Rosanna Bonaccorsi

Molecular SCF Calculations for the Ground State of Some Three‐Membered Ring Molecules: (CH2)3, (CH2)2NH, (CH2)2NH2+, (CH2)2O, (CH2)2S, (CH)2CH2, and N2CH2

LCAO SCF MO calculations with minimum basis sets of Slater‐type orbitals are performed for some three‐membered ring compounds. A reinterpretation of the resulting wavefunctions in terms of localized (exclusive) orbitals is presented and discussed: The bent bonds in the rings are closely evidenced. The electrostatic potentials produced in the neighboring space by the nuclear and electronic charge distributions are evaluated and employed to evidence the molecular sites more likely subject to electrophilic attacks.

Theoretical chemistry in solution. Some results and perspectives of the continuum methods and in particular of the polarizable continuum model

Abstract A concise overview of the theoretical methods based on a continuous distribution of solvent molecules (the continuum dielectric model is the most important example) is accompanied by the examination of a few subjects, selected as examples of the recent developments of the method. The subjects are: (1) electron correlation and dispersion; (2) optimization of the solute geometry; and (3) boundary surfaces in solutions.

N- versus O-proton affinities of the amide group: Ab initio electrostatic molecular potentials

Abstract The electrostatic molecular potentials arising from ab initio LCAO SCF wavefunctions with two different gaussian basis sets are used to discuss the protonation of formamide. Oxygen protonation is clearly favoured.

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.ZusammenfassungEin 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.

On the free energy changes of a solution in light absorption or emission processes

Abstract A simple model to evaluate energy changes in dilute solutions produced by absorption or emission of light is proposed. The computational scheme exploits some characteristics of the continuum model of liquids and is inserted into ab initio quantum-mechanical programs for isolated molecules. The model is addressed to evaluate changes in thermodynamic properties of the whole solution — while earlier models often consider the solute only — and it presents a detailed decomposition of the phenomenon into successive steps.

Counterpoise corrections to the interaction energy components in bimolecular complexes

A new definition of counterpoise corrections (CP) to the components of ΔE evaluated with the Kitaura and Morokuma method is presented and tested on the (H2O)2 and (NH3·HF) molecular systems with two different basis sets. The CP corrections are further decomposed into subsystems contributions, facilitating their interpretation and the elaboration of computationally simplified procedures.

“Ab Initio” Calculation of the Quadrupole Coupling Constant of 14N in HCN, FCN, ClCN, HC2CN, CN−, OCN−, SCN− and the Examination of the Townes–Dailey Interpretation in Terms of Exclusive Orbitals

The electric field gradient at 14N nucleus in some RCN molecules or RCN− ions has been calculated using the accurate SCF wavefunctions obtained by McLean and Yoshimine. From these computed values and the experimentally known 14N quadrupole coupling constant of four molecules of this series, a value of 1.66e × 10−26cm2 for the quadrupole moment of 14N nucleus has been estimated. A check for the semi‐empirical interpretative theory of Townes and Dailey is made in terms of localized orbitals (Boys exclusive orbitals). Some other characteristics of these exclusive orbitals are also displayed.

Theoretical study of reaction mechanisms for the ketonization of vinyl alcohol in gas phase and aqueous solution

Theoretical ab initio calculations are done on different mechanisms for the conversion of vinyl alcohol to acetaldehyde, both in gas phase and in solution. Several basis sets are used in order to assess the accuracy of the results in gas phase and a continuum model of the solvent is employed to mimic reactions in water solution. The results indicate a catalytic action of water in hydrated clusters in gas phase, whereas in solution, and within the error limits of our calculations, both neutral water-chain and ionic mechanisms appear to be equally probable. Finally, the action of acids or bases is tested through the analysis of the reaction of vinyl alcohol with H3O+ and HO−. The results of the calculations are shown to be in qualitative agreement with the experimental facts when 6-31++G basis set is used but not when either STO-3G or 4-31G basis sets are employed.

The influence of the basis set on the evaluation of conformational energies for small organic solutes in aqueous solutions

Abstract Conformational energies of a small set of double rotor molecules (CH 3 CH 2 OH, CH 3 CHFOH(S), CH 2 FCH 2 OH, NH 2 CH 2 OH, CH 3 CH 2 OF) computed in vacuo and in aqueous solution with three basis sets (STO-3G, 3-21G, 4-31G) are compared in order to examine the basis set dependence of the results. Solvent effects are introduced with an SCF algorithm which relies on a continuum description of the solvent. The interaction operator depends on the description of the solute charge distribution and, consequently, on the basis set employed in the calculations.

Semiclassical models in theoretical chemistry. Some results and future prospects

Abstract We present and justify the use of a set of related semiclassical models in the interpretation and prediction of a wide variety of molecular interaction phenomena. The semiclassical model uses ab initio quantum mechanical descriptions of the charge distribution of molecular subunits, interacting according to classical rules. The considered interaction phenomena mainly regard chemical processes (formation of bound aggregates, disruption and formation of covalent bonds) viewed under the energetic as well as the structural point of view, and solvent effects on the solute properties (conformation, reactivity, expectation value of physical observables), as examples of a more general interpretation scheme. Attention is paid to the transferability properties of subunits, and to the description of non-linear effects induced by the specific field acting on these subunits in the various situations. The paper summarizes results obtained in the preceding years, with the addition of fresh numerical data referred to the 3-21G basis set, and presents some new applications among which we signal a simplified procedure to compute the electrostatic contribution to the solvation free energy.