This document compares VSEPR theory and quantum chemistry in predicting molecular geometries and reactivity indices, highlighting their strengths and limitations.
Extract
[...] Then, we will analyze the reactivity indices of molecules by studying their atomic charges and electron densities, in order to better understand the factors influencing their chemical behavior. To do this, we will examine two TP; The TP1 which explores the predictable molecular geometries by the VSEPR method and compares them to the results obtained by quantum chemistry and the TP2 which focuses on molecular reactivity indices through the analysis of electronic densities and atomic charges. Material and Scientific Methodology: TP1 : Molecular Models - Software : GAUSSVIEW, Hartree-Fock method with STO-3G functions. - Steps : ? [...]
[...] with 0.31%), reveal the limitations of VSEPR, which does not take into account fine electronic interactions. These discrepancies are better explained by quantum mechanical calculations. 2.2 Results of TP2: Reactivity Indices Atomic Charges and Electron Density : The atomic charges and electron density of hydrogen atoms in the studied carboxylic acids are presented in Table 2. This table shows the impact of halogen substituents on the reactivity of carboxylic acids. Acid Atomic Charge H pKa Acetic +0.3 4.76 Chloroacetic +0 .5 2.9 Dichloroacetic +0.7 1.3 Trichloroacetic +0.9 0.7 Table 2 : Influence of halogen substituents on the atomic charge and pKa of carboxylic acids. [...]
[...] Furthermore, the analysis of reactivity indices revealed the impact of inductive and mesomeric effects on the chemical properties of molecules, particularly their acidity. These results demonstrate the importance of digital tools in deepening theoretical models and exploring the electronic interactions that influence molecular structure and reactivity. References Handbook of TP: Microscopic Structure of Matter, University XYZ, 2024. Organic Chemistry - Lycée Faidherbe. (S.d.). VSEPR Method. Hartree, D. R., & Fock, V. (1935). A method for the approximate solution of the Schrödinger equation. [...]
[...] Construction of molecules (methane, ammonia, water, ethanoic acid). ? Determination of geometries by the VSEPR method. ? Calculs of geometric optimization. ? Measurement of distances and angles around the central atom. ? Comparison of experimental results with VSEPR predictions. TP2: Reactivity Indices - Software : GAUSSVIEW, Hartree-Fock method with 3-21G* functions. - Steps : ? [...]
[...] Analysis of electron densities and atomic charges. ? Classification of compounds according to their reactivity. Results & Discussions 2-1 Results of TP1: Molecular Models Distances and angles measured : The interatomic distances and angles measured during the geometric optimization calculations are presented in the Table 1. This table compares the predictions of the VSEPR method, the results obtained by quantum chemistry, and the experimental values from the literature. Molecule Distance X-H (Å) Angle H-X-H predicted Angle H-X-H measured Écart CH? [...]
