J. Phys. Chem. 1995,99, 12998-13001
12998
NMR Relaxation Studies of Lithium- Aluminum Interaction in 1:2 LiC1-Ethylaluminum Dichloride Melts Charles E. Keller,' Bernard J. Persma? and W. Robert Carper*9* Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, and Department of Chemistry, Houghton College, Houghton, New York 14744 Received: January 19, 1995; In Final Form: June 12, 1995@
7Li NMR relaxation studies provide evidence for the existence of scalar coupling between aluminum and lithium in 1:2 LiC1-ethylaluminum dichloride room temperature melts (mp % -95 "C). Spin-lattice and spin-spin (7Li) relaxation times (TI and T2) are determined from -15 to 80 "C. At 5 "C and below, scalar relaxation of the second kind dominates the spin-spin relaxation mechanism, providing evidence that lithium is coupled to aluminum. The contribution of scalar coupling to spin-spin relaxation rates (R2) at higher temperatures is approximated by an Arrhenius function. The scalar R2 values are used to calculate 7Li quadrupolar spin-spin relaxation rates which are then used with R I values to determine correlation times and 7Li nuclear quadrupole coupling constants.
Introduction Ethylaluminum dichloride (EtAlC12) exists as a C2h dimer and has a mp of ~ 3 0C.1-4 2 NaCl is virtually insoluble in EtAlC12 , whereas LiCl is soluble up to a mole ratio of 1:2 LiClEtAlCl2 (mp % -95 0C).5 The existence of a 1:2 complex in solution is supported by I3C and 27AlNMR relaxation ~ t u d i e s ~ . ~ Figure 1. MOPAC(PM3)-optimized structure of the 1:2 LiCl-EtAIC12 complex. and by a PM3-optimized structure (MOPAC)8 using the most recent parameters for l i t h i ~ m .The ~ theoretical structure of the 1:2 LiCl-EtAlC12 contains LiCl in its center with two ethylmeasured by the inversion-recovery method ( 18Oo-r-90"aluminum dichloride molecules located on opposite sides (Figure Td) with Td > lOTl. For all melt samples, at least 12 delay 11.7 times (t)were used and relaxation times (in duplicate) obtained from a three-parameter exponential fit of magnetization as a In this study, the increasing contribution of scalar relaxation function o f t . We observed single exponential behavior for all of the second kind6,I0.l1provides evidence that lithiummeasurements. T2's for 7Li were determined from half-height aluminum coupling exists at subambient temperatures. The peak widths (=l/nT2*, where T2* M T2). liquid state nuclear quadrupole coupling constant 01) of 7Li in the 1:2 LiCl-EtAlC12 complex is determined from +80 to -15 "C using an equation developed for viscous solution^.^^'^ This Theoretical Section method7.I2makes use of the fact that highly viscous solutions Relaxation Mechanisms. The relaxation mechanisms that seldom meet the condition of "extreme narrowing", as ut, > may contribute to the spin-lattice (longitudinal, RI)and spin1. Consequently, quadrupolar spin-lattice and spin-spin spin (transverse, R2) 7Li relaxation rates (R) include quadrupole relaxation times are not equal and their ratio is a function of (RQ),dipole-dipole (Rdd), spin-rotation (RSr), and scalar the correlation time, 7,. This allows the detennination of the relaxation (RSC): 10.13.14 correlation times (tc)which are then used to calculate liquid state nuclear quadrupole coupling constants for the nuclei R = 1/T = RQ Rdd Rsr Rsc in (1)
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Experimental Section Materials. Ethylaluminum dichloride (mp 32 "C) was purified by as described previ~usly.~ All manipulations were performed in a helium atmosphere drybox where 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. NMR Measurements. 7Li NMR spectra were recorded on a Varian XL-300 spectrometer at 116.61 MHz. Temperature measurements were calibrated against methanol or ethylene glycol and are accurate to within 0.5 "C. Pulse widths were typically 5- 10 ,us, and longitudinal relaxation times were
Spin-rotation relaxation can be eliminated in this and most other cases due to its positive temperature dependence. Scalar relaxation is usually not observed; however, this case is an interesting exception and will be discussed later. Quadrupolar Relaxation. The first relaxation mechanism and generally the most important for 7Li is that of quadrupolar relaxation. With nonviscous solutions the "extreme narrowing condition" is met (ut
