Refraction, atomic

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. 

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. 

Atom-kem, m. atomic nucleus, -kette,/. chain of atoms, atomic chain, -lage, /. atomic layer atomic position, -lehre, /, doctrine of atoms, atomic theory, -mechanik, /. mechanics of the atom, -modell, n, atomic model, -nummer, /, atomic number, -ord-nung, /. atomic arrangement, -refraktion, /. atomic refraction, -rest, m. atomic residue (= Atomrumpf). -ring, m. ring of atoms, -rumpf, m. atomic residue or core (remainder of an atom, as after removal of some electrons), -schale, /, atomic shell, -strabl, m. atomic ray, -tafel, /, atomic table, atomtbeoretisch, a. of or according to the atomic theory,... 

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-... 

Atomic Refractivities Atomic refractivities may be attributed by virtue of ... 

A few representative atomic refractivities and bond contributions are given in Table 18.1 below ... 

Based on the atomic refractivities given in Table 18.1, it may be possible to calculate the molar refractivities of various pharmaceutical substances theoretically and compare the same with values found experimentally. A few typical examples are cited below ... 

The index of refraction of bromine gas at 0° and 760 mm. for the /Mine is 1 001132 according to E. Mascart. The atomic refraction of liquid bromine, according to J. H. Gladstone, is 15 3 and, according to J. W. Briihl, 8 455 by the /t2-formula. The specific refraction, according to A. Haagen, is 01918. The refractive indices of liquid bromine, selected from measurements by C. Riviere, for rays of different wave lengths, at different temp, as indicated in Table I. These data show that the... 

According to F. P. le Roux, like all vapours with a large selective absorption, iodine has an anomalous dispersion since it increases with a fall of temp., being about 0 06 from A. Hurion s measurements—approximately as large a negative number as glass is positive. The atomic refraction of solid iodine is 24-5 by the //.-formula, and 14-12 by the /t2-formula. 

MR (calc.) Molal refraction calculated from atomic refractive indices. See page 8. 

Atomic Refractive Indices Used for Computing Molecular Refractive Index... 

From the determination of the molecular refractions of a large number of organic compounds containing tervalent arsenic, the atomic refraction of arsenic in each compound has been calculated,10 the values obtained varying from 9-2 to 14-39. Hydrogen, chlorine and alkyl groups in an arsine exert about the same influence on the atomic refraction of arsenic, but replacement of any of these by aryl groups causes an increase in the atomic refraction. The opposite effect results from substitution by a cyanide, oxalate or alkoxyl radical. 

Atomic refraction is the product of the specific refraction of an element by its atomic weight. 

The molecular refraction rmol is an additive function, i.e. it is a function which is equal to the sum of the atomic refraction of carbon, rCi and that of hydrogen, rn, each multiplied by its number of atoms in the average" molecule ... 

Amino Acids. The values for the refractive indices and molar refractions of the amino acids calculated from the refractive indices by Lorenz-Lorentz Equations 1 and 2 are recorded in Table I. Values for molar refractions of the aliphatic amino acids are in good agreement with values calculated from atomic refraction factors. However, the molar refractions of tryptophan, tyrosine, phenylalanine, and histidine are larger than those calculated from atomic refraction factors and larger than might be expected from their comparative specific volumes. 

The atomic refraction shows considerable variability with constitutive influences, whether it is determined by Gladstone s formula... 

Note.—A= atomic weight, nj=refraetivity according to Gladstone s formula, n, = refractivity according to Lorentz and Lorenz s formula.) The atomic refraction ArG of the element is 18-68 (solid), 18-89 (liquid),4 or 18-69 (mean of solid and liquid),5 while the value Arn was 9-10 (mean of solid and liquid).5... 

Atomic refractivities of phosphorus in its compounds are calculated as the difference between the molar refractivities and the sums of the refractivities of the other atoms. They vary according to the structure assigned, namely, whether oxygen is to be considered as singly- or doubly-linked. Molar refractivities appear to be affected by con-... 

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