Catalysis of Organic..

272 Acidic Ionic LiquidsThe effect upon the 27 Al-NMR spectra for changing the Al/RMIM + ratio andadding HCl to the system was attributed to either one (7) or more (8) equilibriainvolving anionic aluminum species.O + [Al 2 Cl 7 ] - + O + Cl - 2O + [AlCl 4 ] - andO + [Al 2 Cl 7 ] - + HCl O + [AlCl 4 ] - + H + [AlCl 4 ] -We (9) reported predictions, after the manner of Chandler and Johnson (10), toshow the effect of HCl upon the Al-speciation in the IL and these results suggestedthat two structures could exist for the anion having the following stoichiometry:[Al 2 Cl 8 H] - . Subsequent molecular modeling suggested that the HCl molecule couldreside in three different environments and these sites showed different free energiesof formation (11). The decreasing reactivity of these IL’s as the structure of thecation was changed to show increasing chain length of R [RMIM] + , R = C n H 2n+1 ; n =2, 4, 6, 8, and 12, could not be related to the amount of HCl adsorbed in the IL’s. Itbecame apparent that the molecular environment of these IL’s was more complicatedthan what had been reported in the literature.The aim of this manuscript is to examine the molecular structure of the IL as theratio of Al/RMIM + was changed with and without HCl added. The workinghypothesis is that the aluminum speciation determines the reactivity of the IL andthis speciation depends upon the structure of RMIM + .Results and DiscussionAl-NMR data of the [n-BuMIM] + /[Al n Cl 3n+1 ] - IL’sfor n = 1, 3/2, and 2 areshown in Fig. 1 withoutany HCl added to the IL’s.The observed resonancesdownfield from Al(NO 3 ) 3(aq) are shown in Table 1.n = 1n = 3/2These observations ofpeak shape are similar tothose reported earlier (7,n = 28); however, neither of these earlier studies used aninternal standard, nor did they employ quantummechanical methods to assist in the peakassignment. The association of the organic halidewith the AlCl 3 was modeled by transition statemethods to give the species shown in Figure 2. Theproducts of this transition state were two dinuclearAl-species showing different structures but similarfree energies of formation. Figure 2-ashows the species before the transitionstate optimization and Figure 2-b is theFigure 1. 27 Al-NMR of IL’s with R = butyland n = 1; 3/2; and 2 mol/mol (clockwise)