Dra. Duran-Guajardo, Rocio

2024, Dra. Duran-Guajardo, Rocio, Barrales-Martínez, César, Jaque, Pablo

Context: This study investigates the energetic and polarizability characteristics of three 1,3-dipolar cycloaddition reactions between diazene oxide and substituted ethylenes, focusing on the transition from synchronous to asynchronous mechanisms. Synchronicity analysis, using the reaction force constant, indicates that the bond evolution process becomes increasingly decoupled as the number of cyano groups increases. Polarizability analysis reveals that isotropic polarizability reaches its maximum near the transition state in all cases, while anisotropy of polarizability shifts from the transition state toward the product direction as asynchronicity increases. The larger the shift, the more asynchronous the mechanism, as refected by the weight of the transition region. A detailed examination of the parallel and perpendicular polarizability components to the newly formed sigma bonds shows that the evolution of the parallel component is closely aligned with the energetic changes along the reaction coordinate, particularly in the synchronous reaction. We have also identifed a relationship between the displacement in the maximum state of the parallel component from the transition state and the synchronicity of the mechanism. The larger the displacement, the more asynchronous the mechanism. These fndings suggest that asynchronous 1,3-dipolar cycloaddition mechanisms are characterized by a decoupling of isotropic and anisotropic polarizabilities and a shift in the maximum polarizability state of the parallel component toward the product direction. Methods: Density functional theory calculations were performed at the B3LYP/6–311+ +G(d,p)//B3LYP/6-31G(d,p) level of theory. The polarizability was calculated at each point of the reaction path, obtained using the intrinsic reaction coordinate method, as implemented in Gaussian 16.

2024, Dra. Duran-Guajardo, Rocio, Barrales-Martínez, César, Caballero, Julio

Context: The negative of the Shannon entropy derivative is proposed to account for electron density contraction as the chemical bonds are breaking and forming during a chemical reaction. We called this property the electron density contraction index, EDC, which allows identifying stages in a reaction that are dominated by electron contraction or expansion. Four diferent reactions were analyzed to show how the EDC index changes along the reaction coordinate. The results indicate that the rate of change of Shannon entropy is directly related to the rate of change of the electron density at the bond critical points between all the atomic pairs in the molecular systems. It is expected that EDC will complement the detailed analysis of reaction mechanisms that can be performed with the theoretical tools available to date. Methods: Density functional theory calculations at the B3LYP/6-31G(d,p) level of theory were carried out using Gaussian 16 to analyze the reaction mechanisms of the four reactions studied. The reaction paths were obtained via the intrinsic reaction coordinate method, which served as the reaction coordinate to obtain the reaction force and the EDC profles in each case. Shannon entropy and electron density at the bond critical points were calculated using the Multiwfn 3.7 package.