Discrete Solvent Models Research Articles

On the origin of the regioselective hydrolysis of a naphthoquinone diacetate: a molecular orbital study

The regioselectivity found in the mild basic hydrolysis of the 2,5-dimethyl-1,4-naphthohydroquinone diacetate (Nunes, R.L.; Bieber, L.W.; Longo, R.L. J. Nat. Prod. 1999, 62, 1600) has been studied with ab initio and semiempirical molecular orbital methods. In the gas phase (isolated systems), these methods were not able to provide results that could explain the observed selectivity. However, when the solvent effects were included in the AM1 method using the discrete solvation model it was possible to establish that this selectivity is due to the relative stability of the tetrahedral intermediates and their transitions states. The origin of this relative stability and thus of the observed selectivity is due to the repulsive interactions between the 2-methyl substituent in the naphthalene ring and the methyl group in the 4-acetate substituent, as well as their hindrance towards the hydration of the ionic group in the tetrahedral intermediates.

MCSCF Study of the SN2 Menshutkin Reaction in Aqueous Solution within the Polarizable Continuum Model

The Menshutkin reaction NH3 + CH3 Cl → CH3NH3+ + Cl- in aqueous solution is studied using the CASSCF method combined with the polarizable continuum model in a version that includes electrostatic, repulsion, and dispersion solute−solvent interactions. The solvent reaction field is inserted in the CASSCF Hamiltonian resorting to a mean-field approximation. The C3v symmetry is maintained for all the geometries considered and the active space is generated distributing four electrons in four orbitals of a1 type. The results of the present study are in excellent agreement with recent ab initio calculations, which use both continuum and discrete solvation models, and with available experimental data. The analysis of the electronic structure of the transition state by the VB method explains the mechanism of this reaction in terms of a Linnett-type nonpaired spatial orbital representation as in a four-electron, three-center bonding unit.

On the role of solvent electronic polarization in charge transfer reactions

The effect of a solvent’s electronic polarization on the rate of a charge transfer reaction is studied in both continuum and discrete solvent models. An effective system Hamiltonian that contains the equilibrium solvation from the solvent electronic polarization is obtained, and leads to an effective matrix element Veff coupling the charge transfer states that is smaller than the gas phase value. Both the effective Hamiltonian and Veff are dependent on the solvent’s instantaneous nuclear configuration, and liquid state theory is used to carry out the configuration average. The solvent electronic polarization reduces the transition rate for both adiabatic and nonadiabatic reactions. A standard relation between the equilibrium solvation energy of the reactants and the solvent reorganization energy is established that permits evaluation of the effect of a molecular solvent (using a Drude model for the electronic degrees of freedom) on the rate by evaluating a partition function. This permits use of a path integral formulation for the mixed quantum (electronic polarization) classical (slow nuclear configuration) solvent that leads to the information required for the rate constant. In a strong coupling regime, where the coupling between the charge-transfer species would be so large as to preclude reactant and product species, we show that a new mechanism for charge localization arising from the solvating effect of the electronic polarization may occur, and formulate a rate constant expression for this regime. It has the form of a quantum Kramers rate and shows that the solvent provides a friction effect that will reduce the rate relative to the no-friction rate.

Hypernetted chain theory for electrolyte solutions: a new approach between continuous and discrete solvent models

A new type of HNC calculation is proposed in which the variation of the solvation energies with salt and solvent concentration is taken into account. This is done by means of a self-consistent calculation of the distance dependent permittivity function around a central ion. The theory yields ion-ion as well as ion-solvent correlation functions and concentration dependent permittivities. In contrast to HNC calculations with a completely discrete-solvent model, the program requires only a small amount of computing power.

A critique of the adsorption isotherms used in electrochemical processes involving the adsorption of organic compounds

The main adsorption isotherms used in electrochemical processes involving the adsorption of organic compounds have been analysed both theoretically and experimentally in order to clarify the models, the limits of their applicability and the validity of the results obtained. The isotherms are divided into two categories: (a) isotherms based on continuous-solvent models and (b) isotherms based on discrete-solvent models. The isotherms of category (a) have, in general, low applicability, while their models do not give a realistic description of the adsorbed phase. However, the discrete-solvent models are closer to the physical reality of the electrode/solution interface, although only the generalized Flory–Huggins isotherms seem to provide both high applicability and believable conclusions concerning the particle–particle interactions of the adsorbed phase.