J. Phys. Chem. B 1998, 102, 1717-1723
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and 27Al NMR Relaxation, Viscosity, and 1H Diffusion Studies of an Ethylaluminum Dichloride Melt Cynthia K. Larive,† Mengfin Lin,† Brian S. Kinnear,‡ Bernard J. Piersma,‡ Charles E. Keller,§ and W. Robert Carper*,⊥ Department of Chemistry, UniVersity of Kansas, Lawrence, Kansas 66045; Department of Chemistry, Houghton College, Houghton, New York 14744; Department of Chemistry, P.O. Box 1901, Mississippi State UniVersity, Mississippi State, Mississippi 39762; and Department of Chemistry, Wichita State UniVersity, Wichita, Kansas 67260 ReceiVed: August 4, 1997; In Final Form: October 16, 1997
13C
and 27Al NMR relaxation studies of a 1:1 1-ethyl-3-methylimidazolium chloride (MEICl)-ethylaluminum dichloride (EtAlCl2) melt provide rotational correlation times for MEI+ and EtAlCl2 which are compared with viscosities and diffusion coefficients. Diffusion coefficients for MEICl and EtAlCl2 in a 1:1 MEIClEtAlCl2 melt are determined simultaneously using 1H diffusion ordered spectroscopy (DOSY). All MEI+ diffusion coefficients (ring, methyl, and ethyl protons) are virtually identical at a given temperature as are the methylene and methyl protons of EtAlCl2. The MEI+ and EtAlCl2 diffusion coefficients are viscosity dependent and correlate with 13C and 27Al NMR rotational correlation times, indicating that the transport properties of the 1:1 MEICl-EtAlCl2 melt are determined by the molar quantities of salt and not by the properties of the individual ions. Microviscosity factors provide reasonable values for the radius of MEI+ from both viscosity and 13C NMR rotational correlation times. The “slip” model is used to describe the movement of MEI+ in the 1:1 MEICl-EtAlCl2 melt.
Introduction Room-temperature chloroaluminate melts provide excellent model systems for NMR relaxation studies.1-4 In addition to the room-temperature melts containing AlCl3, similar molten salts using ethylaluminum dichloride (EtAlCl2) instead of AlCl3 have been reported and investigated by NMR.5-7 The structure of ethylaluminum dichloride is that of a trans dimer with a mp of ≈32 °C.8-11 The addition of solid MEICl to EtAlCl2 produces a viscous melt whose mp is ≈-70 °C. The primary species in the 1:1 MEICl-EtAlCl2 melt has been identified by 27Al NMR as the EtAlCl dimer which has an 27Al NMR peak 2 at 129 ppm relative to Al(H2O)63+ .6,7 The combined use of NMR 1H diffusion ordered spectroscopy (DOSY) and NMR relaxation methods provides useful information about the dynamics and structure of various chemical systems and chloroaluminate systems in particular.3,4,12 This approach has been used to determine diffusion coefficients as a function of temperature for 1-ethyl-3-methylimidazolium chloride (MEICl) in 0.5, 0.8:1, and 1:1 AlCl3-MEICl melts where the predominate cation and anions include MEI+ (Figure 1) in all melts, AlCl4- in the 0.8:1 and 1:1 melts, and Al2Cl7in the 0.8:1 and 0.5:1 melts. The MEI+ diffusion coefficients for these AlCl3-MEICl melts are viscosity dependent and correlate with conductivities and 13C correlation times.3,4 In this study we report 27Al and 13C relaxation rates and 1H diffusion coefficients obtained from 22 to 62 °C for a 1:1 EtAlCl2-MEICl melt. The 27Al and 13C NMR rotational * To whom correspondence
[email protected]. † University of Kansas. ‡ Houghton College. § Mississippi State University. ⊥ Witchita State University.
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Figure 1. MEI cation (positions are labeled).
correlation times obtained for EtAlCl2 and MEI+ in the 1:1 AlCl3-MEICl melt are compared with viscosity and 1H diffusion coefficients. In this study diffusion coefficients of more than one species are determined simultaneously and then used with NMR rotational correlation times and viscosity data to test various physical models as suggested by Boere and Kidd.13 Experimental Section Materials. The 1-ethyl-3-methylimidazolium chloride and chloroaluminate molten salts were prepared as previously described.14 All materials were stored under anhydrous helium gas atmosphere in a drybox. All molten salt preparations and manipulations were performed in the drybox. Samples were loaded into 5 mm sample tubes, capped in the drybox, removed, and sealed immediately with a torch. NMR Relaxation Measurements. 27Al and 13C NMR spectra were recorded on a Varian XL-300 spectrometer (78.15 and 75.43 MHz). Temperature measurements were calibrated against methanol or ethylene glycol and are accurate to within 0.5 °C. Pulse widths were typically 7-10 µs, and longitudinal relaxation times were measured by the inversion-recovery method (180°-τ-90°-T) with T > 10T1. At least 12 delay times (τ) were used, and relaxation times (in duplicate) were
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obtained from a three-parameter exponential fit of magnetization as a function of τ. We failed to observe nonexponential behavior in any of the relaxation measurements. All R1 values reported herein (estimated error
