Effect of Structure on Absorption

Spectroscopic characteristics are held in common by molecules with some of the same chemical features. It may then be reasonable to expect that the correlation can be extended, and that the presence of a certain chemical feature may be implied by the presence of a certain spectral characteristic. 

The most common types of electronic transitions are illustrated in Table 7.3. 

The a- a transitions are very energetic and are found only below 200 nm, in the far-ultraviolet region. This is often termed the vacuum ultraviolet region because the normal constituents of air, Ng and Og, also absorb strongly below about 160 and 200 nm, respectively and spectra of other substances must be obtained in a vacuum. The transitions are also high-energy transitions and generally 

The molar absorptivity (e) of a compound is a function of the cross-sectional area (0) of the absorbing species and of the transition probability (P)  

Using this relation, a molar absorptivity of the order of 10 has been calculated for the average organic molecule with an assumed cross-section of about 10 cm and a unit transition probability. The highest values known for e are a few hundred thousand any value above 10,000 is considered high, and one under 1000 low. 

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Ultraviolet absorptions ofvinylogous lactams were found by MOLCAO calculations and compared with experimental values (663). Infrared spectroscopic studies of vinylogous amides (664) and some fifty vinylogous urethanes (665) allowed configurational and structural assignments. The effect of enamine-imine equilibrium in a series of benzophenone derivatives was established (666) and the effect of structure on enamine basicity studied (667). 

Effects of Structure on Electronic Absorption Corresponding to the Benzenoid Band... 

Temperature An examination has been made of the effect of temperature on the structural changes in polymer films produced from the three vehicles described earlier s. Three methods were used dilatometry, water absorption and ionic resistance. 

When he interviewed with Meyer in 1926, Mark outlined a typically thorough program. He proposed a team of organic and physical chemists, and physicists who would evaluate the influence of structure on properties such as rigidity, elasticity, melting point, and water absorption. Work, he proposed, would shift toward new material development and into the manufacturing facilities to evaluate the effects of processing... 

A number of investigators have studied the effect of ozone on the ultraviolet absorption spectra of proteins and amino acids. A decrease in the absorption of 280-nm light in a number of proteins was originally reported ly Giese et aV to be a consequence of ozone exposure they suggested that this was due to an interaction of ozone with the ring structures of tyrosine and tryptophan. Exposure of a solution of tryptophan to ozone resulted in a decrease in 280-nm absorption, whereas the extinction coefficient of tyrosine increased. Similar results with tyrosine were reported by Scheel et who also noted alterations in the ultraviolet spectra of egg albumen, perhaps representing denaturation by ozone. 

Studies on the effect of pH on peroxidase catalysis, or the heme-linked ionization, have provided much information on peroxidase catalysis and the active site structure. Heme-linked ionization has been observed in kinetic, electrochemical, absorption spectroscopic, proton balance, and Raman spectroscopic studies. Kinetic studies show that compound I formation is base-catalyzed (72). The pKa values are in the range of 3 to 6. The reactions of compounds I and II with substrates are also pH-dependent with pKa values in a similar range (72). Ligand binding (e.g. CO, O2 or halide ions) to ferrous and ferric peroxidases is also pH-dependent. A wide range of pKa values has been reported (72). The redox potentials of Fe3+/Fe2+ couples for peroxidases measured so far are all affected by pH. The pKa values are between 6 and 8, indicative of an imidazole group of a histidine residue (6, 31-33),... 

Martin H) has written a perceptive analysis of the possible ways in which an ionized species may behave in various models and contribute to or be responsible for a given activity. QSAR studies that have dealt with ion-pair partitioning include a study of fibrinolytics ( ) and the effect of benzoic acids on the K ion flux in mollusk neurons ( ). Schaper (10) recently reanalyzed a large number of absorption studies to include terms for the absorption of ionized species. Because specific values were not available for log Pj, he let the relation between log Pi and log P be a parameter in a nonlinear regression analysis. In most cases he used the approximation that the difference between the two values is a constant in a given series. This same assumption was made in the earlier studies (, ) Our work suggests that the pKa of an acid can influence this differential (see below). The influence of structure on the log P of protonated bases or quaternary ammonium compounds is much more complex (11,12) and points out the desirability of being able to easily measure these values. 

The long lifetimes and high redox potentials of a range of ruthenium(II) complexes and in particular [Ru(bpy)3] " have important consequences for their use as photoactive redox catalysts. This area of research is extremely active and we now focus on the decay of the excited state of [Ru(bpy)3] + ( [Ru(bpy)3] " ) and its quenching. Braterman et al. have described the electronic absorption spectrum and structure of the emitting state of [Ru(bpy3] +, and the effects of excited state asymmetry. The effects of solvent on the absorption spectrum of [Ru(bpy)3] " have been studied. In H2O, MeCN and mixtures of these solvents, the value of e(450 nm) remains the same ((4.6 0.4) x 10 dm mol cm ). The ground state spectrum is essentially independent of... 

As is evident from Eqs. (1.36) and (1.37), and from the classical treatment as well, the effect of resonance on the intensity of X-ray scattering is pronounced when E E0, that is, in the vicinity of the absorption edges. Even for data collected at other wavelengths, it is necessary to correct the structure factors for anomalous scattering before the electron density can be calculated by the Fourier inversions of Eqs. (1.22) and (1.26), as further discussed in chapter 5. The anomalous scattering factors needed for this purpose are available in the literature (International Tables for X-ray Crystallography 1974, Kissel and Pratt 1990). 

The different C=0 stretching frequencies of the two systems is a reflection of the stronger basicity of the pyran-4-one. The effects of substitution on this absorption band are smaller in pyran-4-ones, and this feature has been attributed to a more significant contribution of the aromatic structure (100) to the hybrid structure than the contribution of (101) to the structure of pyran-2-one (B-77MI22201). 

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