Refractivity, atomic molecular

The shift of fringes at any wave-length together with a knowledge of the concentration of atomic hydrogen and the index of refraction of molecular hydrogen serves to determine the dispersion of atomic hydrogen. 

In atomic-molecular media the damping of plasmon states is due to the interaction of plasmon waves with electrons, lattice vibrations, and impurities. The electron-plasmon interaction is a long-range one. With absorption of a plasmon, the momentum q is transferred to the electron, resulting in a decay of the collective state into a single-particle one. The latter process is identical with absorption of a photon with the same energy. Wolff102 (see also Ref. 103) has shown that in this case the lifetime can be expressed in terms of two optical constants the absorption coefficient k and the refractive index nT, namely,... 

ALOGP, Ghose-Crippen-Viswanadhan log P AMR, Ghose-Crippen-Viswanadhan molar refractivity MW, molecular weight A, number of atoms P%, the percentage of covering. 

Pi, to O Such relationships are called additivity laws, and are fundamental in many areas of physical chemistry. An additive property is one whose value is determined by adding up the contributions from smaller units. An almost trivial example (though it wasn t a century and a half ago) is the relationship between the molecular weight of a compound and the atomic weights of its constituent atoms. Molecular weights and molecular volumes are additive melting points or refractive indexes are not. 

The descriptor uses readily calculable physicochemical properties from the topological structure. The descriptors used in this study were atomic weight, hydropho-bicity, molecular refractivity, atomic charge, polar surface area, hydrogen bond acceptors, and hydrogen bond donors. The authors note that Martin et al. [32] applied a similar approach for the design of diverse combinatorial libraries. 

The simplest structural descriptors are just the number of carbon atoms, or molecular weight, that is linearly related to the increase in gas chromatographic retention among a homologous series of compounds. This simple relationship led to the development of the Kovits retention index scale. A linear relationship between Kovits retention index and carbon number for homo-logues has been shown to hold for a number of chemical groups. Molar volume, molar refractivity, and molecular polarizability are other simple descriptors of molecular bulk that have been used in QSRR studies. 

The RID profile can be controlled by conduction of simultaneous diffusion of several monomers copolymerized with the matrix, which possess different atomic refractions and molecular masses ... 

The next step towards increasing the accuracy in estimating molecular properties is to use different contributions for atoms in different hybridi2ation states. This simple extension is sufficient to reproduce mean molecular polarizabilities to within 1-3 % of the experimental value. The estimation of mean molecular polarizabilities from atomic refractions has a long history, dating back to around 1911 [7], Miller and Sav-chik were the first to propose a method that considered atom hybridization in which each atom is characterized by its state of atomic hybridization [8]. They derived a formula for calculating these contributions on the basis of a theoretical interpretation of variational perturbation results and on the basis of molecular orbital theory. 

Here, k is a factor which converts to units (kcal/mol in this case where the distances are in A and the polarisabilities in A ). G, and Gj are constants chosen to reproduce the well depths for like-with-like interactions. The atomic polarisability values are obtained from an examination of appropriate molecular experimental data (such as measurements of molar refractivity). 

Uric acid is odourless in spite of three carbonyl groups, four trivalent nitrogen atoms and a double bond, and that it is similarly colourless in spite of four chromophores. Measurements of its refractive and dispersive properties indicate that it is a saturated body which suggests that molecular attraction exists between the various groups. 

The molecular refraction is a constant frequently quoted for individual chemical compounds, and is of considerable value as evidence of constitution, since it is generally true that the molecular refraction of a compound is composed additively of the refractive powers of the atoms contained in the-mmolecular refraction is the value obtained by multiplying the refractive power by the molecular weight. 

To indicate the value of this constant in deciding the constitution of a compound, the case of geraniol, Cj HigO, may be examined. Calculated from the above values the molecular refraction would be the Sum of the atomic refractions, as follows — ... 

But by experimental determination the molecular refraction is found to be 48 71, which is 3-235 in excess of the value calculated from the atomic refractions. Two double bonds between carbon atoms would account for 3-414 in excess, so that it is evident that geraniol contains two such double linkages. No alcohol of the formula Cj HjgO with two double linkages can contain a ring, so that geraniol must belong to the open chain series, a conclusion entirely supported by its chemical characters. 

With the development of accurate computational methods for generating 3D conformations of chemical structures, QSAR approaches that employ 3D descriptors have been developed to address the problems of 2D QSAR techniques, that is, their inability to distinguish stereoisomers. Examples of 3D QSAR include molecular shape analysis (MSA) [26], distance geometry,and Voronoi techniques [27]. The MSA method utilizes shape descriptors and MLR analysis, whereas the other two approaches apply atomic refractivity as structural descriptor and the solution of mathematical inequalities to obtain the quantitative relationships. These methods have been applied to study structure-activity relationships of many data sets by Hopfinger and Crippen, respectively. Perhaps the most popular example of the 3D QSAR is the com-... 

Electromagnetic radiation has its origins in atomic and molecular processes. Experiments demonstrating reflection, refraction, diffraction and interference phenomena show that the radiation has wave-like characteristics, while its emission and absorption are better explained in terms of a particulate or quantum nature. Although its properties and behaviour can be expressed mathematically, the exact nature of the radiation remains unknown. 

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