Ionization potential, semiempirical molecular

The He photoelectron spectra of thiazole, benzothiazole, and naphtho[l,2-<( thiazoles have been reported <93JST(296)115>. This study confirms most of earlier results <84CHEC-I(4)240>. The assignments of ionization potentials to molecular orbitals have been made on the basis of semiempirical SCF MO calculations. 

A multitude of semiempirical and semiclassical theories have been developed to calculate electron impact ionization cross sections of atoms and atomic ions, with relatively few for the more complicated case of molecular electron impact ionization cross sections. One of the earlier treatments of molecular targets was that of Jain and Khare.38 Two of the more successful recent approaches are the method proposed by Deutsch and Mark and coworkers12-14 and the binary-encounter Bethe method developed by Kim and Rudd.15,16 The observation of a strong correlation between the maximum in the ionization efficiency curve and the polarizability of the target resulted in the semiempirical polarizability model which depends only on the polarizability, ionization potential, and maximum electron impact ionization cross section of the target molecule.39,40 These and other methods will be considered in detail below. 

A critical comparison between experiment and theory is hindered by the range of experimental values reported in the literature for each molecule. This reflects the difficulty in the measurement of absolute ionization cross sections and justifies attempts to develop reliable semiempirical methods, such as the polarizability equation, for estimating the molecular ionization cross sections which have not been measured or for which only single values have been reported. The polarizability model predicts a linear relationship between the ionization cross section and the square root of the ratio of the volume polarizability to the ionization potential. Plots of this function against experimental values for ionization cross sections for atoms are shown in Figure 7 and for molecules in Figure 8. The equations determined... 

Semiempirical methods, on the other hand, utilize minimum basis sets to speed up computations, and the loss in rigor is compensated by the use of experimental data to reproduce important chemical properties, such as the heats of formation, molecular geometries, dipole moments, and ionization potentials (Dewar, 1976 Stewart, 1989a). As a result of their computational simplicity and their chemically useful accuracy, semiempirical methods are widely used, especially when large molecules are involved (see, for example, Stewart, 1989b Dewar et al., 1985 Dewar, 1975). 

Empirical, semiempirical, and ab initio methods have been used extensively to calculate molecular descriptors. These molecular property descriptors help capture important characteristics of compounds such as bioavailability and receptor affinity. Descriptors such as octanol-water partition coefficient (log P), HOMO/LUMO energies, hammett a, total energy, heats of formation, ionization potential, atomic charges, electron densities, dipole/quadrupole moments, volume, and polar surface area are common examples. For an excellent review of physicochemical descriptors, the reader is directed to the following reference. ... 

Based on semiempirical SCF MO calculations, the ionization potentials of thiazole and several benzothiazole derivatives obtained by photoelectron spectroscopy, have been assigned to molecular orbitals <93JST(296)115>. 

A semiempirical method only gives accurate results for molecular properties when the parameters for all elements and combinations of elements in a specific compound have been optimized before. A parameter optimization is performed on a set i) of test molecules for which reliable experimental reference values /,rel are available for the properties of interest. In general, these are heats of formation AfH, bond lengths R, angles 0, torsion angles ionization potentials IP, and dipole moments /t. For this... 

Ionization potentials of perfiuorinated diamines were calculated as energies of the highest occupied molecular orbitals using the MNDO-PM3 semiempirical molecular orbital approximation (26). Calculations were performed with MOPAC Ver.6 (27) program with a Sony News-830 work station. Bond lengths, bond angles, and edral angles were fully optimized within the MNDO-PM3 framework. 

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