Correlation, effects transition-state geometries

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

Computational studies investigate reaction mechanisms and pathways by constructing potential energy profiles. This involves exploring reaction thermodynamics and kinetics, by examining reactants and products as well as the transition states geometries and activation energy barriers. Like those seen in structure prediction, most current studies implement effective core potentials and density functional theory to perform calculations.However, ECPs can be paired with MP2 to account for electron correlation thus far, this approach has only been used for smaller chemical systems. " Eurthermore, solvation methods such as the polarizable continuum model can be employed to examine... 

Glad SS, Jensen F (1996) Basis set and correlation effects on transition state geometries and kinetic isotope effects. J Phys Chem 100 16892-16898... 

Further illustrations of the effects of basis set enlargement and correlation recovery are given in Table 3 for a stable intermediate geometry and a transition state geometry. The errors are seen to be roughly comparable to the errors associated with the energy difference between reactants and produas. 

Cycloisomerization of methyl-substituted 1,6-diene is also catalyzed by tin(rv) triflate (Equation (8.8)). DFT computations proposed that the mechanism does not involve the direct addition of the tin(IV) cation to a double bond because the catalyst regeneration step would be energetically unfeasible [24]. The active catalyst is a hydrated triflate salt where water molecule plays a decisive role to enable the smooth completion of the catalytic cycle. The diastereoselectivity observed in the cycloisomerization was associated with the transition-state geometries. DFT calculations also showed that protonation and deprotonation occur on a single face of the substrate. These considerations, correlated to the experiments, showed that Brpnsted superacids are not effective in Lewis superacid catalysis. 

Usually, geometries of transition states are significantly more sensitive with respect to method than are stmctures of stable species. Since electron correlation effects are of particular importance for these stmctures, the determination of transition states at the Hartree-Fock level should be avoided. It is recommended to compare the stmctural parameters of transition states obtained from different methods (for instance DFT and MP2) in order not to be misled. 

Geometries were fully optimized at the HF/6-31G level of theory, and single point energies were evaluated at the MP2/6-31G level to indude the effects of electron correlation. Transition states were characterized by harmonic frequency analysis. 

If we consider the [JI2S + (n2s + 2S)] mechanism, then the lateral overlap between Nl and C4 pAOs leads to a Ci-symmetric transition structure and a nonzero value for the co = N1-C2-C3-C4 dihedral angle (Fig. 5), as well as to larger N1-C4 distances. In contrast, the ri2s + (n2s + m2s)] interaction leads to a Cs transition state for the reaction between ethylene and isocyanic acid. Both geometries were found for this reaction at different theoretical levels [109-111] (Fig. 6). Thus, at the RHF level the transition structure was found to be Cs-symmetric [110, 111], whereas the RHF(SCRF, MP2 and MP2(SCRF) leads to to values of ca. 40 deg. Thus, inclusion of solvent effects and/or electron correlation favor the [n2s + (k2s + n2s)] mechanism. This behavior was also observed with substituted alkenes [109]. 

At the same time the selectivity of COD formation decreases. Too much reliance cannot, of course, be placed on this qualitative correlation, and it is not to be assumed that the transition state of the cyclization reaction has the same geometry as a Lig2Ni(CO)2 complex. It can be seen from Table IV that the selective formation of COD using ligands with o-sub-stituents is not associated with electronic factors. In these cases steric effects play an important role. This is illustrated clearly in Table V. 

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