Quantum Chemical Study of the Thermodynamic and Kinetic Aspects of the S_N2 Reaction in Gas Phase and Solution Using a DFT Interpretation

Abstract

The SN2 reactions X- + CH3Y → Y- + CH3X have been investigated in the gas phase and in solution using the Hartree−Fock level with 6-31+G* and 3-21G* bases, respectively. In the gas phase the interpretation has been done in the context of the HSAB principle using Gázquez's formalism using the polarizability (proportional to softness) of the two minima corresponding to the two ion−molecule complexes, and the TS, the softness of the nucleophile and the charge on the leaving group. The thermodynamic study shows that the reaction energy, ΔE r - p and the energy differences between the two ion−molecule complexes, ΔE m, are close to the experimental data available. The application of the MHP indicates that in these reactions the molecules arrange themselves to be as hard as possible. The polarizable continuum model (PCM) has been used to study the influence of the solvent on the kinetics of the SN2 reaction. The application of Gázquez's formula provides fundamental information in a case study, with X = Cl and Y = I: the activation energy calculated with the PCM model yields a very good correlation with the values obtained with Gázquez's formula. Finally, we concentrate on the influence of solvation on the nucleophilicity and the kinetics using the (PCM) model with X = F, I and Y = Br and compare the results with the gas phase. The order of reactivity in the gas phase is F- > I-, which is the same order as in the solvent using the polarizable continuum model, but opposed to experiment. If, however, we take into consideration the interaction energy caused by the hydrogen bond, the order of reactivity in solution is reversed yielding the same results as experiment.

A G2(+) level investigation of the gas-phase non-identity SN2 reactions of halides with halodimethylamine

“…In the aliphatic S N 2 reactions, nucleophilicity of nucleophile in the solvent may be different from in the gas phase because of the solvation energy. The observed [23] and predicted [24,25] reactivity sequences of nucleophiles in the gas-phase S N 2 reaction at carbon follow the order: F Ϫ Ͼ Cl Ϫ Ͼ Br Ϫ Ͼ I Ϫ , which will be reverse in the dipolar solvent, such as water and the alcohol. Here, we'll discuss the nucleophilicity and leaving-group ability of different halides in the gasphase S N 2 reactions at nitrogen using our G2(ϩ) energetics in Table 2.…”

Conceptual, Qualitative, and Quantitative Theories of 1,3‐Dipolar and Diels–Alder Cycloadditions Used in Synthesis

“…Conceptual DFT uses no orbitals, although it can be shown to be quantitatively related to FMO ideas. This theory has been applied to carbene reactivity, [11] S N 2 reactions, [12] enolate formation, [13] tautomerizations, [14] metal complexes, [15] enzymatic catalysis, [16] and many other organic reactions including cycloadditions. [17] HSAB theory began as a classification of Lewis acids and bases as hard or soft based on properties such as ionization energies and polarizabilities.…”

Quantum Information-Theoretical Analyses of Systems and Processes of Chemical and Nanotechnological Interest

“…In mass-weighted coordinates the IRC and the steepest-descent path are the same. Several computational techniques which calculate energy gradients and Hessians have been developed to follow such reaction paths (González & Schlegel, 1990;Peng et al, 1996;Fan & Ziegler, 1992;Safi, 2001;Pople et al, 1978;González-García et al, 2006;Ishida, et al 1977;Schmidt, et al , 1985;Baskin et al, 1974).…”