Stewart atomic charges

As to the direction of the dipole moments in the ground state of the molecules, two kinds of experimental measurements are available. Stewart has calculated the X-ray dipole moment of uracil within the point-charge approximation using the atomic charges for a standard STO L-shell. The calculated dipole moment is 4.0 1.3 D and its direction of 71° + 12° from N-l-C-4 toward N-3 atom. These values obtained within a point-charge model have an estimated standard error of about 30%. Nevertheless, the X-ray dipole moment is of reasonable magnitude and in close agreement with the value of 4.1 D obtained by solution measurement of uracil in dioxan (Table XX). 

When doing the molecular fitting of ASA coefficients compared with promolecular ASA, the ASA coefficients no longer sum to 1 for all atoms in the molecule, because the sum of the coefficients reflects the effect of polarization. In other words, rather than having Pa = Za in promolecular ASA, the values of Pa Za in fitted ASA reflect the internal electron density redistribution. In such a way, fitted ASA coefficients are reminiscent of the Stewart atoms idea, where atoms in molecules are considered as radially distorted atoms and are connected with so-called Stewart charges. Contrary to common promolecular ASA where the atomic densities of Eq. [41] integrate to Za ... 

Quantum-chemical methods (Dewar and Thiel, 1977 Dewar et al, 1985 Stewart, 1989) may be applied to obtain a large variety of stereo-electronic descriptors such as ionization potential, polarizability, electron affinity, dipole moment, charge densities, HOMO- and LUMO-energies or atomic charges using semi-empirical or ab initio calculations (Table 1.4). 

Fig. 12. Total net charges in uracil calculated by different methods (from top to bottom w-HMO + o-Del Re, tt-SCF MO + a-Del Re, EHT, lEHT, CNDO/2, nonempirical). Data taken from Pullman and Pullman and from Stewart (nonempirical). Experimental net gross valence atomic populations for uracil are underlined [two sets of experimental numbers were obtained by the use of the L-shell standard STO s and L-shell SCF AO s scattering factors (in brackets), respectively]. The estimated standard deviations referring to the last decimal place are given in parentheses.

A. O. Madsen, H.O. Sorensen, C. Flesburg, R. F. Stewart and S. Larsen, Modeling of the nuclear parameters of H atoms in X-ray charge density studies, Acta Cryst. A60, 550-561 (2004). 

M. Yanez, R. F. Stewart, and J. A. Pople, Acta Crystallogr., Sect. A, 34, 641 (1978). The Projection of Molecular Charge Density into Spherical Atoms. I. Density Basis Functions for First-Row Atoms. 

Finally, we would like to mention a derivative of OG spectroscopy, which is conducted not in a discharge cell but in a (hot) flame the technique is often referred to as laser-enhanced ionization (LEI). The technique is used for sensitive detection of trace atoms and molecules. The excitation of the species under investigation populates high-lying energy levels the thermal heat of the hot flame is sufficient to ionize the species out of their excited levels. Electrodes placed around the flame detect the charge carriers generated in this way. Further details on the principles and applications of OG spectroscopy and LEI can be found, for example, in Stewart et al. (1989). 

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