A combined Bonding Evolution Theory (BET) and Interacting Quantum Atoms-Relative Energy Gradient (IQA-REG) study is carried out on a non-polar zw-type [3+2] cycloaddition (32CA) reaction. BET is the joint use of Catastrophe Theory and the topology of the Electron Localization Function (ELF) to characterise molecular mechanisms, while IQA is a quantum topological energy partitioning method and REG is a method to compute chemical insight at atomistic level, usually in connection with energy. This 32CA reaction involves the simplest nitrone with ethylene and has been studied here at B3LYP/6-311G(d,p) level within the context of Molecular Electron Density Theory (MEDT), which is based on the idea that changes in electron density, and not molecular orbital interactions, are responsible for chemical reactivity. We aim to determine the origin of the high activation energy of 32CA reactions involving zwitterionic three-atom-components. The BET study and IQA-REG method are applied to the overall activation energy path. While BET suggests that the barrier is mainly associated with the rupture of the nitrone CN double bond, IQA-REG indicates that it is mainly related to the rupture of the ethylene CC double bond. The present study shows that activation energies can be accurately and easily described by IQA-REG, and its complementary use with BET helps achieving a more detailed description of molecular mechanisms.