Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and the bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We have decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species are the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results actually show that the enthalpy of BF3 adduct formation in DCM solution is clearly influenced by specific interactions, DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allows us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain because of identified reasons can be complemented by the computed values.

Enthalpies of Adduct Formation between Boron Trifluoride and Selected Organic Bases in Solution: Toward an Accurate Theoretical Entry to Lewis Basicity

The Lewis basicity of selected organic bases, modeled by the enthalpies of adduct formation between gaseous BF3 and the bases in dichloromethane (DCM) solution, is critically examined. Although experimental enthalpies for a large number of molecules have been reported in the literature, it may be desirable to estimate missing or uncertain data for important Lewis bases. We have decided to use high-level ab initio procedures, combined with a polarized continuum solvation model, in which the solvated species are the clusters formed by specific hydrogen bonding of DCM with the Lewis base and the Lewis base/BF3 adduct. This mode of interaction with DCM corresponds to a specific solvation model (SSM). The results actually show that the enthalpy of BF3 adduct formation in DCM solution is clearly influenced by specific interactions, DCM acting as hydrogen-bonding donor (HBD) molecule in two ways: base/DCM and adduct/DCM, confirming that specific solvation is an important contribution to experimentally determined Lewis basicity scales. This analysis allows us to conclude that there are reasons to suspect some gas-phase values to be in error by more than the stated experimental uncertainty. Some experimental values in DCM solution that were uncertain because of identified reasons can be complemented by the computed values.

Kinetics and mechanism of acyl transfer reactions. Part 16. Quantum chemical simulation of mechanism of N-methylaniline sulfonation in aqueous 1,4-dioxane

The RHF/6-31G(d) quantum chemical simulation of the mechanism of the secondary fatty aromatic amine N-methylaniline interaction with benzenesulfonyl chloride under conditions of N-methylaniline specific solvation by one water molecule and one 1,4-dioxane molecule, and under conditions of N-methylaniline specific solvation by two water molecules and one 1,4-dioxane molecule. Three-dimensional potential energy surfaces of the processes pointed out are computed. It is shown that in the both cases a single route of the reactions is realized, starting as an axial nucleophilic attack, which goes further with decreasing of the attack angle as reagent molecules approach each other. It was established that both simulated reactions proceed in accordance with bimolecular concerted mechanism of nucleophilic substitution SN2, which implies the formation of a single transition state in the reaction path. It was found that geometrical configuration of the reaction center in the transition states of the reactions is medium between the trigonal-bipyramidal and tetragonal-pyramidal, which is associated with the change in the angle of N-methylaniline attack as the reactant molecules approach each other. In the benzenesulfonyl chloride reaction with N-methylaniline, solvated by one water molecule and one 1,4-dioxane molecule, the transition state is solvated only by 1,4-dioxane molecule, while water molecule moves away from the reaction center, whereas in the benzenesulfonyl chloride reaction with N-methylaniline, solvated by two water molecule and one 1,4-dioxane molecule the transition state is solvated by 1,4-dioxane molecule and one water molecule that forms hydrogen bond with chlorine atom and promote the S–Cl-bond loosening. The activation energies of the reactions were calculated; it is shown that specific solvation increases the reactions energetic barrier as compared with the reaction in gaseous phase, that is caused by the partial dehydratation of N-methylaniline molecule before the transition state formation. A decrease of the activation energy of the reaction with participation of N-methylaniline, solvated by two water molecule and one 1,4-dioxane molecule as compared with the cases of non-specific solvation of the reactants and N-methyl-aniline solvation by one water molecule and one 1,4-dioxane molecule is caused by the existence of the second water molecule in the system, forming a bond with amine group and facilitating N–H bond break.

Kinetics and mechanism of acyl transfer reactions. Part 15. Quantumchemicalsimulation of mechanisms of reactions of N-ethylaniline sulfonation

The RHF/6-31G(d) quantum chemical simulation of the mechanism of the interaction of the secondary fatty aromatic amine N-ethylaniline with benzenesulfonyl chloride under conditions of non-specific water solvation, using the continuum model of the solvent, as well as of sulfonylation reactions of N-ethylaniline solvation complexes containing one water molecule, modeled specific solvation of N-ethylaniline with water, and one molecule of water and one of dioxane, which simulate the solvation of the amine with aqueous dioxane. Three-dimensional potential energy surface of these processes is calculated. It is shown that in the case of a reaction proceeding under conditions of non-specific solvation of reagents, the route with axial attack of the N-ethylaniline molecule to the sulfonyl reaction center is realized, in the two other cases the reactions proceed along a single route, starting as an axial attack of the nucleophile, which goes further with decreasing of the attack angle as reagent molecules approach each other. It was established that all the simulated reactions proceed in accordance with bimolecular coordinated mechanism of nucleophilic substitution SN2, which implies the formation of a single transition state in the reaction path. It was found that geometrical configuration of the reaction center in the transition state of N-ethylaniline reaction with benzenesulfonyl chloride under non-specific solvation by water is close to trigonal-bipyramidal, which is determined by the axial direction of the nucleophilic attack, in the two other cases it is medium between the trigonal-bipyramidal and tetragonal-pyramidal, which is associated with the change in the angle of N-ethylaniline attack as the reactant molecules approach each other. In a reaction involving N-ethylaniline monohydrate, a water molecule forms a 6-membered cyclic structure with reagent molecules in the transition state, in which the transfer of a proton from N-ethylaniline amino group to a hydrogen chloride molecule occurs via a relay mechanism involving the water molecule. The activation energy values of the studied processes were calculated; it is shown that both specific and universal solvation significantly lower the energy barrier of the reaction compared to the reaction occurring in gas phase, which is consistent with the data obtained earlier for related processes.