Direct observation of denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, using a visible 5-fs pulse laser

Curly arrows, electron flow, and reaction mechanisms from the perspective of the bonding evolution theory

Despite the usefulness of curly arrows in chemistry, their relationship with real electron density flows is still imprecise, and even their direct connection to quantum chemistry is still controversial. The paradigmatic description - from first principles - of the mechanistic aspects of a given chemical process is based mainly on the relative energies and geometrical changes at the stationary points of the potential energy surface along the reaction pathway; however, it is not sufficient to describe chemical systems in terms of bonding aspects. Probing the electron density distribution during a chemical reaction can provide important insights, enabling us to understand and control chemical reactions. This aim has required an extension of the relationships between the concepts of traditional chemistry and those of quantum mechanics. Bonding evolution theory (BET), which combines the topological analysis of the electron localization function (ELF) and Thom's catastrophe theory (CT), provides a powerful method that offers insight into the molecular mechanism of chemical rearrangements. In agreement with the laws of physical and aspects of quantum theory, BET can be considered an appropriate tool to tackle chemical reactivity with a wide range of possible applications. In this work, BET is applied to address a long-standing problem: the ability to monitor the flow of electron density. BET analysis shows a connection between quantum mechanics and bond making/forming processes. Likewise, the present approach retrieves the classical curly arrows used to describe the rearrangements of chemical bonds and provides detailed physical grounds for this type of representation. We demonstrate this procedure using the test set of prototypical examples of thermal ring apertures, and the degenerated Cope rearrangement of semibullvalene.

Inquiry of the electron density transfers in chemical reactions: a complete reaction path for the denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives

A detailed study on all stages associated with the reaction mechanisms for the denitrogenation of 2,3-diazabicyclo[2.2.1]hept-2-ene derivatives (DBX, with X substituents at the methano-bridge carbon atom, X = H and OH) is presented. In particular, we have characterized the processes leading to cycloalkene derivatives through migration-type mechanisms as well as the processes leading to cyclopentil-1,3-diradical species along concerted or stepwise pathways. The reaction mechanisms have been further analysed within the bonding evolution theory framework at B3LYP and M05-2X/6-311+G(2d,p) levels of theory. Analysis of the results allows us to obtain the intimate electronic mechanism for the studied processes, providing a new topological picture of processes underlying the correlation between the experimental measurements obtained by few-optical-cycle visible pulse radiation and the quantum topological analysis of the electron localization function (ELF) in terms of breaking/forming processes along this chemical rearrangement. The evolution of the population of the disynaptic basin V(N1,N2) can be related to the experimental observation associated with the N=N stretching mode evolution, relative to the N2 release, along the reaction process. This result allows us to determine why the N2 release is easier for the DBH case via a concerted mechanism compared to the stepwise mechanism found in the DBOH system. This holds the key to unprecedented insight into the mapping of the electrons making/breaking the bonds while the bonds change.

Substituent effect on reactivity of triplet excited state of 2,3-diazabicyclo[2.2.1]hept-2-enes, DBH derivatives: α C-N bond cleavage versus β C-C bond cleavage

The photoreaction of a series of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, in which various substituents (X) were introduced at the methano bridge carbon of C(7), was investigated under direct (>290 nm) and triplet-sensitized (Ph2CO, >370 nm) irradiation conditions of the azo chromophore (−Cβ–Cα–N═N–Cα–Cβ−). The azo compounds offered a unique opportunity to see the substituent X effect at the remote position of the azo chromophore on the reactivity of the triplet excited state of bicyclic azoalkanes. The present study led to the first observation of the unusual Cα–Cβ bond-cleavage reaction without the ring stiffness, that is, the structural rigidity, in the triplet state of the cyclic azoalkanes. The stereoelectronic effects were found to play an important role in lowering the activation energy of the Cα–Cβ bond-cleavage reaction. NBO analyses at the M06-2X/cc-PVDZ level of theory confirmed the stereoelectronic effect.

How a Quantum Chemical Topology Analysis Enables Prediction of Electron Density Transfers in Chemical Reactions. The Degenerated Cope Rearrangement of Semibullvalene

Recent works on the reaction mechanism for the degenerated Cope rearrangement (DCR) of semibullvalene (SBV) in the ground state prompted us to investigate this complex rearrangement in order to assign experimentally observed contrast features in the simulated electron distribution. We present a joint use of the electron localization function (ELF) and Thom's catastrophe theory (CT) as a powerful tool to analyze the electron density transfers along the DCR. The progress of the reaction is monitored by the structural stability domains of the topology of ELF, while the change between them is controlled by turning points derived from CT. The ELF topological analysis shows that the DCR of SBV corresponds to asynchronous electron density rearrangement taking place in three consecutive stages. We show how the pictures anticipated by drawing Lewis structures of the rearrangement correlate with the experimental data and time-dependent quantum description of the process.