Catalysis of Organic..

Angueira and White 273optimizedgeometry at thetransition state.Product 1(Figure 2-c)was formed bycutting two Alμ−Clbondsindicated byvertical lines;whereas,product 2(Figure 2-d)was formed bycutting one Alμ−Clbond asshown by thehorizontal line.The predicted27 Al-NMRchemical shiftsfrom Al(NO 3 ) 3(aq) are alsoshown in thesame figures.acFigure 2: a-reactants, b-optimized transition state geometry,c-product 1, d-product 2dbThe free energies for forming products 1 and 2 were 99.68 and 99.69 kcal/mol,which are indistinguishable, considering that the uncertainty in these predictions is 4-5 kcal/mol (12). The predicted 27 Al-NMR chemical shifts downfield from Al(NO 3 ) 3(aq) were 69.8, 69.7 ppm for product 1 and 62.7, 60.0 ppm for product 2. While theabsolute values of the predicted chemical shifts are ~30 ppm smaller than theobserved chemical shifts, these calculations permit the assignment shown in Table 1.The success of this modeling can be ascertained by the ability to replicate theobserved peak shapes using Gaussian peaks centered at peak positions suggested bythe modeling (Fig. 3). For the case where n = 3/2, five peaks were needed: 1 for themonomeric Al and 2 peaks each for each of the two dinuclear Al species. Thesepeaks were combined to successfully replicate the observed NMR peaks recorded forthis sample. When n = 2, the data were reproduced using only 4 peaks, two each foreach of the two dinuclear Al species. Our earlier predictions (11) showed that HClcould combine with either of the dinuclear Al-species in three different positions,which showed different acid strengths.We attempted to model the 1 H-NMR of HCl in these three positions to showchemical shifts of 15, 14, and 2-3 ppm (Fig. 4). These predictions suggest that three