Abstract
An information-theoretical complexity analysis of the SN2 exchange reaction for CH3Cl + F− is performed in both position and momentum spaces by means of the following composite functionals of the one-particle density: D-L and I-J planes and Fisher-Shannon’s (FS) and López-Ruiz-Mancini-Calbet (LMC) shape complexities. It was found that all the chemical concepts traditionally assigned to elementary reactions such as the breaking/forming regions (B-B/F), the charge transfer/reorganization and the charge repulsion can be unraveled from the phenomenological analysis performed in this study through aspects of localizability, uniformity and disorder associated with the information-theoretical functionals. In contrast, no energy-based functionals can reveal the above mentioned chemical concepts. In addition, it is found that the TS critical point for this reaction does not show any chemical meaning (other than the barrier height) as compared with the concurrent processes revealed by the information-theoretical analysis. Instead, it is apparent from this study that a maximum delocalized state could be identified in the transition region which is associated to the charge transfer process as a new concurrent phenomenon associated with the charge transfer region (CT) for the ion-complex is identified. Finally it is discussed why most of the chemical features of interest (e.g., CT, B-B/F) are only revealed when some information-theoretic properties are taken into account, such as localizability, uniformity and disorder.
Highlights
Bimolecular nucleophilic substitution (SN2) reactions at carbon centers continue to be among the most studied chemical reactions
In order to associate the aforementioned information-theoretical measures along the course of the reaction path, we have employed some selected physical descriptors: the atomic charges obtained from the molecular electrostatic potential (MEP) and the atomic electric potentials associated with the basins of the MEP
Rx ~ +1, charge transferring continues until the molecule stabilizes electrostatically reaching a null dipole moment, (iv) from Rx ~ +3 to +4 the process inverts and the buffer accepts charge from fluorine up to the bond forming between these two species occurs, and (v) from Rx ~ +4 up to the end of the reaction, the whole process gets inverted and repulsive forces between the [CH3···F]* and Cl− occur
Summary
Bimolecular nucleophilic substitution (SN2) reactions at carbon centers continue to be among the most studied chemical reactions. Information theoretical analyses have shown to be useful for the phenomenological description of the course of elementary chemical reactions This is achieved through the localized/delocalized behavior of the electron densities in position and momentum spaces by revealing important chemical regions that are not present in the energy profile, such as the ones in which bond forming and bond breaking occur and where bond cleavage energy regions (BCER) appear. The usefulness of each definition depends on the type of system or process under study, the level of the description, and the scale of the interactions among either elementary particles, atoms, molecules, biological systems, etc Fundamental concepts such as uncertainty or randomness are frequently employed in the definitions of complexity, some other concepts like clustering, order, localization or organization might be important for characterizing the complexity of systems or processes.
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