Theoretical investigations on the solvation process

A model to facilitate the computation of the most stable conformer of associated M · H₂O (M being a polar molecule) which depends upon the electrostatic interaction energy between the two associated molecules is proposed and tested. SCF electrostatic potentials for the M molecule and a suitable point charge distribution for H₂O were employed in the model computations. Energies predicted by the model are found to be in good agreement with those resulting from an ab initio minimal STO basis SCF treatment of some conformations of the H₂O dimer.

---

Zusammenfassung Ein Modell zur Durchführung der Berechnung des stabilsten Konformeren eines Assoziationskomplexes M · H₂O, wobei M ein polares Molekül ist, wird vorgeschlagen und untersucht. Es basiert auf der elektrostatischen Wechselwirkung zwischen beiden Partnern, und zwar wird für das Molekül M der elektrostatische Anteil seines SCF-Potentials und für H₂O eine angemessene Punktladungsverteilung zugrunde gelegt. Die resultierenden Energien sind in guter Übereinstimmung mit denen, die sich bei einer ab initio Rechnung mit minimaler STO Basis ergeben.

---

---

Work performed with the financial support of the Consiglio Nazionale delle Ricerche, through its Laboratorio di Chimica Quantistica ed Energetica Molecolare.     

An approximate expression of the electrostatic molecular potential for benzenic compounds

A method for getting approximate but realistic evaluations of the electrostatic molecular potential for benzene derivatives is presented and discussed. The method is based on a partition of the whole molecular observable into directly transferable contributions, which can be calculated without a previous knowledge of the molecular wavefunction. The algorithm, which probably is applicable also to other aromatic compounds, implements a preceding study concerning only unconjugated molecules.     

A representation of the polarization term in the interaction energy between a molecule and a point-like charge

The interaction energy of a molecule M with a point-like charge q can be partitioned into simpler contributions, two of which can be expressed in terms of the charge distribution _M of the sole M. The first term, qV(r), represents the interaction of q with the undistorted charge _M ⁰ of M while the second q ² P(r) gives the additional contributions due to the polarization of _M ⁰ under the influence of the charge q placed at the point r. In this paper we investigate the possibility of getting an inexpensive and sufficiently accurate analytical representation of P(r) over the whole space outside the van der Waals volume of M.     

Alternative paths in the ring opening of oxadiaziridine: The diimide N-oxide versus the oxodiimide rearrangement

The ring opening of the oxadiaziridine by cleavage of the N-N bond has been theoretically investigated by SCF calculations improved with limited CI. The possible competition of this reaction with the better known ones (N-O bond cleavage, inversion of the N atom) is discussed. The chemical implications of the formation of a new type of 1,3-dipole are examined.     

A polarizable continuum model for molecules at diffuse interfaces

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green's function is known. In the present case, such a Green's function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.     

Neutral organic lewis acids of π type

The interaction of some neutral acids of π type, bearing appropriate unsymmetrical substitutions at the C?C group with some selected bases (H₂O, NH₃, OH^?), is compared with that of the parent compound of a new set of neutral π acids, bearing symmetrical substitutions at the C?C group with the same bases. The analyses of the interaction energy, performed according to two decomposition schemes, with and without the counterpoise corrections, make clear the similarity of symmetrically and unsymmetrically substituted neutral organic acids.     

Solvation dynamics in acetonitrile: a study incorporating solute electronic response and nuclear relaxation

The solvent reorganization process after electronic excitation of a polar solute in a polar solvent such as acetonitrile is related mainly to the time evolution of the solute-solvent electrostatic interaction. Modern laser-based techniques have sufficient time resolution to follow this decay in real time, providing information to be confirmed and interpreted by theories and models. We present here a study aimed at the investigation of the different steps involved in the process taking place after a vertical S(0) --> S(1) excitation of a large size chromophore, coumarin 153 (C153), in acetonitrile, from both the solute and the solvent points of view. To do this, we use accurate quantum mechanical calculations for the solute properties within the polarizable continuum model (PCM) and classical molecular dynamics (MD) simulations, both equilibrium and nonequilibrium, for C153 in the presence of the solvent. The geometry of the solute is allowed to change in order to study the role of internal motions in the time-dependent solvation process. The solvent response function has been obtained from the simulation data and compared to experiment, while the comparison between equilibrium and nonequilibrium MD results for the solvation response confirms the validity of the linear response approximation in the C153-acetonitrile system. The MD trajectories have also been used to monitor the structure of the solvation shell and to determine its change in response to the change in the solute partial charges.     

On the direct calculation of the time evolution of excited molecular states in the presence of nonadiabatic interactions

The possibility is explored of calculating the time evolution of a given initial molecular state, in the presence of sufficiently strong nonadiabatic interactions, with a fully quantum-mechanical approach. Two methods are presented. The first one is based on the determination of the molecular eigenstates, with expansion of the nuclear wavefunctions on a Hermite basis. The second method is based on the Padé 1,1 approximation of the time evolution operator and on a finite difference representation of the time-dependent nuclear wavefunctions. Both methods are applied to simple models of a diatomic molecule.