Molecular refraction additivity

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. 

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 ... 

Molecular refractivity. Molecular refractivity is an additive quantity which is often used to assist in determining molecular structures via the Lorentz-Lorentz equation. This generally gives good agreement between values derived from experimental measurements of density and refractive index and values... 

Thus Bruhl3 concluded, as the result of an investigation of the molecular refractivity of water, that water is an unsaturated compound containing tetravalent oxygen. This view is m entire harmony with the results described below, and supplies an explanation for the readiness of water to form additive compounds, especially with salts, m which case the added water is commonly known as water of crystallisation.4... 

Bennett and Mitchell found AEa an additive property of normal liquids, but this result is only another form of Kopp s law of additivity of molecular volumes ( 2.VIII B), or of the additivity of molecular refractions, since AEa is proportional to the electric polarisability, which is additive. According to Einstein, no heat content is to be ascribed to the surface film and the whole of its energy is potential. 

Organic compounds— For a large number of organic compounds it is found that the bond refraction is an additive quantity. As an example of this relationship between the number and type of bonds and the molecular refraction, Table LXXI gives the differences in the values of the molecular... 

The refraction of molecules may be calculated by the addition of the appropriate atomic refractions. It is necessary to add to the sum so obtained additional quantities corresponding to the multiplicity of the bonds involved. For a large number of molecules, the calculated and measured values of the molecular refraction agree, within the limits of experimental error. In molecules containing a conjugated system of double bonds, however, deviations from the additivity rule occur, the experimental value always being greater than the calculated. The difference, may... 

Any structural modification which tends to destroy coplanarity inhibits resonance. The introduction of alkyl groups ortho to a dialkyl-amino, an acyl, or a nitro group hinders the formation of a structure like XXXV, and as a result these substituted derivatives resemble their aliphatic analogs more closely. In addition to arguments based upon dipole moments,16 molecular refractivity,17 and ultraviolet absorption spectra,18 there is other chemical evidence which indicates the steric in-... 

The analysis of observed binding energies 13,14,15) and other properties (molecular refractivities, magnetic susceptibilities, etc... ) of saturated hydrocarbons shows that it is possible to reproduce the experimental data to a high degree of accuracy, by adding up a number of the C—C and C—H bonds. A simple additivity scheme giving bond... 

Rc=0 = O-S-ficarbon + ketonicoxygen and so on. The imperfect additivity of the earlier atomic and structural refractivities is, of course, carried over to bond refractivities by such derivations. The bond refractions of Denbigh or Vogel depend directly upon molecular refractions determined by experiment fundamentally the value found for the methylene group in a homologous series can be written as f CHa = i c—c + 2-RC—h and, correspondingly, the molecular refraction for any n-alkane is... 

Notwithstanding that negative refractions have no physical meaning, and that additivity is never perfect, the fact remains that bond refractions still have usefulness for the calculation of molecular refractions when structures are known for such purposes the few negative values may be viewed as empirical increments . Vogel s data, as summarized by Cresswell et al. (1952), covered thirty-five bonds between the atoms H, C, O, S, N, P, F, Cl, Br, and I. The information on the C—F linkage was annotated as preliminary, and details for many bonds of interest were omitted. 

Dispersive power is more constitutive than refractivity—a fact first recognized by Gladstone (1886, 1887) when investigating the quotients (w — nXl) d and M nXl — nX2)jd for additivity. Briihl (1891), using the Lorentz-Lorenz expression for the specific or molecular refractions, considerably extended the subject and prepared lists of atomic dispersions for the a and y hydrogen lines these were later revised by von Auwers and Eisenlohr. Tables such as those produced by the last-named author (1912, 1923) have always included values for Rp — Ra and Ry — Ra. Predicted dispersions are sometimes satisfactory when absorption wavelengths are well away from the visible region (e.g. from Table 2 Ry — Ra for acetyl chloride and pentyl alcohol are 0-44 and 0-64 cm3 the observed differences are 0 48 and 0-64 cm3 2 ... 

The constitutive corrections should also be taken into consideration, as is done for all kinds of additive properties (e.g., in the case of molecular refraction). The value Q cni = 14 kcal is suitable for reactions in which no double bond is formed and broken, for example, in ethane hydrogenolysis. The value Q=cni =27 kcal refers to C bound by the double bond with the adjacent atoms C, N, and 0 in the molecule both inside and outside the index. This value should be used for the hydrogenation of the double bond as well as for dehydrogenation because the reverse reaction passes through the same intermediate state as the direct one. It is quite feasible that in the next approximation similar corrections for the double bond also for N, 0, and S atoms should be introduced. 

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