Peter Politzer

On the problem of defining the charge on an atom in a molecule

Abstract It is shown that the Mulliken and the Lowdin definitions of atomic charge lead to significantly different wave functions and physical interpretations, when they are used as criteria of self-consistency in computing the wave functions.

On the validity of the core-point-charge approximation for inner subshells of atoms

Abstract In order to test the validity of approximating the effects of various atomic inner subshells by means of point charges at the nuclei, the electron repulsion and exchange integrals involving these inner electrons and valence electrons were computed for a series of atoms. The point-charge approximation was found to be very poor for the 3s, and 3p subshells of the first-row transition elements.

Properties of atoms in molecules. The position of the center of electronic charge of an atom in a molecule

Abstract The positions of the effective centers of electronic charge have been calculated for the atoms in a group of homonuclear diatomic molecules. For all of the molecules studied except Li 2 , these centers of charge were found to be outside the internuclear region.

An investigation of the high-frequency form of carbon monoxide chemisorbed on nickel oxide

Abstract Semi-empirical calculations have been carried out with the aim of achieving a better understanding of the CO-NiO interaction which results in an absorption band at a frequency higher than that of gaseous CO. It was concluded that the adsorbed form responsible for this band bonds through the carbon to a nickel ion of the oxide. Rearrangements of both σ and π electronic charge occur which strengthen the C-O bond and increase its vibration frequency (as measured by the C-O overlap population). Charge transfer takes place between the CO and the nickel ion, σ charge shifting to the ion and π charge to the CO. In the cases of Ni (II) and Ni (III), the σ shift considerably outweighs the π, resulting in net transfers of approximately 0.3 electrons from the CO to the Ni (II) and 0.6 electrons from the CO to the Ni(III).

On the electronic density distribution in diborane

Abstract A study has been made of the effects of including hydrogen 2p basis orbitals in computing semi-empirical wave functions for diborane. The charges predicted by the Mulliken and the Lowdin definitions are compared with the electronic density distributions computed from the wave functions.

Properties of atoms in molecules. a proposed method for calculating the extent of distortion of an atom

Abstract A procedure for calculating the extent of distortion of an atom in a molecule is proposed and applied to the first-and second-row diatomic hydrides. The atomic charges in these molecules were also computed.

Energy calculations with the extended-hückel method

Abstract The reliability of various modifications of the extended-Huckel method in calculating several energy-related quantities was investigated for a group of diatomic molecules. It was found that the total energy can be computed fairly accurately, the accuracy increasing as the charge distribution is more uniform. Relatively good estimates of dissociation energies can also be obtained, using the molecular orbital energies and the elements of the H matrix used to compute the molecular wavefunction.

A charge-transfer interpretation of the interactions of hemoglobin with oxygen and carbon monoxide.

It is shown that the use of the charge-transfer theory to describe the interaction between hemoglobin and O2 or CO leads to the prediction of a linear relationship between log K and the quantity (λαHbO2 − λαHbC), where K is the equilibrium constant of the reaction HbO2 + CO af HbCO + O2 and λαHbO2 and λαHbCO are the wavelengths of the so-called α-bands in the absorption spectra of oxyhemoglobin and carboxyhemoglobin. Such a linear relation was long ago observed experimentally for a number of vertebrate hemoglobins, but was never satisfactorily explained. The fact that the charge-transfer theory does explain this seemingly unlikely experimental relationship provides support for the view that the interaction between hemoglobin and O2 or CO may be regarded and treated as a charge-transfer process.

Orbital sizes and dipole moments in diatomic hydrides

Abstract It is suggested that the dipole moment of A1H (recently calculated) may be anomalous, due to an unusual degree of penetration of the aluminum outer shell by the hydrogen. It is shown that the sizes of the outer shells of Mg, Al, Tl, and Pb deviate from general trends.