Energetics and electronics of polar Diels-Alder reactions at the atomic level: QTAIM and IQA analyses of complete IRC paths.

The mechanism of Diels-Alder reactions between cyclopentadiene and several cyanoethylenes was studied by means of Density Functional Theory calculations using QTAIM and IQA (Interacting Quantum Atoms) analyses along complete IRC paths. Each geometry from the IRC had its wavefunction computed and the topology of the electronic density for it was then evaluated. By means of IQA, the global energetic profile was partitioned among the various atoms in the molecule, providing insight into what atoms are the main ones responsible for the magnitude of the energy barriers. The (a)synchronicity of the reaction mechanisms featuring non-symmetrically substituted dienophiles was characterized, from QTAIM, by the electron densities and Laplacians over the LCP's as well as by the different atomic energy barriers obtained from IQA. The magnitude of the atomic barrier nicely explains the (a)synchronicity of the reaction mechanisms, and the degree of (a)synchronicity is nicely revealed by the difference between the earlier and later bond breaking and bond formations. The main conclusion is that important energetic and electronic changes are occurring before and after the position of the transition state structure, mainly for those asynchronous mechanisms, and although these provide essential insight into the reaction mechanism, most studies cannot assess this kind of information because they are focusing solely on reactants, transition states, and products. We advocate that the additional computational effort required for such analyses is more than compensated by the great amount of useful information it provides.