Local ionization potential

In the spirit of Koopmans theorem, the local ionization potential, IPi, at a point in space near a molecule is defined [46] as in Eq. (54), where HOMO is the highest occupied MO, p( is the electron density due to MO i at the point being considered, and ej is the eigenvalue of MO i. 

This quantity is found to be related to the local polarization energy and is complementary to the MEP at the same point in space, making it a potentially very useful descriptor. Reported studies on local ionization potentials have been based on HF ab-initio calculations. However, they could equally well use semi-empirical methods, especially because these are parameterized to give accurate Koopmans theorem ionization potentials. 

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Wavelet descriptors that describe the same basic value as DGNH6. Wavelet descriptors that describe the same basic value as DGNH6. Wavelet descriptors that describe the same basic value as DGNH6. Descriptor that conveys information about the local ionization potential of the molecule. 

Descriptor that conveys information about the local ionization potential of the molecule. 

This chapter introduces a number of useful graphical models, including molecular orbitals, electron densities, spin densities, electrostatic potentials and local ionization potentials, and relates these models both to molecular size and shape and molecular charge distributions. The chapter also introduces and illustrates property maps which simultaneously depict molecular size and shape in addition to a molecular property. Properties include the electrostatic potential, the value of the LUMO, the local ionization potential and the spin density. 

Among the quantities which have proven of value as graphical models are the molecular orbitals, the electron density, the spin density (for radicals and other molecules with unpaired electrons), the electrostatic potential and the local ionization potential. These may all be expressed as three-dimensional functions of the coordinates. One way to display them on a two-dimensional video screen (or on a printed page) is to define a surface of constant value, a so-called isovalue surface or, more simply, isosurface. ... 

This chapter introduces and illustrates isosurface displays of molecular orbitals, electron and spin densities, electrostatic potentials and local ionization potentials, as well as maps of the lowest-unoccupied molecular orbital, the electrostatic and local ionization potentials and the spin density (on top of electron density surfaces). Applications of these models to the description of molecular properties and chemical reactivity and selectivity are provided in Chapter 19 of this guide. 

Another quantity of some utility is the so-called local ionization potential, I(r). This is defined as the sum over orbital electron densities, pi(r) times absolute orbital energies, e i, and divided by the total electron density, p(r). 

The local ionization potential is intended to reflect the relative ease of electron removal ( ionization ) at any location around a molecule. For example, a surface of low local ionization potential for sulfur tetrafluoride demarks the areas which are most easily ionized. 

A more important application of the local ionization potential is as an alternative to the electrostatic potential as a graphical indicator of electrophilic reactivity. This is in terms of a property map rather than as an isosurface. Further discussion is provided later in this chapter. 

The local ionization potential does not fall to zero with increasing distance from the molecule. 

Mapping the local ionization potential onto a size surface reveals those regions from which electrons are most easily ionized. 

Such a representation is referred to as a local ionization potential map. Local ionization potential maps provide an alternative to electrostatic potential maps for revealing sites which may be particularly susceptible to electrophilic attack. For example, local ionization potential maps show both the positional selectivity in electrophilic aromatic substitution (NH2 directs ortho para, and NO2 directs meta), and the fact that TC-donor groups (NH2) activate benzene while electron-withdrawing groups (NO2) deactivate benzene. 

The author wishes to thank Dr. Denton Hoyer at Pfizer for pointing ont the ntility of local ionization potential maps for this purpose. 

Local Ionization Potential. A function of the relative ease of electron removal (ionization) from a molecule. 

Local Ionization Potential Map. A graph of the Local Ionization Potential on an Isodensity Surface corresponding to a van der Waals Surface. 

Graphical Models are introduced and illustrated in Chapter 4. Among other quantities, these include models for presentation and interpretation of electron distributions and electrostatic potentials as well as for the molecular orbitals themselves. Property maps, which typically combine the electron density (representing overall molecular size and shape) with the electrostatic potential, the local ionization potential, the spin density, or with the value of a particular molecular orbital (representing a property or a reactivity index where it can be accessed) are introduced and illustrated. 

In particular, combinations of PARASURF descriptors with molecular surface-based descriptors were of interest. Descriptors like the local ionization potential or the local electron affinity, provided by PARASURF, should capture aspects of chemical reactivity, while surface or pharmacophore-related descriptors could have the potential to describe the recognition of the molecule by cytochromes. Hence, various descriptor sets were combined with quantum-chanical descriptors from PARASURF yielding improved models. For example, the sole set of 2D-MOE descriptors shows an f of 0.6 for the training set, while rises to 0.68 for the MOE/PARASURF combinations. 

Murray, Abu-Awwad, and Politzer computed the average local ionization energy and electrostatic potentials on the surfaces of nine aromatic hydrocarbons.This average local ionization potential, originally defined in the framework of Hartree-Fock theory, is given as... 

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