J. Phys. Chem. 1994,98, 6865-6869
6865
13C NMR Relaxation and Dual Spin Probe Studies of Molten Salts Containing Ethylaluminum Dichloride Charles E. Kellert and W. Robert Carper’ Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, and The Frank J. Seller Laboratory, United States Air Force Academy, Colorado Springs, Colorado 80840-6528 Received: February 2, 1994’
The microdynamics of binary and ternary room-temperature molten salts containing ethylaluminum dichloride (EtAlClZ) are examined by l3C N M R relaxation methods as a function of melt composition and temperature. The EtAlClz melts include 1-ethyl-3-methylimidazolium chloride (MEICl) and AlC13. Dual spin probe (DSP) results indicate that interactions exist between the ME1 cation and both AlCb- and the EtAlClz dimer. Liquidstate z7Alquadrupolar coupling constants of complexes between MEI+ and either AlC14- (1.1 1-1 -20MHz) or the EtAlClz dimer ( 3 1 4 3 MHz) are obtained from DSP analyses.
Introduction Room-temperature chloroaluminate melts contain charged speciesidentifiable by variousspectroscopic techniquesincluding NMR.I-I1 Recently, new room-temperature molten salts containing ethylaluminum dichloride instead of AlC13 have been reported.l2-l5 These melts are liquid over a wide range of temperatures and melt compositions. The species found in one of these melts include the EtAlC12 dimer and EtAlCl3- which have 27Al NMR peaks at 129 and 102 ppm relative to that of Al( H ~ 0 ) 6 ~14*15 +. The use of NMR relaxation methods provides useful information about the dynamics and structure of various chemical systems and chloroaluminatesystems in particular. In a previous study? 13C NMR relaxation measurements were used to investigate the motion and interactions of the ME1 cation (Figure 1). The results indicate that AlCld- in a Na+o.zzMEI+o.7eAlC14melt interacts with the C-2, C-4, and C-5 hydrogens on the MEI+ ring. This investigation9 preceded studieslOJ1 in which the dual spin probe (DSP) method16 supported the existence of MEI(AlC~)n(*l~ complexes in neutral (AlC13 = MEICl) and NaC1-buffered (to neutrality) .melts. 27Al,23Na, and l3C NMR relaxation results confirmed the presence of the chloroaluminate-MEI+ complexes and yieIded z7Al and z3Na liquid-state quadrupole coupling constants.10J1 In the present study, we use both 13C dipolar relaxation rates and the DSP method16 (1) to establish that interactions are occurring in binary (MEICl and EtAlC12) and ternary (MEICl, EtAlC12, and AlCl3) melts and (2) to identify which species are interacting in these melts. The 13C dipolar relaxation rates of MEI+ indicate the relative mobilities of the MEI+ ring and its methyl and ethyl groups. These results establish that possible interactions between MEI+ and either EtAlCl3- or the EtAlCl2 dimer in the binary melt. The DSP method is then used to determine that there is interaction only between MEI+ and the EtAlClz dimer and not between MEI+ and EtAlC13-. The methyl and ethyl groupsof MEI+ are separated into the fragments NCH3 (methyl), NCH2 (ethyl), and CH3 (ethyl) for the purposes of discussion. Experimental Section Materials. The 1-ethyl-3-methylimidazolium chloride, AlCl3, and EtAlCl, were prepared as previously described.9J3s14 All
* Address correspondenceto this author at the Department of Chemistry, Wichita State University, Wichita, KS 67260-0051. t Wichita State University. *Abstract published in Aduance ACS Abstracts, June 1, 1994. 0022-365419412098-6865$04.50/0
Figure 1. ME1 cation (positions are labeled).
materials were stored under an anhydroushelium gas atmosphere in a drybox. All molten salt preparations and manipulations were performed in a drybox. Samples were loaded into 5-mm sample tubes, capped, and sealed with parafilm. They were then removed from the drybox and sealed immediately with a torch. N M R Measurements. I3C NMR spectra were recorded on a Varian XL-300 spectrometer at 75.43 MHz. Temperature measurements were calibratedagainst methanol or ethylcneglycol and are accurate to within 0.5 OC. Pulse widths were typically 8-10 ps, and longitudinalrelaxation times were measured by the inversion-recovery method (l8Oo-~-9O0-T)with T > 1OT1. For all melt samples, at least 12 delay times ( 7 ) were used and relaxation times (in duplicate) obtained from a three-parameter exponential fit of magnetization as a function of 7 . We failed to observe nonexponential behavior for any of these melts. All R1 values reported herein (estimated error
