Bond angles refractive index

Instrumental methods in chemistry make it possible to characterize any chemical compound by a very large number of different kind of measurements. Such data can be called observables. Examples are provided by Spectroscopy (absorbtions in IR, NMR, UV, ESCA. ..) chromatography (retentions in TLC, HPLC, GLC. ..) thermodynamics (heat capacity, standard Gibbs energy of formation, heat of vaporization. ..) physical propery measures (refractive index, boiling point, dielectric constant, dipole moment, solubility. ..) chemical properties (protolytic constants, ionzation potential, lipophilicity (log P)...) structural data (bond lengths, bond angles, van der Waals radii...) empirical structural parameters (Es, [Pg.34]

Instrumental methods in chemistry have dramatically increased the availability of measurable properties. Any molecule can be characterized by many different kinds of data. Examples are provided by Physical measures, e.g. melting point, boiling point, dipole moment, refractive index structural data, e.g. bond lengths, bond angles, van der Waals radii thermodynamic data, e.g. heat of formation, heat of vaporization, ioniziation energy, standard entropy chemical properties, e.g. pK, lipophilicity (log P), proton affinity, relative rate measurements chromatographic data, e.g. retention in HPLC, GLC, TLC spectroscopic data, e.g. UV, IR, NMR, ESCA. 

As mentioned above, the effect of refractive index is more complex than the Lorenz correction can deal with it varies in different ways with different compounds and may reflect the properties of Rydberg transitions. Since the refractive index of many compounds is of the order of (2), we approximate this term as 4/3 and pay it no further attention. A right-handed helix illustrating these terms is shown in Figure 2. For simple four-atom systems, the term LA/D can be calculated from bond lengths (d, d2, d ), bond angles (a and P) and the dihedral angle (0) ... 

Other structural methods that we have discussed depend upon the nuclear positions, and this difference may be important. X-ray structures also suffer from what is called rigid-body motion. This has little effect upon angles bnt can canse bond lengths to artifically appear shortened by 0.005-0.015 A if determined at room temperatnre (less if determined at low temperatures). Vibrational amplitndes and, hence, things like the unit cell size and refractive index are temperatnre dependent. 

There are well defined maxima for the cyclic polymers in both plots at n == 22. No such maxima are shown by the chain molecules. As a results of the different bond angles at the silicon and oxygen atoms of the polysiloxane chain, a cyclic molecule with n = 22 can adopt a low energy conformation in the all-trans state. Consequently, the ring has a planar disc-sh ed character and such rings can pack more efficiently than spherical molecules, giving rise to the increase in density and refractive index. 

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