4724 13C
J. Phys. Chem. 1996, 100, 4724-4728
NMR Relaxation and 1H Diffusion (DOSY) Studies of an Acidic Chloroaluminate Melt W. Robert Carper*,† Department of Chemistry, Wichita State UniVersity, Wichita, Kansas 67260
Gilbert J. Mains Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078
Bernard J. Piersma Department of Chemistry, Houghton College, Houghton, New York 14744
Shawn L. Mansfield and Cynthia K. Larive* Department of Chemistry, UniVersity of Kansas, Lawrence, Kansas 66045 ReceiVed: October 30, 1995; In Final Form: December 26, 1995X
13C
NMR relaxation studies of a 2:1 AlCl3:1-ethyl-3-methylimidazolium chloride (MEICl) melt between 26 and 57 °C provide rotational correlation times for MEI+ which are compared with viscosities and diffusion coefficients. Diffusion coefficients for MEICl in a 2:1 AlCl3-MEICl melt are determined using 1H diffusion ordered spectroscopy (DOSY) between 26 and 57 °C. All MEI+ diffusion coefficients (ring, methyl, and ethyl protons) are virtually identical at a given temperature. The MEI+ diffusion coefficients are viscosity dependent and correlate with conductivity and 13C NMR rotational correlation times, indicating that the transport properties of the 2:1 AlCl3-MEICl 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 2:1 AlCl3-MEICl melt where Al2Cl7- is the predominant anion. The ab initio structures of AlCl4- and Al2Cl7- have been calculated at the HF/6-31G(d) level.
Introduction In a recent report,1 1H diffusion ordered spectroscopy (DOSY)2,3 was used to determine diffusion coefficients as a function of temperature for 1-ethyl-3-methylimidazolium chloride (MEICl) in 0.8:1 and 1:1 AlCl3-MEICl melts where the predominate cation and anion are MEI+ and AlCl4-, respectively. The MEI+ diffusion coefficients for the 0.8:1 and 1:1 AlCl3-MEICl melts are viscosity dependent and correlate with conductivities and 13C correlation times.1 In this study we report MEI+ 13C relaxation rates and 1H diffusion coefficients obtained from 25 to 60 °C for an acidic 2:1 AlCl3-MEICl melt where the predominate ions are MEI+ (Figure 1) and Al2Cl7-. The 13C NMR rotational correlation times obtained for MEI+ in the 2:1 AlCl3-MEICl melt are compared with viscosity, conductivity, and 1H diffusion coefficients. This is one of the few studies in which all of these various physical properties can be used to test various physical models as suggested by Boere and Kidd.4 Diffusion ordered spectroscopy (DOSY) is a pulsed field gradient method5,6 which uses the LED (longitudinal encodedecode) pulse sequence7 and actively shielded gradient coils7-9 to produce high-resolution NMR spectra. In this method, a series of LED NMR spectra are acquired differing only in the amplitude and duration of the magnetic field gradient pulses. Self-diffusion is measured directly from the dependence of resonance intensity on gradient area. In DOSY, the diffusion coefficients are extracted from the LED spectra using data inversion programs for the analysis of discrete or continuous † X
E-mail:
[email protected]. Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4724$12.00/0
Figure 1. MEI cation (positions are labeled).
distributions of diffusion coefficients. The results of the DOSY analysis are displayed in a two-dimensional spectral format with NMR chemical shift along one dimension and diffusion coefficient in the other dimension. Experimental Section Materials. The 1-ethyl-3-methylimidazolium chloride and chloroaluminate molten salts were prepared as previously described.10 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. 13C NMR spectra were recorded on a Varian XL-300 spectrometer at 75.43 MHz. Temperature measurements were calibrated against methanol © 1996 American Chemical Society
Microdynamics of an Acidic Chloroaluminate Salt
J. Phys. Chem., Vol. 100, No. 12, 1996 4725
or ethylene glycol and are accurate to within 0.5 °C. Pulse widths were typically 8-10 µs, and longitudinal relaxation times were measured by the inversion-recovery method (180°-τ90°-T) with T > 10T1. At least 12 delay times (τ) were used and relaxation times (in duplicate) 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
