Investigation of pericyclic reactions with the unified reaction valley approach: Van der Waals complexes, bifurcation points, and biradicaloid formatio

The study of chemical reaction mechanisms is advanced with a newly developed Unified Reaction Valley Approach (URVA2016). URVA is utilized to follow the reaction complex from far out in the entrance channel to the transition state and then to the location of the final products in the exit channel. Since electronic structure changes of the reaction complex are related to the curvature of the reaction path, URVA examines the reaction path curvature using the curvature coupling coefficients, which are third order response properties. Curvature peaks along the path indicate electronic structure changes of the reaction complex such as rehybridization or bond cleavage (formation). The reaction mechanism can be partitioned into phases dominated by curvature peaks (separated by curvature minima). Each peak is associated with changes in the electronic structure of the reaction complex. URVA2016 includes a new decomposition analysis of the path curvature that is based on newly defined local vibrational modes, which are robust in even in cases of path instabilities, and provides an extremely detailed insight into the chemical changes of the reaction complex. This use of this decomposition analysis was demonstrated for the Diels-Alder reaction of 1,3-butadiene and ethene. Improving upon an early study of this reaction with URVA, it was possible to extend the investigated path by 300% so that beside the chemical phases of the reaction valley, the prechemical and postchemical phases could also be analyzed. One important result of this study was the discovery that the fate of the reaction is already decided in the van der Waals region of the entrance channel. Butadiene transfers just a few milli-electron of charge to ethene, which initiates the planarization of gauche 1,3-butadiene to its cis form and latter causes the pyramidalization of the terminal CH2 groups that initiate the bond formation. For the first time, it was possible to identify the bifurcation points along the reaction path and to develop a new procedure that distinguishes curvature peaks caused by path bifurcations from those caused by chemical events. In addition to the Diels-Alder reaction, 1,3-dipolar cycloadditions of ten 1,3-dipoles with ethene or acetylene have been investigated as the understanding of their mechanisms is of utmost importance for chemical synthesis. Again, the mechanism is strongly determined in its first stages by the strength of van der Waals complexes that substantially contribute to the energetics of the cycloadditions. Despite differences in initial reactant configurations and differences in elecrophilicity and nucleophilicity of the 1,3-dipoles, the reaction complex follows a similar sequence of bond forming steps.;It is noteworthy that, after initial orientation, the shorter contact distance always leads to a delay in bonding. It is proved that charge transfer and charge polarization leading to an ansynchronous rehybridization at the reactive centers are responsible for this mechanistic feature. Finally, the role of the dipolarophile was analyzed and predictions were made with regard to more effective 1,3-dipolar cycloadditions.