Reactions B3LYP

TABLE 19. Calculated B3LYP reaction energies (kcalmol 1) of reactions of H2II4 and EH2 ... 

Compare your results to the experimental values of -34.0 2 kcal mol for the lithium reaction and -3.6 .5 kcal mol" for the water dimer reaction. Use the same model chemistry as in Example 8.2 B3LYP/6-311+G(2df,2p) // B3LYP/6-31G(d). 

Compute AH for each reaction, using the B3LYP/6-31G(d) model chemistry for structures and zero-point energies and the B3LYP/6-311+G(3df,2p) model chemistry for the final energy calculations. 

Sju could expand the discussion of the isodesmic reactions by considering additional similar reactions (e.g., X= NH2, SiH, PHj, CN, SH, CF3). You could also run the computations at the MP4 level to discuss the relative merits of the MP2, MP3 and MP4 levels of theory and to compare the B3LYP results to them. 

Substituted pyrazolin-5-ones have only three and -substituted pyrazolin-3-ones only two tautomers, since now the corresponding 19c and 19d structures are isomers. The calculations involved l-methylpyrazolin-5-one (PM3/6-3H-G, anions and cations), l-phenyl-3-methyl-2-pyrazolin-5-one (DFT, radical reactions) [97JPC(A)3769], and l-(2, 4 -dinitrophenyl)-3-methyl-2-pyrazolin-5-one [B3LYP/6-31G and the crystal structure (Section V,D,2)] (98NJC1421). 

Further studies by Garcia, Mayoral et al. [10b] also included DFT calculations for the BF3-catalyzed reaction of acrolein with butadiene and it was found that the B3LYP transition state also gave the [4+2] cycloadduct, as happens for the MP2 calculations. The calculated activation energy for lowest transition-state energy was between 7.3 and 11.2 kcal mol depending on the basis set used. These values compare well with the activation enthalpies experimentally determined for the reaction of butadiene with methyl acrylate catalyzed by AIGI3 [4 a, 10]. 

As the formation of betaines from amide-stabilized ylides is known to be reversible (in contrast with aryl- or semistabilized ylides, which can exhibit irreversible anti betaine formation see Section 1.2.1.3), the enantiodifferentiating step cannot be the C-C bond-forming step. B3LYP calculations of the individual steps along the reaction pathway have shown that in this instance ring-closure has the highest barrier and is most likely to be the enantiodifferentiating step of the reaction (Scheme 1.16) [25]. 

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

They reported that the DFT calculations of 114 at the B3LYP/6-31G level showed that the ji-HOMO lobes at the a-position are slightly greater for the syn-n-face than for the anti face. The deformation is well consistent with the prediction by the orbital mixing rule. However, the situation becomes the reverse for the Jt-LUMO lobes, which are slightly greater at the anti than the syn-n-face. They concluded that the iyn-Jt-facial selectivity of the normal-electron-demand Diels-Alder reactions... 

Figure 15. Calculated potential energy surface and geometries of intermediates of the V" + CO2 reaction. The energy of the lowest energy state for the quintet (solid hnes) and triplet (dotted lines) stationary points are shown. Energies are calculated at the CCSD(T)/6-311+G(3df) level, at the B3LYP/6-311+G(d) geometry and include zero-point energy at the B3LYP/6-311+G(d) level.

The TS proposed for these proline-catalyzed reactions is very similar to that for the proline-catalyzed aldol addition (see p. 132). In the case of imines, however, the aldehyde substituent is directed toward the enamine double bond because of the dominant steric effect of the (V-aryl substituent. This leads to formation of syn isomers, whereas the aldol reaction leads to anti isomers. This is the TS found to be the most stable by B3LYP/6-31G computations.199 The proton transfer is essentially complete at the TS. As with the aldol addition TS, the enamine is oriented anti to the proline carboxy group in the most stable TS. 

Fig. 2.6. Free-energy profile (B3LYP/6-31 + G with ZPE correction) for intermediates and transition structures for Wadsworth-Emmons reactions between the lithium enolate of trimethyl phosphonoacetate anion and formaldehyde in the gas phase and in tetrahydrofuran or ethanol. Adapted from J. Org. Chem., 63, 1280 (1998), by permission of the American Chemical Society.

Figure 2.P25 shows the calculated [B3LYP/6-31G(4,/f)] reaction energy profile for the aldol addition of benzaldehyde and cyclohexanone catalyzed by alanine. The best TSs leading to (S,R) (R,S) (S,S) and (R,R) products are given. What factors favor the observed (R,S) product ... 

According to B3LYP/6-31G computations of the intermediates and TSs, there are no large barriers to the reaction and it is strongly exothermic.156 Measured Ea values are around 10kcal/mol.157 The complexation of borane to the catalyst shifts electron density from nitrogen to boron and enhances the nucleophilicity of the hydride. The... 

In one study, the mechanisms of the reaction of methyl cinnamate and cyclopen-tadiene with BF3, A1C13, and catecholborane bromide as catalysts were compared.29 According to these computations (B3LYP/6-31G ), the uncatalyzed and BF3- and AlCl3-catalyzed reactions proceed by asynchronous concerted mechanisms, but a... 

Fig. 8.2. Computational energy profile (B3LYP/631 A) for reaction of (CH3)2CuLi-LiCl with CH3Br including one solvent (CH3OCH3) molecule. Adapted from./. Am. Chem. Soc., 122, 7294 (2000), by permission of the American Chemical Society.

The experimental isotope effects have been measured for the reaction of 2-methylbutene with formaldehyde with diethylaluminum chloride as the catalyst,27 and are consistent with a stepwise mechanism or a concerted mechanism with a large degree of bond formation at the TS. B3LYP/6-31G computations using H+ as the Lewis acid favored a stepwise mechanism. 

There have been a number of computational studies of the epoxidation reaction. These studies have generally found that the hydrogen-bonded peroxy acid is approximately perpendicular to the axis of the double bond, giving a spiro structure.75 Figure 12.8 shows TS structures and Ea values based on B3LYP/6-31G computations. The Ea trend is as expected for an electrophilic process OCH3 < CH3 CH = CH2 < H < CN. Similar trends were found in MP4/6-31G and QCISD/6-31G computations. 

Kinetics and isotope effects are consistent with this mechanism.92 The reagent is electrophilic in character and reaction is facilitated by ERG substituents in the alkene. A B3LYP/6-31G computation found the transition structures and Ea values shown in... 

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