Additivity of Molecular Refraction

By starting from such values, and reversing the arguments through which they were obtained, a way of solving problems of molecular constitution was opened. As the approach involved no destruction of materials or disturbance of equilibria, molecular refractions quickly became extensively applied to a variety of structural questions, particularly to those difficultly resolved by the ordinary chemical techniques of the time, e.g. the positions of double bonds in terpenes, the recognition of 

His constants were revised and extended by von Auwers and Eisenlohr from 1910 onward Table 2 is an extract from Eisenlohr s 1923 data. Although the amendments to Briihl s figures appear slight it is important to remember that the R s shown are mean values. 

Notwithstanding these and other objections the fact remains that tables such as Table 2 have an empirical usefulness for the testing of, 

Briihl (1886a) had noted that the effect of unsaturation on molecular refraction could not always be represented by the increments previously deduced two olefinic bonds when situated conjugatively increased the refraction abnormally, the difference between observed and calculated R s being regarded as optical anomaly . In 1907 he introduced the terms optical exaltation and optical depression to refer respectively to cases where the observed R exceeded or fell short of that calculated numerically, depressions are commonly small but exaltations may be very large (see Smiles, 1910 von Auwers, 1924)  

The occurrence of positive exaltation has been frequently cited when fixing the relative positions of C=C and C=0 units in structures containing two or more of these bonds many examples are to be found in terpene chemistry (cf. Semmler, 1906 Gildemeister and Hoffman, 1928-31 or Simonsen, 1947-9). 

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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. 

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. 

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. 

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. 

The responses of the usual concentration detectors may be influenced by the copolymer composition as well as the concentration. An RI detector is a concentration detector, and a response obtained from it must be corrected before the calculation of molecular mass and MMD. The additivity of the refractive index increment may be assumed, and the refractive index increment for a copolymer AB can be written as [32] ... 

In addition to the refractive index (already seen to be closely linked with molecular structure) there are a number of other optical properties of importance... 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

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... 

Similar conclusions have been arrived at in an SPR model designed as a DNA probe (24). The DNA probe sequence was immobilised to a thiol modified silver film deposited on a waveguide in a SPR format. The probe showed a characteristic change in the SPR excitation on binding complementary DNA. In this instance the use of an antibody to double stranded DNA could be used in a subsequent step to check the specifity of the result and allow an independent estimation of the level of detection. Without the addition of high molecular weight or refractive index labels however, this technique could not achieve a desirable level of detection, nor eliminate non specific interactions. 

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