Diels-Alder cycloaddition is one of the most powerful chemical reaction used by organic chemists. In most cases, it involves a concerted mechanism with two CC bonds formed during the same elementary step. This explains the stereoselectivity observed. Nonetheless the concerted mechanism can be either synchronous with the two bonds forming exactly concomitantly or asynchronous with the two bonds created in two stages along the same elementary step. In extreme cases, a stepwise mechanism can be observed. In this work we studied by DFT 16 Diels-Alder reactions covering a large range of degree of synchronicity using Gaussian09 (M06-2X/6-31G(d,p)). As expected, reaction profiles of the reaction force (F(ξ)) and more specifically of the reaction force constantκ(ξ)[1] highligthed these various degrees of synchronicity. More quantitatively, the width of the transition state region (∆ξ) revealed to be a good measure of the asynchronicity. We introduced a new index Θ from molecular orbital theory to predict the degree of synchronicity from the reactant’s structures. Then we proposed a topological study based on QTAIM to analyze the bond’s formations. We focused on the sign of r2ρ at the bond critical point for each new bond[2].Interestingly, r2ρ changes of sign coincide with kappa minima. It may help to analyze process composing a mechanism of concerted reactions. References [1] JAQUE, P., TORO-LABBÉ, A., POLITZER, P., AND GEERLINGS, P. Reaction force constant and projected force constants of vibrational modes along the path of an intramolecular proton transfer reaction. Chemical Physics Letters 456, 4-6 (May 2008), 135–140. [2] POPELIER, P. L. A. The QTAIM Perspective of Chemical Bonding. In The Chemical Bond, G. Frenking and S. Shaik, Eds. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, May 2014, pp. 271–308.