Eolo Scrocco

Electronic Molecular Structure, Reactivity and Intermolecular Forces: An Euristic Interpretation by Means of Electrostatic Molecular Potentials

Publisher Summary This chapter discusses a model that relies on the knowledge of the molecular electrostatic potential, which is derived from a molecular wavefunction by using the usual methods for calculating the mean expectation value of an operator. In its basic premises the model employs quantum mechanics, with only the approximations necessary in molecular quantal calculations. The model is also discussed regarding its relationships with the Hellmann–Feynman theorem. The electrostatic potential V itself is examined in order to show how the electrostatic potential reflects the characteristics of the electronic distribution of a molecule and then the reliability of V is discussed as a reactivity index. The shape of the electrostatic potential and its relationship to the electronic molecular structure is discussed with the aid of various examples. One of them includes the glycine tautomers and the corresponding anion example. The chapter also discusses the electrostatic molecular potential in terms of local group contributions.

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.

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.

Ab initio molecular electrostatic potentials

The ab initio isopotential map of guanine is given and compared to that of adenine.It shows that in contrast to the situation in adenine, the most basic site of guanine is N7 with a secondary potential minimum at O6. These results as well as those concerning the secondary out-of-plane attractive regions over the NH2 group and C8 H bonds of the two molecules are discussed in connection with the available experimental knowledge concerning the bonding of alkylating carcinogens and mutagens.

“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.

Minimal‐Basis‐Set LCAO–SCF–MO Calculations for the Ground State of O3, NO2–, NOF, and OF2 Molecules

Ground‐state wavefunctions for the nonlinear triatomic molecules O3,NO2–, NOF, and OF2 are computed. The calculation was performed in two versions both using the minimal set of STOs in the LCAO–SCF–MO approximation and differing only in the choice of the orbital exponents. The results of a population analysis are also presented.

Double‐ζ LCAO SCF MO Calculations for NO2 and OF2

The ground‐state wavefunctions for NO2− and OF2 are obtained by using the free‐atom‐ζ double‐ζ STO set in the LCAO SCF MO approximation. Comparisons are made with the corresponding minimal STO set SCF wavefunctions and, for OF2, also with a Gaussian set SCF wavefunction.

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 ϱM of the sole M. The first term, qV(r), represents the interaction of q with the undistorted charge ϱM0 of M while the second q2P(r) gives the additional contributions due to the polarization of ϱM0 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.

Multipole expansions of the electrostatic molecular potential

Multipole expansions of the electrostatic molecular potential up to the hexadecapole terms are examined for H2O, NH3 and C2H4NH. A reasonable approximation to get unexpensive first order representations of the electrostatic potential for regions outside the van der Waals volume is found.