B3LYP/6-31G calculations

The basis of the enantioselectivity of the BOX catalysts has been probed using B3LYP/6-31G calculations.112 It has been proposed that in the case of the f-butyl... 

FIGURE 3.26. Potential energy profiles (from B3LYP/6-31G calculations) for the cleavage of 4-cyanobenzyl chloride (a) 3- and 4-cyanobenzyl bromides (b and c) anion radicals in the gas phase (top) and in a solvent (middle and bottom) (from COSMO solvation calculations with a dielectric constant of 36.6 and 78.4, respectively). Dotted and solid lines best-fitting Morse and dissociative Morse curves, respectively. Adapted from Figures 4 and 5 of reference 43, with permission from the American Chemical Society. 

The fate of A-alkoxy-Af-aminoamides from Af-methylaniline and Af-acyloxy-Af-alkoxy-amides has been modelled at semiempirical, ab initio and density functional levels of theory " . B3LYP/6-31G calculations predicted the HERON rearrangement of the model intermediate 214 to methyl formate (173) and l,l-dunethyldiazene (220) to be exothermic by 5.5 kcalmoG and have an activation energy of 21.4 kcalmoG in the gas phase (Scheme 32). 

In summary, transition structures with dioxirane and dimethyldioxirane are unsymmet-rical at the MP2/6-31G level, but are symmetrical at the QCISD/6-31G and B3LYP/6-31G levels. The transition states for oxidation of ethylene by carbonyl oxides do not suffer from the same difficulties as those for dioxirane and peroxyforaiic acid. Even at the MP2/6-31G level, they are symmetrical (Figure 17). The barriers at the MP2 and MP4 levels are similar and solvent has relatively little effect. The calculated barriers agree well with experiment . In a similar fashion, the oxidation of ethylene by peroxyformic acid has been studied at the MP2/6-31G, MP4/6-31G, QCISD/6-31G and CCSD(T)/6-31G and B3LYP levels of theory. The MP2/6-31G level of theory calculations lead to an unsymmetrical transition structure for peracid epoxidation that, as noted above, is an artifact of the method. However, QCISD/6-31G and B3LYP/6-31G calculations both result in symmetrical transition structures with essentially equal C—O bonds. 

The most convincing evidence for an essentially synchronous peracid epoxidation of simple alkenes came from a combined experimental and theoretical study by Singleton, Honk and coworkers. Experimental KIEs for the reaction of m-CPBA with 1-pentene, determined by the clever methodology developed by Singleton and Thomas utilizing the combinatorial high-precision determination of C and H KIEs at natural abundance, confirmed the symmetrical or nearly symmetrical nature of this epoxidation TS. These data were corroborated by B3LYP/6-31G calculations on propylene that supported a synchronous transition state for peroxyformic acid epoxidation. 

Cyclic allylic alcohols have different steric requirements than the acyclic substrates discussed above. Sarzi-Amade and coworkers addressed the mechanism of epoxida-tion of 2-cyclohexen-l-ol by locating all the transition structures (TSs) for the reaction of peroxyformic acid (PFA) with both pseudoequatorial and pseudoaxial cyclohexenol con-formers. Geometry optimizations were performed at the B3LYP/6-31G level, and the total energies were refined with single-point B3LYP/6-311- -G //B3LYP/6-31G calculations. 

MP2/6-31G bond distances are comparable to those from B3LYP/ 6-31G calculations, although errors for bonds involving one or more second-row elements are generally somewhat smaller. As stated earlier, it is difRcult to justify use of the more costly MP2/6-31G model (over B3LYP/6-31G ) for geometry determinations. 

Table 15-2 Effect of Choice of Geometry on Activation Energies from B3LYP/6-31G Calculations... 

Use B3LYP/6-31G calculations (as above), and apply a correction of 4 kcal/mol (in favor or the singlet). 

Phenylcyclopropane radical cation (9 ) has divergent hyperfine coupling constants for the secondary cyclopropane protons (aptrans = 0.78 mT upcis = 0.07 mT CIDNP, B3LYP/6-31G calculations), apparently because the cis protons are located in a nodal plane. " Similarly, vinylcyclopropane radical cation is... 

In the ET-catalyzed Diels-AIder reaction of indole with 1,3-cycIohexadiene a stepwise mechanism was derived on the basis of kinetic isotope effects. In light of B3LYP/6-31G calculations, these effects support a stepwise mechanism, initiated by attack of the diene on the 3-position of indole. ... 

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