Theoretical and Experimental Study of the ¹³C Chemical Shift Tensors of Acetone Complexed with Brønsted and Lewis Acids

Abstract

We calculated the chemical shift tensors for acetone and 12 complexes of acetone with Brønsted and Lewis acids. Each complex was optimized at the MP2/6-311+G* or B3LYP/DZVP2 level of theory. Chemical shift tensors were calculated with the gauge including atomic orbital (GIAO) approach at the RHF and MP2 levels of theory. We discovered a strong correlation between the MP2 and RHF 13C isotropic shifts of the carbonyl carbon in the complexes studied. Linear regression indicates that the MP2 isotropic shifts can be predicted by δMP2 = [1.12 (±0.04)]δRHF − 42.1 (±10.5) ppm (R2 = 0.9974). We were thus able to study two systems that are currently intractable at the GIAO-MP2 level. One is the complex of acetone with a large model of a zeolite, (H3SiO)3SiOHAl(OSiH3)3. The RHF shift of the carbonyl carbon of acetone on the zeolite model (238.4 ppm) is in poor agreement with the experimental (223 ppm) value. However, the MP2 result predicted by the linear correlation (224.0 ppm) is in much closer agreement. We were also able to study acetone adsorbed on aluminum chloride powder models. We find that acetone·AlCl3 is a poor adsorption model, as demonstrated by a large discrepancy between experimental and calculated MP2 shifts. However, the MP2 shift from the regression equation (244.7 ppm) for acetone complexed to the larger Al2Cl6 cluster is in excellent agreement with the experimental result (245 ppm). We also report experimental measurements of the principal components of the carbonyl 13C shift tensor for a variety of solid acids, including frozen oleum and frozen SbF5.