A Critical Look at Bonding Evolution Theory: Re-examining the NH3 Umbrella Inversion and the H3C-CH3 and H3B-NH3 Dissociations

This paper revisits the assignment basis of elementary catastrophes proposed by Silvi and collaborators [J. Phys. Chem. A, 1997, 101, 7277–7282] to characterize the inversion of ammonia as well as the dissociation of both ethane and ammonia borane molecules. Herein, we stress the need and significance of considering Thom’s works fundamentals for a correctly assigning the parametric function describing cleavage and forming of bonds along a reaction pathway. Monitoring the determinant of the Hessian matrix at potentially degenerate critical points and the relative distance between them revealed an unexpected finding: no elliptic umbilic occurs along the reaction pathway for any of the systems, in contrast to the previous report. This result significantly simplifies the rationalization of electron rearrangements in the ground state. The requirement of some conditions encompassing the symmetry of the electron density becomes a tool for, a priori, predicting the type of catastrophic bifurcations describing any reaction mechanism, which is consistent with recent observations. Therefore, elementary catastrophes constitute a fingerprint of the existence and persistence (or of the lack of any) of the electron density symmetry along a reaction path. These results challenge the past and most actual applications of BET original version for describing chemical reactions in systems ranging from the gas phase to the solid state.