Molar atomic refractions

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 ... 

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. 

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-... 

Consider 1-methylnaphthalene and 1-methoxynaphthalene, which differ by a single oxygen atom. The molar refractions, 48.8 cm3 and 55.4 crp, respectively, differ by 6.6 crn, while the normal atomic refraction of an ether oxygen is only 1.6 cm. ... 

An alternative to improving atomic/ionic refractions was to express molecular refraction through bond increments. A system of bond refractions is definitely superior to the system of atomic refractions, as it allows to account for chemical interactions explicitly. The concept of bond refraction was introduced by Bachinskii [183] who suggested that the molar refraction (as well as volumes, heats of combustion, etc) of organic compounds can be calculated of bond increments. According to Bachinskii, Rc-c =l/4f c + l/4 c. = l/47Jc + etc. This method is not quite con-... 

The formation heats of the organic compounds correlate with differences between experimental molar refractions and sums of the corresponding atomic refractions (AR) [198]. Hohm [199] obtained a similar relationship between the atomization energy of inorganic compounds and AR /. However, this correlation can be used only... 

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). 

From the atomic and group refractions in Table 5.19, the molar refraction is computed as follows ... 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

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