2990
Inorg. Chem. 1995, 34, 2990-2993
Molecular Orbital Calculations and Raman Measurements for 1-Ethyl-3-methylimidazolium Chloroaluminates S. Takahashi,?L. A. Curtiss, D. Gosztola, N. Koura? and Marie-Louise Saboungi" Argonne National Laboratory, Argonne, Illinois 60439-48 14 Received August 4, 1994@
Raman spectroscopic data are presented for the room-temperature molten salts (AlC13)x(1-ethyl-3-methylimidolium chloride)l-,, for x ranging from 0.46 to 0.67. Ab initio molecular orbital theory has been used to compute the structure and vibrational frequencies of some of the anionic and cationic species present in these melts; the results are compared with semiempirical calculations and used to interpret the experiments.
Introduction Mixtures of aluminum chloride with either alkali chlorides (MC1-AlCl3) or organic halides have fascinating chemistry and are used in a variety of applications, such as electrolytes in highenergy, high-density batteries;'-3 nonaqueous solvents for studying ionic component^;^.^ catalytic solvents for FriedelCrafts reaction@* and industrial electrodeposition processes, Le., deposition of A1 on high-quality steel surface^.^ These mixtures are referred to as chloroaluminates. The MCl-AlC13 mixtures have been thoroughly investigated by a variety of techniques, including electrochemical and therm~dynamic,'~-'~ infrared and Raman,I5-l7 X-ray,I8 and neutron ~cattering.'~ In basic solutions for 0.33 -= XAQ < 0.5, the dominant anionic species is AlCld-. In acidic solutions, + Permanent address: Nisshin Steel Co., Ltd., 7-1 Koya-shinmachi, Ichikawa 272, Japan. Permanent address: Tokyo University of Science, Tokyo, Japan. Abstract oublished in Advance ACS Abstracts. Mav 15. 1995. Dymek, 'c. J., Jr.; Williams, J. L.; Groeger, 'D. J:; Aubom, J. J. J. Electrochem. SOC. 1984, 131, 2887. Reynolds. G. F.; Dymek, C. J., Jr. J . Power Sources 1985, 15, 109. Carlin, R. T.; Fuller, J.; Wilkes, J. S. Air Force Report FJSRL-TR90-0003: Frank J. Seiler Research Laboratory, USAF Academy: Colorado Springs, CO, 1990. Laher, T. M.; Hussey, C. L. Inorg. Chem. 1982, 21, 4019. Barnard, P. A.; Hussey, C. L. J. Electrochem. SOC. 1990, 137, 913. Uribe, F. A.; Osteryoung, R. A. J . Electrochem. SOC. 1988, 135, 378. Janiszewska. L.; Osteryoung, R. A. J . Electrochem. Soc. 1988, 135, 116. Boon, J. A.; Levisky, J. A,; Pflug, J. L.; Wilkes, J. S. J . Urg. Chem. 1986, 51, 480. Carlin, R. T.; Wilkes, J. S. J. Mol. Catal. 1990, 63, 125. Takahashi, S.; Saeki, I.; Akimoto, K. J . Surf. Finish. Soc. Jpn. 1989, 40 (4), 42. Torsi, G.; Mamantov, G. Inorg. Chem. 1972, 11, 1439. Fannin, A. A., Jr.: King, L. A,; Seegmiller, D. W. J . Electrochem. SOC.1972. 119. 801. Boxall, L. G.; Jones, H. L.; Osteryoung, R. A. J . Electrochem. SOC. 1973, 120. 223. 0ve. H. A.; Gruen. D. M. Inora. Chem. 1964, 3, 836. V h a , J . T.; King, L. A.; Fannin, A. A,; Seegmiller, D. W. J . Chem. Eng. Data 1978, 23, 122. CYvin, s. J.: Klaeboe, P.; Rytter, E.; @ye,H. A. J . Chem. Phys. 1970, 52. 2776. Torsi, G.: Mamantov, G.; Begun. G. M. Inorg. Nucl. Chem. Lett. 1970, @
6. 553.
(a) Hvistendahl, J.; Klaeboe, P.; Rytter, E.; (aye, H. Inorg. Chem. 1984, 23, 706. (b) Rytter, E.; Dye, H. A,; Cyvin, S. J.; Cyvin, B. N.; Klaeboe, P. J. Inorg. Nucl. Chem. 1973. 35, 1185. Takahashi, S.; Maruoka, K.; Koura, N.; Ohno, H. J . Chem. Phys. 1986. 84, 408. Rytter, E.; Rytter, B. E. D.;0ye, H. A,; Krogh-Moe. J. Acta Cnstallogr. 1973. 829, 1541.
