Subject semi-empirical molecular

In the period under review both ab initio and semi-empirical molecular orbital calculations of the electric field gradients at 14N nuclei have been reported. Pyrazole and imidazole have been the subjects of an ab initio approach using double zeta functions. (185) The calculated values of x an<3 7 are 111 g°°d agreement with experimental results obtained from microwave studies. The agreement is considerably better... 

The aim of this chapter is to review the current status of the quantum-mechanical calculation of electric and magnetic properties of isolated atoms and molecules. In view of the rapid advances made during the past decade in the calculation of ab initio molecular wavefunctions, we will clearly concentrate for the most part on the calculation of such properties using standard ab initio methods such as gaussian orbital LCAO-MO-SCF (linear combination of atomic orbital-miolecular orbital-self-consistent field), configuration interaction (Cl), coupled Hartree-Fock, and the like, but will also review similar calculations at the semi-empirical and empirical level where appropriate. For readers unfamiliar with the theory of electric and magnetic properties, the books by Davies and by Atkins review the subject thoroughly, whilst the more technical details of quantum-mechanical calculations on atoms and molecules have been described in many other places. ... 

The calculation methods for pore distribution in the microporous domain are still the subject of numerous disputes with various opposing schools of thought , particularly with regard to the nature of the adsorbed phase in micropores. In fact, the adsorbate-adsorbent interactions in these types of solids are such that the adsorbate no longer has the properties of the liquid phase, particularly in terms of density, rendering the capillary condensation theory and Kelvin s equation inadequate. The micropore domain (0.1 to several nm) corresponds to molecular sizes and is thus especially important for current preoccupations (zeolites, new specialised aluminas). Unfortunately, current routine techniques are insufficient to cover this domain both in terms of the accuracy of measurement (very low pressure and temperature gas-solid isotherms) and their geometrical interpretation (insufficiency of semi-empirical models such as BET, BJH, Horvath-Kawazoe, Dubinin Radushkevich. etc.). 

In such studies the form of the N=XY fragments was also subjected to refinement It has been shown that deviations from linear structures, if they occur, should not exceed 10°. However, even such a possibility makes the choice of molecular models quite a complicated problem. The electron diffraction data have been found to be consistent with two sets of parameters for the C1N=C=0 molecule, depending on the assumption of linear or bent geometry for the N=C=0 group. The authors of prefer the bent model since it gives a better fit with the microwave data and the results of semi-empirical CNDO/2 calculations. 

Photosynthetic water oxidation to dioxygen takes place in a Mn-containing protein in PSH via Kok s intermediates, labeled Sq-S [1]. These S-states probably reflect different redox states of some or all of four Mn ions involved in the enz3rme-substrate(H20) complex. The molecular structure and oxidation states of this Mn-cluster in each S-state has been the subject of considerable recent spectroscopic studies involving EPR[2-5]and X-ray absorption[6,7]. In this paper, we present high-quality Mn K-edge XANES spectra measured for spinach PSH membranes in Si and S2 states, which exhibit informative pre-edge feature due to Is to 3d transition as well as four fine structures superimposed on the principal absorption band. These features are semi-empirically analized in comparison with those of authentic Mn complexes. Based on the results obtained, we propose a hypothetical model for the Mn-cluster which is compatible with EPR data[2-5] in S2-state. 

For the simulation of more complex flows, one needs a constitutive equation or a rheological equation of state. Nearly all of the many equations that have been proposed over the past fifty years are basically empirical in nature, and only in the last twenty-five years have such models been developed on the basis of mean field molecular theories, e.g., tube models. Although the early models were often developed with a molecular viewpoint in mind, it is best to think of them as continuum models or semi-empirical models. The relaxation mechanisms invoked were crude, involving concepts such as network rupture or anisotropic friction without the molecular detail required to predict a priori the dependence of viscoelastic behavior on molecular structure. While these lack a firm molecular basis and thus do not have universal validity or predictive capability, they have been useful in the interpretation of experimental data. In more recent times, constitutive equations have been derived from mean field models of molecular behavior, and these are described in Chapter 11. We describe in this section a few constitutive equations that have proven useful in one or another way. More complete treatments of this subject are given by Larson [7] and by Bird et al. [8]. 

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