Specific refractive dispersion

As indicated, the specific refractive index increment is best measured by differential refractometry or interferometry. Experimental procedures as well as tabulated values of dn/ dc for many systems have been presented elsewhere40,63K The relevant wavelength and temperature are those used for LS. The value of X0 is invariably 436 or 546 nm, but with the advent of laser LS, values of dn/dc at other wavelengths are required. These can be estimated with good reliability using a Cauchy type of dispersion (dn/dc a 1/Xq). For example the values of dn dc for aqueous solutions of the bacterium T-ferrioxidans at 18 °C are 0.159, 0.141 and 0.125 ml/gm at X0 = 488, 633 and 1060 nm respectively64 ... 

Zeiss GD, Meath WJ (1975) The H20-H20 dispersion energy constant and the dispersion of the specific refractivity of dilute water vapour. Mol Phys 30 161—169... 

Table 5 shows the experimental specific refractivities, K X) = n(l) l]/ p, and the average polarizability as calculated from equation (1) at a number of frequencies for liquid and vapour phases. The values of the specific refractivity of the vapour have been obtained from the Cauchy dispersion formula of Zeiss and Meath.39 In this paper the authors assess the results of a number of experimental determinations of the refractive index of water vapour and its variation with frequency. Even after some normalization of the data to harmonize the absolute values from different determinations there is a one or two percent spread of results at any one wavelength. Extrapolation of the renormalized data for five independent sets of data leads to zero frequency values of K(7.) within the range (2.985-3.013) x 10-4 m3 kg 1, giving, via equation (1), LL — 9.63 0.10 au. Extrapolation of the earlier refractive index data of Cuthbertson and Cuthbertson40 by Russell and Spackman41 from 8 values of frequency between 0.068 and 0.095 au, leads to a zero frequency value, of y.i, 1,(0) = 9.83 au. While the considerable variation between the raw experimental data reported in different determinations is cause for some uncertainty, it appears that the most convincing analysis to date is that of... 

The specific optical dispersion (ASTM D-1807) serves as an indication of aromatic content. The difference in refractive index of the oil as measured at two wavelengths is determined, divided by the relative density (all measurements are at the same temperature) and multiplied by 100 values above 97 are stated to bear a direct relationship to the aromatic content of the oil. 

ASTM D1807 Test Methods for Refractive Index and Specific Optical Dispersion of Electrical Insulating Liquids... 

In principle, absolute values of the refractive index can be used to identify chemical species in the same fashion as boiling and melting points, and the density of light absorption properties. Specific refraction and dispersion spectra are used to characterize the chemical structure of analytes. However, it is very difficult to apply refractometry for this purpose because of small differences in refractive index between substances, and relatively large dependence of the index with temperature, dissolved gas, and impurities. This method has been used more successfully in combination with microscopy to differentiate between components of inhomogeneous solid samples. 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

Optical activity comes from the different refractions of right and left circularly polarized light by chiral molecules. The difference in refractive indices in a dissymmetric medium corresponds to the slowing down of one beam in relation to the other. This can cause a rotation of the plane of polarization or optical rotation. The value of specific rotation varies with wavelength of the incident polarized light. This is called optical rotatory dispersion (ORD). 

Rather, correlations are presented with the dielectric constant or with solvent polarity . It is true that the magnitude of the effects observed is frequently in the range of 0.2 -0.5 Hz as predicted by Raynes. Some evidence for specific interaction is suggested by the collision complex model 50> and by the deviations observed in chloroform solutions, acids and gases. A few investigators have noted that no correlation was found with the refractive index of the solvents suggesting that dispersion forces are not important for 2/H H, at least in those compounds studied. 

Let us first consider the various compound-specific parameters in Eq. 11-3. In Chapters 4 to 7, we used the refractive index to quantify the dispersive vdW properties of a given compound [i.e., vdW( < (nf - 1)/( , + 2)], see Section 3.2]. We also noted that other parameters such as the air-hexadecane partition constant, K, would have been more appropriate to describe vdW, however, due to lack of experimental Kiah values, we have chosen not to do so here. Instead, we use the... 

Circular dichroism arises from the same optically active transitions responsible for the Cotton effects observed in ORD curves, but unlike ORD it is an absorption, not a dispersion, phenomenon. Hence, the CD effect is restricted to the region of the transition and can be interpreted more straightforwardly. Both ORD and CD can best be understood if one imagines the incident plane-polarized beam resolved into two in-phase circularly polarized beams whose vectors rotate in opposite directions. A difference in index of refraction between the left and right circularly polarized beams results in rotation of the transmitted plane polarized beam while differential absorption of the two circularly polarized beams results in depolarization of the transmitted beam, so that an incident plane-polarized beam whose frequency is within that of an optically active absorption band becomes both rotated and elliptically polarized upon passage through the sample. This depolarization effect is CD, and the measured parameter is (et — er), the difference in extinction coefficient between the left and right circularly polarized beams. The data is usually recorded as the specific ellipticity, defined as ... 

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