Daniel Rinaldi

A quantum chemical approach to dielectric solvent effects in molecular liquids

Abstract The SCF equations for a system of electrons and nuclei surrounded by a dielectric continuum are derived by using a multipolar expansion of the interaction energy The system is used as a model for the dielectric solvent effect. The case of a series of fluorinated is treated as an example in the CNDO/2 approximation. Ethyl fluoride and 1,1,1-trifluoroethane illustrate the importance of the moments other than the dipole one. The solvent effects on conformational equilibria of 1,2-difluoro, 1,1,2,2-tetrafluro, and 1,1,2-trifluoroethane are in reasonable agreement with other data owing to experimental incertainties.

Quantum mechanical computations on very large molecular systems: the local self-consistent field method

Quantum chemical computations on a subset of a large molecule can be performed, at the neglect of diatomic differential overlap (NDDO) level, without further approximation provided that the atomic orbitals of the frontier atoms are replaced by parametrized orthogonal hybrid orbitals. The electrostatic interaction with the rest of the molecule, treated classically by the usual molecular mechanical approximations, is included into the self‐consistent field (SCF) equations. The first and second derivatives of energy are obtained analytically, allowing the search for energy minima and transition states as well as the resolution of Newton equations in molecular dynamics simulations. The local self‐consistent field (LSCF) method based on these approximations is tested by studying the intramolecular proton transfer in a Gly‐Arg‐Glu‐Gly model tetrapeptide, which reveals an excellent agreement between a computation performed on the whole molecule and the results obtained by the present method, especially if the quantum subsystem includes the side chains and the peptidic unit in between. The merits of the LSCF method are examplified by a study of proton transfer in the Asp69—Arg71 salt bridge in dihydrofolate reductase. Simulations of large systems, involving local changes of electronic structure, are therefore possible at a good degree of approximation by introducing a quantum chemical part in molecular dynamics studies. This methodology is expected to be very useful for reactivity studies in biomolecules or at the surface of covalent solids.

Polarisabilites moléculaires et effet diélectrique de milieu à l'état liquide. Étude théorique de la molécule d'eau et de ses diméres

The model of Onsager in which a polar molecule undergoes a reaction field due to the polarization of the molecular surroundings is used to evaluate by a S.C.F. calculation (CNDO/2 approximation) the modifications of a molecular structure in the liquid state.Application to water molecule and to three polar dimers for values of the dielectric constant varying between 3 and 78, shows that most of geometric parameters and dipoles moments vary of few per cent when the molecule is inserted in a liquid. In the liquid state dipole moments do not depend very much on the dielectric constant but energies and relative stabilities of isomers are strongly dependent on the medium.[/p]

Ab initio SCF calculations on electrostatically solvated molecules using a deformable three axes ellipsoidal cavity

The analytical expression of the free energy of solvation of a molecule interacting with a dielectric continuum through a three axes ellipsoidal cavity is used to derive the SCF equations of such a solvated molecule. In this paper, the shape of the cavity is defined after the principal values of the electronic polarizability tensor. Applied to two sets of rotational isomers (trans and gauche 1,2 difluoroethane, and E and Z N methyl formamide), this method confirms that the electronic structure, the molecular geometry, and the equilibrium constant are influenced by the solvent. The energy of solvation depends strongly on the shape of the cavity although the electronic structure appears to be less influenced by a modification of the geometrical characteristics of the boundary surface at constant volume of cavity. Therefore, the computation of the electronic wave function of a molecule interacting with a solvent by electrostatic and induction forces appears to be quite feasible.

Reaction field factors for a multipole distribution in a cavity surrounded by a continuum

Abstract The reaction field factors appear in the generalization of the Kirkwood model of solvation for a molecule imbedded in a cavity surrounded by a dielectric continuum. A general algorithm is proposed to compute these factors for a distributed multipole analysis of the charge distribution of the solute.

Liquid state quantum chemistry : A cavity model

Abstract A cavity model for quantum chemical calculations on physically solvated molecules is described. The cavity is defined by a surface on which the electronic contribution to the electrostatic potential is constant and the value of this quantity is fixed by the volume of the cavity which is equal to the molecular volume in the liquid. The solvent is represented by its static dielectric constant. Although the thermodynamic quantities computed with this model are strongly dependent on the shape of the cavity, the electronic structure can be obtained with a good accuracy by approximating the surface by an ellipsoid which allows an analytical calculation of the electrostatic term and even the dispersion term of the interaction free energy. A general scheme is proposed for a liquid state quantum chemistry.

Electrostatic component of solvation: Comparison of SCRF continuum models

We report a systematic comparison of the electrostatic contributions to the free energy of solvation from three different kinds of quantum mechanical self‐consistent reaction field (SCRF) methods. We also compare the liquid‐phase dipole moments as a measure of the solutes response to the reaction field of the solvent. In particular, we compare these quantities for the generalized Born model as implemented in the SM5.42R method, the multipolar expansion model developed at Nancy, and the MST version of the polarizable continuum model. All calculations are carried out at the HF/6‐31G(d) level. The effects of various choices of solute cavities and representations of the charge density are examined. The test set consists of 18 molecules containing prototypical polar groups, and three different values of the dielectric permittivity are considered.

Analytical energy derivatives for a realistic continuum model of solvation: Application to the analysis of solvent effects on reaction paths

Analytical expressions for the first and second derivatives of the Hartree–Fock energy have been derived in case of a solvated system simulated by a multipolar charge distribution embedded in a cavity of arbitrary shape and a solvent represented by a dielectric continuum. A computer code has been written on these bases. It allows geometry optimizations and more generally the determination of the critical points of the potential energy surface for a molecular system interacting with a solvent as easily as in the case of an isolated molecule. The use of this code is illustrated by the computation of the main features of the reaction path of a Menshutkin‐type reaction in various solvents. The results compare pretty well with those obtained by a full Monte Carlo simulation of the solvent by Gao. This agreement supports the idea that solvents, including water, can be safely modeled by a continuum. The advantage of such models rests in the fact that they allow refined computations on the solute at a minimum com...

Influence of dispersion forces on the electronic structure of a solvated molecule

Abstract Equations giving the dispersion contribution to the free energy of solvation of a molecule in a dielectric continuum are used to establish the SCF equations of the solvated molecule. The electronic structure obtained by minimizing the overall energy (molecular + solvation) differs slightly from that obtained when the solute-solvent interaction is limited to the electrostatic term. The difference is negligible in the case of a strongly polar molecule (N-methylformamide), the role of dispersion being a slight reduction of the polarization by the solvent. In the case of a non-dipolar molecule (methane), the effect of dispersion is of the same order of magnitude as the electrostatic one and increases the polarization.

Intramolecular electron correlation in the self-consistent reaction field model of solvation. A MP2/6-31G** ab initio study of the NH3HCl complex

Abstract The effects of intramolecular electron correlation on the electronic and molecular properties of a solvated molecule have been considered in this paper throughout a general 6-31G** ab initio study of the ammonia—hydrogen chloride complex. Calculations have been performed both in vacuo and in a non-polar solvent, at the Hartree—Fock and at the second-order Moller—Plesset levels of approximation. In contrast with the isolated complex, the self-consistent reaction field model of solvation reveals the presence of two minima corresponding to an ionic and a molecular complex. The shape of the potential energy curve is substantially modified by the MP2 correction.