Spectra optical, free molecules

Figure 2.6-2 Variation of the frequencies by the incorporation of a tetraatomic molecule with two degenerate vibrational states ( ) in a crystal lattice, a spectrum of the free molecule, R = rotations, T = translations b static influence of the crystal lattice. The degenerate states split, the free rotations change into librations L c dynamic coupling of the vibrations of molecules within a primitive unit cell with z = 2 molecules. Each vibrational level of a molecule splits into z components and 3 z - 3 translational vibrations TS and 3 z librations L appear d dependence of the vibrational frequencies on the wave vector k of the coupled vibrations of all unit cells in the lattice. The three acoustic branches arise from the three free translations with = 0 (for k 0) of the unit cell all vibrations of the unit cells with / 0 (for k 0) give optical branches .

The equilibrium and rate constants for NADH binding to the three isozymes EE, ES, and SS of the horse enzyme have been determined 305). Differences in binding to the two types of chains were found both for the binding strength and the pH dependence. Changes in the absorption spectrum 306,307), the fluorescence polarization spectrum (305), the optical rotatory dispersion spectrum 309), and the effect of DzO on the fluorescence spectrum 310) have been studied for the binary enzyme coenzyme complexes compared to the free molecules. 

Fig. 12.18 In the molecule porphyrin (free base form), the two central H atoms can be switched back and forth between two configurations by light (photo-isomerisation). This leads in a solid solution to a hole in the absorption spectrum. Optical hole-burning with organic molecules was first demonstrated...

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra (71PMH(3)397) are very sensitive to the spatial disposition of the atoms in a molecule, and conformational changes may yield rather dramatic changes in the appearance of a CD or ORD spectrum of a chiral molecule. The analysis of the temperature dependence of the CD spectrum may give information on populations and free energy differences. Except for nucleosides, the use of the chiroptical method in conformational analysis is rather limited, which may be accounted for by the complexity of the theory for optical activity. 

Some solvents are able to filter parts of the optical spectrum — an effect that may provide some protection of the dissolved or suspended drug. However, absorption of photons may lead to formation of free radicals from solvent molecules, resulting in chain reactions that are well documented for unsaturated fats and oils (Florence and Attwood, 1998). Drugs dissolved or suspended in such vehicles may participate in the free radical chain reactions, leading to photosensitized decomposition of the drug induced by the solvent. 

The UV spectrum of the unsubstituted compound 10 at pH 7 has three maxima, at 208, 290, and 300 nm, with an inflection at 320 nm. At Ho = -2.8, the spectrum consists of two bands at 212 and 302 nm. Measurements of the change in optical density at 275 nm indicated that the parent heterocycle is a weak base of pK = -0.64. In M-deuterohydrochloric acid the PMR spectrum exhibits the expected pattern with a singlet at 8 8.72 (H-3) and an AB quartet (H-5, H-6) centered at 8 9.00 and 9.16 (/s o = 3 Hz). As the acidic solvent used was not strong enough to effect complete protonation of the molecule, these PMR measurements must have been made on a mixture of the free base and cation of compound 10. Brief details have been reported of the PMR spectrum of the free base in methanol. More recently the free base and its 4-oxide have been compared using UV and PMR spectroscopy. ... 

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