1CI3 - EMIC melts in the range 46 - 67 mol% A1C13.The significant shifts from MNDO calculations in the vibrational frequencies dueto interaction of a single anion (A1CI4-) with EMl + cation are shown in Table 2. Theshifts are small (<20 cm -l) with the exception of the 528 cm -1 band which split into 3bands (499, 515, and 573 cm-1). There is some evidence for a weak band at around510 cm-l in the Raman spectra.ConclusionA better understanding of the physical and chemical properties of AICI3 - EMICmelts has been achieved by combining ab initio calculations with Raman and neutronspectroscopy. AIC14-has a tetrahedral structure, A12CI7- has a C2 structure (A1C1A1bond angle = 123.4°), and EMI+ has a C1 structure. The Raman bands at 181,350 cm-l were assigned to A1CI4- and the Raman bands at 162, 313,429 and 562 cm-! wereassigned to A12C17-.The Raman bands of 241,383, 596 and 700 cm-1 were assignedto EMI +. The neutron results also agree qualitatively with these assignments. Thecalculations on the interaction of a single anion (A1C14-or A12C17-)with a single cation(EMI +) show little effect on the structures; however, some of the vibrationalfrequencies were shifted.AcknowledgmentsThis work was supported by the U. S. Department of Energy, Basic Energy Sciences,under contract No. W-31-109-ENG-38 and by Nisshin Steel Co., Ltd. We would liketo thank the staff of IPNS for their collaboration and assistance.References1. J.S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey, Inorg. Chem., 21,1263 (1982).2. S. Takahashi, I. Saeki, K. Akimoto, J. Surf. Fin. Soc., Japan, 40 (4), 42(1989).3. Z.-J. Karpinski, R. A. Osteryoung, Inorg. Chem., 24, 2259 (1985).4. J.S. Wilkes, J. S. Frye, G. F. Reynolds, Inorg. Chem., 22, 3870 (1983).5. A.A. Fannin, Jr., L. A. King, J. A. Levisky, J. S. Wilkes, J. Phys. Chem.,88, 2609 (1984).6. S. Takahashi, M.-L. Saboungi, R J. Klingler, M. J. Chen, J. W. Rathke,Proceedings of the Eighth International Symposium on Molten Salts, The Electrochem.Soc., Pennington, New Jersey, (1992).7. C.J. Dymek, Jr., J. S. Wilkes, M.-A. Einarsrud, H. A. Oye, Polyhedron, 7(13), 1139 (1988).8. Ab Initio Molecular Orbital Theory, W. J. Hehre, L. Radom, P. V. R. Schleyer,and J. A. Pople, Wiley, New York, (1986).9. M.J.S. Dewar and W. Theil, J. Am. Chem. Soc., 99, 4899 (1977).10. M. Blander, E. Bierwagen, K. G. Calkins, L. A. Curtiss, D. L. Price, and M.-L.Saboungi, J. Chem. Phys. 97, 2733 (1992).11. L.A. Curtiss and R. Nichols, Proceedings of the Fifth International Symposiumon Molten Salts, 1986, The Electrochem. Soc., p. 289.12. L.P. Davis, C. J. Dymek, J. J. P. Stewart, H. P. Clark and W. J. Lauderdale,J. Am. Chern. Soc., 107, 5041 (1985).13. L.A. Curtiss, Proceedings of the Joint International Symposium on Molten Salts,1987, The Electrochem. Soc., p. 185.4!/Table 1. Raman frequencies (cm-1) of the A1C13-EMICsystem46 mol% AICI3 67 mol% AICI3162dp181 dp241 dp 241 dp313p350 p383 p? 387 p?445 p 429 p504 dp562 dp?597 p 593 p680 dp701 dp 695 dp734 dpiiTable 2. Comparison of frequencies (in cm- 1) of isolated ions and theA1CI4- ....EMI+ complex from MNDO calculations, aA1CI4- ...EMl + Isolated AICI4-complex and EMI+New freq. 6.418.420.941.274.493.1A1CI4- 109.9 107.8112.0 107.8499.5 528.2514.8 528.2572.9 528.2EMI+ 135.0 122.5897.0 906.2964.0 945.1964.3 974.11533.0 1518.13380.4 3407.33457.2 3440.53468.9 3452.5a The frequencies correspond to the ions listed in the first column. Only thefrequencies that have the largest changes are listed. (The first six frequenciescorrespond to new modes)."_ T = 298 K._ 67 tool% MCt3_ *_ .L200 300 400 500 600 700(o / cm-1_°Figure 1. Measured Raman spectra of A1CI3 - EMIC. 'Figure 2. The observed neutron-weighted vibrational spectra of 46 mol% AICI3 - 54tool% EMIC and 67 mol% AICI3 - 33 mol% EMIC at 25 K. The upper spectrum isdisplaced by 5 units in the vertical direction.w¢6353 o• ,,m.,a 5 -- I= I,.dI;&" 3- i=_.,IIr'J 188.E 2-_i 121 I1 - I 51170 ..... h _• . ._1 ! ! Io_0 2oo 400.0 600.0 soo.oco./cm"nFigure 3. Ca]culated Raman activity of A]CI4-. tt7310442_5-_ l._ 4-1633t_ 2_ oo i='1loou0.0 200.0 400.0 600.0 800.0-I/ cm..Figure 4. Calculated Raman activity of EMI+.w87- 632oe-45 -,_ 744=: 3-2 - 147 249 4oa°'°TT Ir_-T Tmt_ I _ lm I I t_l _ __00.0 200,0 400.0 600.0 800.0co/cre_,Figure 5 Calculated Raman activity of A12C17-.Figure 6. Structure of EMI cation from HF / 6-31G* calculations7 /

Dialkylimidazolium chloroaluminates: Ab initio calculations, Raman and neutron scattering measurements

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Dialkylimidazolium chloroaluminates: Ab initio calculations, Raman and neutron scattering measurements

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