Transition states B3LYP calculations

DFT calculation was performed using Gaussian 03 package [11]. The geometry optimizations were carried out using the B3LYP hybrid functional and the triple-f basis set. H atoms were kept frozen through all calculations. Transition states were localised... 

This protonated cyclopropane is found at B3LYP/6-31G to be 15.99 kcal/mol above the 1,5-p-H-bridged 23, the most stable structure of the 2-octyl cation. This transition-state is thus significantly smaller than the calculated transition-states that we have obtained for the dehydrocyclization mechanisms (29.60 kcal/mol for structure 24), and therefore in at least qualitative agreement with the observation of Davis (8) that an equilibration of n-octane with at least some other isooctanes is set up prior to significant dehydrocyclization of the feedstock. 

Semiempirical AMI and DFT (B3LYP/6-31G ) calculations were used to investigate the highly diastereoselective 1,3-dipolar cycloaddition of 1,4-dihydropyridine- (g) containing azomethine ylides to [60]fullerene (Prato s reaction). The activation energy for the four calculated transition state structures favours the formation of SSaS and... 

A reaction model for the S-oxidation of 3-amino-substituted isothiazoles with (-f)- or (—)-((8,8-dichlorocamphor-yl)sulfonyl)oxaziridines (see Section 4.05.55, compound 70) was obtained by B3LYP/6-31G(d,p),S(3df) calculations. Transition states relative energies are in concordance with the experimental reaction outcome and the analysis of the geometrical parameters allows a description of the reasons governing the observed enantioselectivity <2006UP1>. 

Density functional transition states and activation energies have their problems. Merrill et al. found that for the fluoride ion-induced elimination of HE fromCH3CH2F none of the 11 functionals tested (including B3LYP) was satisfactory, by comparison with high-level ab initio calculations. Transition states were often looser and stabler than predicted by ab initio, and in several cases a transition state could not even be found. They concluded that hybrid functionals offer the most promise, and that the ability of density functional methods to predict the nature of TSs demands a great deal more attention than it has received to date. [34]. Note that it is assumed here that... 

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]. 

A structural comparison of the calculated (B3LYP/6-311+G ) ts (transition state in the gas phase), ts-wc (transition state in the cluster of five extra water molecules), ts-CPCM (transition state within the CPCM-solvent model (B3LYP(CPCM)/6-311+G )) and ts-PCM (transition state optimized within the PCM-solvent model (B3LYP(PCM)/6-311+G )), shows no large differences (see Fig. 8), which is also valid for the precursor complexes (see Fig. 9). Modeling solvent effects shrinks in all cases the Be-0 bonds of the entering/leaving water molecules (159). 

The optimized geometries for the a-bond species 1, the transition state 2, and the intermediate product 3 from BSl at B3LYP level are shown in Fig. 2. The calculated geometries from BS2 are similar to those from BSl and the calculated bond lengths are listed in parenthesis in... 

Compound 53 has an orifice just wide enough to allow the potential entry of small ions, atoms (He), or molecules (H2, D2, T2) inside its cavity as judged from high-level transition state calculations at the density functionally theory level (B3LYP/6-31G, Figure 19).1791 The low barrier of insertion for helium makes it a specially attractive target, but other spe-... 

This transition-state, 22, has a calculated energy only slightly higher (0.48 kcal/mol at B3LYP/6-31G ) than the transition-state for isomer 18, and the geometry is also slightly different, for example, a somewhat shorter H—H bond. Loss of dihydrogen from this transition-state will lead to a secondary cation intermediate, and after a 1,2-hydride shift, to the same 1-methylcyclohexyl cation product that is directly produced from the other route. 

Scheme 9.31 Cyclic transition states for the (M)-l, 2-butadienylzinc additions to acetaldehyde calculated at the B3LYP/6-31G level of theory.

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