0020- 166919511334-2990$09.00/0
XAICI? > 0.5, more species are reported such as Al2C17-, Al3Cl,o-, and A12C16. It has been shown that AlC14- has a nearly tetrahedral structure, and Al2Cl7- and A12C4 are composed of two AlCb- tetrahedra sharing one and two comers, respectively. In the case of Al2C17-, infrared studiesI7 have indicated that Li+ stabilizes a bent Al-C1-A1 bridge while the larger alkali cations stabilize a linear (or slightly bent) bridge. In crystalline KAlzBr7, a bent bridge has been reported in a careful crystallographic investigation;20 recent neutron scattering results on the structure of liquid KAl2Br7 lend support to the persistence of the bent bridge even in the liquid phase.19 A theoretical study2' using the semiempirical MNDO (modified neglect of differential overlap) method has indicated that, for Al*C17-, the A1-Cl-A1 bridge is bent with an angle of 125". This has also been confirmed by ab initio molecular orbital calculation^^^^^^ using the 3-21G basis. Recently, the structure of Al$2llo- was calculated using the MNDO method and found to consist of three ions sharing three comer^.^^,*^ Mixtures of AlCl3 and 1-ethyl-3-methylimidaoliumchloride (EMIC) form chloroaluminate salts that are liquid well below room t e m p e r a t ~ r e . The ~ ~ chemistry of these systems is quite similar to that of the mixtures with the alkali halides; the stability and relative abundance of the ionic species strongly depend on the acidity or basicity of the melts as shown by numerous experiments including NMR and IR.25-28 In this paper, we present results of Raman scattering experiments on selected melts as well as ab initio and semiempirical molecular orbital calculations of the structure and the vibrational modes associated with AlCb-, A12C17-, and EMI+. As will be discussed, the theoretical calculations are used to assign the absorption bands in the Raman spectra and provide information on the structure of the isolated ions.
Blander, M.; Bierwagen, E.; Calkins, K. G.; Curtiss, L. A,; Price, D. L.; Saboungi, M.-L. J. Chem. Phys. 1992, 97, 2733. Davis. L. P.; Dymek, C. J.; Stewart, J. J.; Clark, H. P.; Lauderdale, W. J. J . Am. Chem. SOC. 1985, 107, 5041. Curtiss, L. A. Proceedings of the Fifth International Symposium on Molten Salts; Electrochemical Society: Pennington, NJ, 1986; p 185. Dymek, C. J., Jr.; Wilkes, J. S.; Einarsrud, M.-A,; 0ye, H. A. Polyhedron 1988, 7, 1 1 39. Wilkes. J. S.; Levisky, J. A,; Wilson. R. A,; Hussey. C. L. Znorg. Chem. 1982, 2 1 . 1263. Fannin, A. A., Jr.: King, L. A,; Levisky, J. A,; Wilkes, J. S. J . Phjs. Chem. 1984,88. 2609. Takahashi, S.; Saboungi, M. L.; Klingler, R. J.; Chen, M. J.; Rathke, J. W. J . Chem. SOC., Faraday Trans. 1993. 19, 3591. Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4352. Dieter, K. M.; Dymek, C. J., Jr.: Heimer, N. D.; Rovang, J. W.; Wilkes. J. S. J . Am. Chem. Sur. 1988, 110. 2722.
0 1995 American Chemical Society
1-Ethyl-3-methylimidazolium Chloroaluminates
Inorganic Chemistry, Vol. 34, No. 11, 1995 2991 Table 1. Raman Frequencies (cm-I) of the AlClj-EMIC System
I
67% ~ 1 ~ 1 3
u,* 150
350 450 550 Raman shift (em-')
250
650
46 mol % AlC1354 mol % EMIC
50 mol %
60 mol %
AlCl350 mol % EMIC
AlC1340 mol % EMIC
181 dp 241 dp
180 dp
350 p 383 p? 445 P 504 dp
347 p 383 dp? 440 P 506 dp 566 p?
597 p
602 p 680 dp 733 dp
46% ~ 1 ~ 1 3
750
Figure 1. Raman spectra for four different AlC13-EM1 melts at 25 "C.
Results A. Raman Spectroscopy. The Raman spectra obtained at 25 "C in the range 150-750 cm-' are shown in Figure 1, and the band positions are listed in Table 1. The number of bands increases with increasing AlC13 content. Two bands (180 and 347 cm-I) show a definite decrease in intensity with increasing content of A1C13, while three bands (157, 310, and 432 cm-I) show a definite increase in intensity with increasing content of AlC13. The remaining bands (241, 383, 504, 566, 597, 680, and 734 cm-I) were present for nearly every melt but did not show any systematic variations with AlCl3 content. (29) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley: New York, 1986. (30) Frisch, M. J.; Head-Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M. A,; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whiteside, R. A,; Seeger,R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian 90; Gaussian, Inc.: Pittsburgh, PA, 1990. (31) Pople, J. A.; Schelgel,H. B.; Krishnan, R.; Defrees, D.J.; Binkley, J. S.; Frisch, M. J.; Whitesides, R. A.; Hout, R. F.; Hehre, W. J. Int. J . Quantum Chem. 1981, S15, 269. (32) Curtiss, L. A.; Nichols, R. Proceedings of the Fifrh International Symposium on Molten Salts; Electrochemical Society: Pennington, NJ, 1986; p 289. (33) Dewar, M. J. S.; Thiel, W. J . Am. Chem. SOC. 1977, 99, 4899.
157 dp 241 dp 310 p 383 p? 432 p 504 dp 566 dp? 597 p 680 dp 734 dp
Table 2. Frequencies (cm-') of AlC14- (Td Structure) and A12C17(CZStructure)"
species
symm
MNDO
HF/6-3 lG*
AlCl4-
E TZ Ai Tz
107.8 169.1 361.9 528.2 10.5 13.7 41.2 77.7 78.4 88.0 108.2 135.6 145.2 148.7 162.8 173.5 184.5 316.5 350.4 403.1 448.4 557.0 560.7 573.5 574.3
121.1 187.6 353.1 511.4 15.5 17.3 39.9 90.4 90.6 98.0 125.0 151.3 162.9 168.2 182.9 196.2 206.8 310.5 332.7 393.2 441.6 553.4 557.7 572.4 572.7
Methods A. Raman Spectroscopy. The Raman spectra were measured using the 457.9 and 476.5 nm lines from a Laser Ionics 1400-15 ion laser. The
