Intrinsic reaction coordinate calculation

The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ringopening of cyclohexadiene. 

The geometries, vibrational frequencies and energies of all the stationary points for the ketene-oxygen reaction have been calculated. Relationships of the reactants, transition states, intermediates, and products are confirmed by intrinsic reaction coordinate calculations and important aspects of the mechanism have been highlighted.266... 

Bosch, E., Moreno, M., Lluch, J.M., Bertran, J. Intrinsic reaction coordinate calculations for reaction paths possessing branching points. Chem. Phys. Lett. 1989,160, 543-8. 

The biosynthesis of cholesterol from squalene was examined. Using model structures and intrinsic reaction coordinate calculations, evidence was reported for the asynchronous, concerted reaction of squalene oxide to the protosterol cation. 

Techniques have been developed within the CASSCF method to characterize the critical points on the excited-state PES. Analytic first and second derivatives mean that minima and saddle points can be located using traditional energy optimization procedures. More importantly, intersections can also be located using constrained minimization [42,43]. Of particular interest for the mechanism of a reaction is the minimum energy path (MEP), defined as the line followed by a classical particle with zero kinetic energy [44-46]. Such paths can be calculated using intrinsic reaction coordinate (IRC) techniques... 

Subsequent calculations at the MP2 level locate the two transition structures like those suggested. In this case, Intrinsic Reaction Coordinate (IRC) calculations were used to confirm that these transition structures do in fact connect the minima in question we ll look at this kind of calculation in detail later in this chapter. 

Although intrinsic reaction coordinates like minima, maxima, and saddle points comprise geometrical or mathematical features of energy surfaces, considerable care must be exercised not to attribute chemical or physical significance to them. Real molecules have more than infinitesimal kinetic energy, and will not follow the intrinsic reaction path. Nevertheless, the intrinsic reaction coordinate provides a convenient description of the progress of a reaction, and also plays a central role in the calculation of reaction rates by variational state theory and reaction path Hamiltonians. 

Figure 10. Potential energy diagram for the dehydrocyclization of 18 obtained from an Intrinsic Reaction Coordinate (IRC) calculation at B3LYP/6-31G. Note that the IRC starts at the TS but does not quite reach either ground-state structure (reactant-products).

Spin-coupled theory has been used to smdy the changes that occur in the electronic wavefunction as a system moves along the intrinsic reaction coordinate for the case of the conrotatory and disrotatory pathways in the electrocyclization of cyclobutene to c/x-butadiene. Against intuitive expectations, conrotatory opening of cyclobutenes was found to be promoted by pressure. Ab initio MO and density functional calculations have indicated that the ring opening of the cyclobutene... 

---