Reaction mechanisms Hartree-Fock theory

MOPAC is a general-purpose semiempirical molecular orbital program for the study of chemical structures and reactions. It is available in desktop PC running Windows, Macintosh OS, and Unix-based workstation versions. It uses semiempirical quantum mechanical methods that are based on Hartree-Fock (HF) theory with some parameterized functions and empirically determined parameters replacing some sections of the complete HF treatment. The approximations in... 

This mechanistic question is one of the examples of the success of density functional theory methods in organometallic chemistry. Earlier work on the reaction mechanism could not discriminate between the two alternatives. Analysis of the different orbitals based on extended Hiickel calculations came to the result that the [3+2] pathway is more likely, but could not exclude the possibility of a [2+2] pathway [13]. Similar conclusions where obtained from the results of Hartree-Fock calculations in combination with QCISD(T) single point calculations [21], Attempts to use Ru04 as a model for osmium tetraoxide indicated that the formation of an oxetane is less favorable compared to the [3+2] pathway, but still possible [22, 23],... 

The description of electron motion and electronic states that is at the heart of all of chemistry is included in wave function theory, which is also referred to as self-consistent-field (SCF) or, by honouring its originators, Hartree-Fock (HF) theory [7]. In principle, this theory also includes density functional theory (DFT) approaches if one uses densities derived from SCF densities, which is common but not a precondition [2] therefore, we treat density functional theory in a separate section. Many approaches based on wave function theory date back to when desktop supercomputers were not available and scientists had to reduce the computational effort by approximating the underlying equations with data from experiment. This approach and its application to the elucidation of reaction mechanisms are outlined in Section 7.2.3. 

It has been shown that the reaction path from the pyran to the most stable open form involved a two-step mechanism through a cis open form (Figure 1). The first step of this mechanism (transition state TS1) is the rate-limiting step, with a barrier of 138 kJmol-1 at the Hartree-Fock 6-31G(d) level, corrected to 92kJmol-1 with Moller-Plesset second-order perturbation theory. This value is in agreement with experimental measurements of the activation energy of the ring... 

The role of CI2 and monochloroacetic acid in the selective chlorination is a difficult problem to understand from the experimental studies. There are several possible orientations for the reactant, product and promoter molecules inside the complex structure of zeolite-L. In this context, it is pertinent to note that molecular modelling techniques are contributing in considerable amount to understand the reaction mechanisms. Molecular modelling includes force field based calculations [3] such as energy minimisation, Monte Carlo, and molecular dynamics calculations and quantum chemical calculations [4 ] such as EHMO, CNDO/INDO, MOPAC, Hartree-Fock and density functional theory calculations. In this study, we have attempted to apply the combination of molecular graphics, force field calculations and quantum chemical calculations to understand the mechanism of selective chlorination of DCB to TCB over zeolite K-L promoted by monochloroacetic acid. 

Combined quantum mechanical/molecular mechanical methods are not, of course, restricted to studies of reactions but can also be used to study association processes and conformational transitions. Most implementations use a two-zone model as described above, but Morokuma and colleagues have described a multilayered approach called ONIOM [Svensson et al. 1996]. ONIOM is a particularly apt name given that a typical calculation is constructed from a series of layers For example, a three-layer ONIOM calculation on the Diels-Alder reaction involved an inner core treated with the B3LYP density functional approach, the intermediate layer with a Hartree-Fock level of theory and the outer layer with MM3. A particular feature of ONIOM and its related methods is that they provide rigorous gradients and second derivatives, so enabling properties such as vibrational frequencies to be calculated [Dapprich et al. 1999]... 

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