Molecular refractive indices

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

Characterization of a Petroleum Cut by Refractive Index, Density, and Molecular Weight (ndM method)... 

As in the case of density or specific gravity, the refractive index, n, for hydrocarbons varies in relation to their chemical structures. The value of n follows the order n paraffins < n naphthenes < n aromatics and it increases with molecular weight. 

With the accumulation of results obtained from various and complex analyses of narrow cuts (Waterman method), correlations have been found f ctween refractive index, specific gravity and molecular weight on one hand, and percentages of paraffinic, naphthenic and aromatic carbon on the other. 

More recently Andrews and Juzeliunas [6, 7] developed a unified tlieory that embraces botli radiationless (Forster) and long-range radiative energy transfer. In otlier words tliis tlieory is valid over tire whole span of distances ranging from tliose which characterize molecular stmcture (nanometres) up to cosmic distances. It also addresses tire intennediate range where neitlier tire radiative nor tire Forster mechanism is fully valid. Below is tlieir expression for tire rate of pairwise energy transfer w from donor to acceptor, applicable to transfer in systems where tire donor and acceptor are embedded in a transparent medium of refractive index ... 

In addition to the numeric data (color, solubility, refraction index, spectra, etc.), these factual databases also include a bibliographic section with references or sources and a section with information for the identification of a compound (e.g., name, CAS Registry Number, molecular weight). 

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystalhsation does not alter it, the compound may be regarded as pure. For a hquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boUing point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

The index of refraction of most polymers is greater parallel to the chain than normal to the molecular axis. Substances showing this anisotropy of refractive index are said to be birefringent. 

Equations (10.17) and (10.18) show that both the relative dielectric constant and the refractive index of a substance are measurable properties of matter that quantify the interaction between matter and electric fields of whatever origin. The polarizability is the molecular parameter which is pertinent to this interaction. We shall see in the next section that a also plays an important role in the theory of light scattering. The following example illustrates the use of Eq. (10.17) to evaluate a and considers one aspect of the applicability of this quantity to light scattering. 

Interpret the slope and intercept values of the line in Fig. 10.12 in terms of the molecular weight and radius of gyration of cellulose nitrate in this solution. At 436 nm the refractive index of acetone is 1.359. 

Using the original Hc2/r values, recalculate M using the various refractive index gradients. On the basis of self-consistency, estimate the molecular weight of this polymer and select the best value of dn/dc2 in each solvent. Criticize or defend the following proposition Since the extension of the Debye theory to large particles requires that the difference between n for solute and solvent be small, this difference should routinely be minimized for best results. 

Compound Molecular formula CAS Registry Number °C kPa Refractive index Density... 

Solution Polymers. Acryflc solution polymers are usually characterized by their composition, solids content, viscosity, molecular weight, glass-transition temperature, and solvent. The compositions of acryflc polymers are most readily determined by physicochemical methods such as spectroscopy, pyrolytic gas—liquid chromatography, and refractive index measurements (97,158). The solids content of acryflc polymers is determined by dilution followed by solvent evaporation to constant weight. Viscosities are most conveniently determined with a Brookfield viscometer, molecular weight by intrinsic viscosity (158), and glass-transition temperature by calorimetry. 

PFC number Formula CAS Registry Number Molecular weight Boiling point, °C Melting point, °C Liquid density, g/mL at °C Liquid refractive index, at°C Critical temp, °C Critical pressure, MPa "... 

Formula GAS Registry Number Molecular weight Boiling poiut, °G Melting poiut, °G Liquid density, g/mL at °G Refractive index, n- at °G... 

Compound CAS Registry Number Molecular weight Melting point, °C Boiling point, °C" Refractive index Specific gravity Viscosity mPa(=cP) Solubihty in water at 25°C F poi... 

In methacrylic ester polymers, the glass-transition temperature, is influenced primarily by the nature of the alcohol group as can be seen in Table 1. Below the the polymers are hard, brittle, and glass-like above the they are relatively soft, flexible, and mbbery. At even higher temperatures, depending on molecular weight, they flow and are tacky. Table 1 also contains typical values for the density, solubiHty parameter, and refractive index for various methacrylic homopolymers. 

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

Cycloaliphatic amine CAS Registry Number Molecular formula Boiling point, °C Flash point, °C Specific gravity, g/mL Refractive index. 

Optical Properties. When light falls on an object, it is either partially absorbed, reflected, or transmitted. The behavior of the object as it relates to each of these three possibiUties determines visual appearance. Optical properties of fibers give useful information about the fiber stmcture refractive indexes correlate well with fiber crystalline and molecular orientation and birefringence gives a measure of the degree of anisotropy of the fiber. 

---