Gas phase B3LYP calculation

Fig. (20). Bond lengths in P-carotene and canthaxanthin and their ion radicals and di-ions, from gas-phase B3LYP calculations Squares, neutral circles, ion radical triangles, di-ion. Reprinted with permission from [94]. Copyright 2001 American Chemical Society.

Andersson and coworkers investigated the role of solvation, dispersion, and steric effects on the enantioselectivity. The authors results agree with the fact that gas-phase B3LYP calculations describe the drop in enantioselectivity of 2,3,4,5,6-pentafluoroacetophenone compared to acetophenone where steric and electrostatic effects are the only major effects, but the approximation is too crude to reproduce quantitatively the extent of enantioselection observed experimentally. In order to extract the isolated contributions of steric effects, the authors correlated the same energetic parameter with respect to an empirical steric parameter called STERIMOL B1 [132]. Their results showed that bulkier alkyl groups tend to decrease the enantioselectivity, which correlates well with the B1 parameter. Based on this observation, it seems that an intrinsic steric factor enhances or depresses the enantioselectivity in the reduction of acetophenone and other n-alkyl aryl ketones. However, a generalized rationalization of this effect is not trivial, and investigation of similar catalysts reveals only a small role for steric effects [105, 109]. 

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