Spectroscopic effects

Distribution function of orientation for spectroscopic effect studied... 

The mechanical, nonbonded, and electrostatic expressions described above are not sufficient to describe some structural and spectroscopic effects. Three specific structural and spectroscopic phenomena have been incorporated into MM3. They are the electronegativity, anomeric, and Bohlmann effects, which essentially can be traced to molecular orbital origins. These chemical effects are a part of the MM3 program.74,75... 

Our earlier research on the coupled binuclear copper proteins generated a series of protein derivatives in which the active site was systematically varied and subjected to a variety of spectroscopic probes. These studies developed a Spectroscopically Effective Model for the oxyhemocyanin active slte.(l) The coupled binuclear copper active site in tyrosinase was farther shown to be extremely similar to that of the hemocyanlns with differences in reactivity correlating to active site accessibility, and to the monophenol coordinating directly to the copper(II) of the oxytyroslnase site.(2) These studies have been presented in a number of reviews.(3) In the first part of this chapter, we summarize some of our more recent results related to the unique spectral features of oxyhemocyanin, and use... 

Figure 7. The Spectroscopically Effective Active Site of hemocyanln and tyrosinase.

Figure 21. Comparison of the spectroscopically effective models for azide binding at the blnuclear copper active site in hemocyanln and the trlnuclear copper cluster site in laccase.

Coleman et al. 2471 reported the spectra of different proportions of poly(vinylidene fluoride) PVDF and atactic poly(methyl methacrylate) PMMA. At a level of 75/25 PVDF/PMMA the blend is incompatible and the spectra of the blend can be synthesized by addition of the spectra of the pure components in the appropriate amounts. On the other hand, a blend composition of 39 61 had an infrared spectrum which could not be approximated by absorbance addition of the two pure spectra. A carbonyl band at 1718cm-1 was observed and indicates a distinct interaction involving the carbonyl groups. The spectra of the PVDF shows that a conformational change has been induced in the compatible blend but only a fraction of the PVDF is involved in the conformational change. Allara M9 250 251) cautioned that some of these spectroscopic effects in polymer blends may arise from dispersion effects in the difference spectra rather than chemical effects. Refractive index differences between the pure component and the blend can alter the band shapes and lead to frequency shifts to lower frequencies and in general the frequency shifts are to lower frequencies. 

Results. Figure 5.6 compares the classical and quantum profiles of the spectral function, VG (o), of He-Ar pairs at 295 K (light and heavy solid curves, respectively), over a wide frequency band. Whereas at the lowest frequencies the profiles are quite similar, at the higher frequencies we observe increasing differences which amount up to an order of magnitude. The induced dipole [278] and the potential function [12] are the same for both computations. Bound state contributions have been suppressed we have seen above that for He-Ar at 295 K, the spectroscopic effects involving van der Waals molecules amount to only 2% at the lowest frequencies, and to much less than that at higher frequencies. 

Practically all computations shown above were undertaken in the framework of the isotropic interaction approximation. For the examples considered, agreement of calculated and observed spectra was found. The most critical comparisons between theory and measurement were made for the H2-X systems whose anisotropy is relatively mild. Nevertheless, some understanding is desirable of what the spectroscopic effects of the anisotropy are. Furthermore, other important systems like N2-N2 and CO2-CO2 are more anisotropic than H2-X. The question thus remains as to what the spectroscopic significance of anisotropic interaction might be. In this Section, an attempt is made to focus on the known spectroscopic manifestations of the anisotropy of the intermolecular interaction. 

P. Gaspard Concerning the issue raised by Prof. Marcus, I should remark that information on the shape of the periodic orbits in the original coordinates is lost at the level of a spectroscopic effective Hamil-... 

The radius thus calculated from the theory of Smith and Symons does not correspond to any known property of halide ions. However, when the acceptable physical model of Franck and Platzman is combined with the concept of a variable radius, as proposed by Smith and Symons, both absolute value and environmental effects can be accounted for. This was done in the theory of Stein and Treinin (18, 19, 47), using an improved energetic cycle to obtain absolute values of r, the spectroscopically effective radius of the cavity containing the X ion. These values were then found to correspond to the known partial ionic radii in solution, as did values of dr/dT to values obtained from other experiments. The specific effects of temperature, solvents, and added salts could be used to differentiate between internal and such CTTS transitions where the electron interacts in the excited state strongly with the medium. These spectroscopic aspects of the theory were examined later in detail and compared with experiment by Treinin and his co-workers (3, 4, 32, 33, 42,48). 

Substituted NH ions Whalley, 28> discussed the spectroscopic effects of orientional disorder about one axis (in contrast to the disorder about three axes as described by Whalley and Bertie 03) and Bertie and Whalley 129> in the a-phases of the methylammonium halides. In principle, all vibrations of an orientational disordered crystal are spectroscopically active, but if the disorder is only about one axis, some restrictions operate, the symmetric bands are sharp in the one-dimensional disordered case, but the bands due to asymmetric vibrations (E) are broad. Whalley use the infra-red results of Sandorfy et al. 130>131> 0f the CH3 -ammonium halides to illustrate the effect which is predicted from interionic coupling of the E-modes. No such effect is visible in the spectrum of the methoxyammonium ion CH3ONH3 reported by Nelson, 32>. 

To avoid the spectroscopic effects induced by adsorbate-adsorbate interactions (frequency shifts and FWHM variations), we deal only with low-coverage spectra (9 —> 0 limit) here. The effect of adsorbate-adsorbate interactions (both static and dynamic) is discussed briefly in Section IV.A.4. 

We discussed the way in which conjugation affects reactivity in Chapter 10. and mentioned its spectroscopic effect there as well. 

A. the methods measuring tunneling rates by observing their spectroscopic effects ... 

Figure 5. A simple spectroscopically effective MO diagram for the (E-/V))MoO(S S) complexes. The MO energies are not to scale. The orbitals are shown as degenerate due to the dominance of the strongly bound oxo ligand on the z axis. [Adapted from (23).]...

Figure 18. Comparison of half-met hemocyanin with the half-met type 3 (in T2D) laccase copper sites. A EPR spectra and binding constants of exogenous azide binding. B Spectroscopically effective structural models for exogenous ligand binding to the half-met derivatives and their relation to differences in dioxygen reactivity.

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