High-temperature spectra

At the highest temperature, the positions of the atoms are no longer fixed. The atoms perform random motions around a local minimum until they move to a neighboring site. 

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Fig. 6. Spectral monitoring of the thermal denaturation of the highly helical, Ala-rich peptide Ac-(AAAAK)3AAAA-YNH2 in D20 from 5 to 60°C, as followed by changes in the amide V IR (left) and VCD (right). IR show a clear shift to higher wavenumber from the dominant a-helical peak (here at an unusually low value, 1637 cm-1, due to full solvation of the helix) to a typical random coil value ( 1645 cm-1). VCD loses the (—,+,—) low-temperature helical pattern to yield a broad negative couplet, characteristic of a disordered coil, at high temperature. Spectra were normalized to A = 1.0 by 45°C.

Thermally denatured proteins have been studied for a variety of systems using FTIR and VCD. The resulting high-temperature spectra often reflect the characteristics seen earlier for random coil peptides as well as that seen for the unstructured casein. Particularly the amide I IR bands show a frequency shift to center on a broadened band at 1645-50 cm-1. The amide I VCD loses its distinctive character (Fig. 11) and tends toward... 

Interestingly, at the higher temperatures and if low densities are employed, the Q branch has a minumum near the Q ) transition frequency at 4155 cm-1, and there are two maxima (in the early days of collision-induced absorption, these were called the Qr and Qp lines ) these and the resulting dip between the maxima are clearly visible in the high-temperature spectra, Fig. 3.31. These maxima have a frequency separation... 

The two high-temperature spectra clearly Indicate the motion of the spin label itself. Compared to the 5-DXSA films the motion is considerably more excited. In an attempt to understand the dynamics in terms of liquid-like motion, the increase of the rotational constants should lead to a shift of the whole structure at the high- and low-field extrema toward the isotropic values. 

High-temperature spectra in situ permit characterization of the OH groups and other structural features as they present themselves to the... 

Fig. 3 shows XANES spectra recorded during temperature rise up to 775 K. First evaluation reveals a drastic change in case of ZnH-[Al]MFI, whereas H-[Zn]MFI is almost not affected. After cooling the cell down to room temperature in nitrogen flow, main characteristics of high temperature spectra of both samples are maintained and, thus, can be interpreted in terms of coordination effects. 

In the high-temperature spectra, where the solvent as well as the dissolved investigated substance are solely ionic in character, an extremely fast mutual exchange of nuclei. 

Hence, high-temperature spectra bands of all structural units could not be observed, as it is the case of low-temperature spectra in molecular systems, and moreover, only one band averaging contributions from all structural units that can be expressively wide-spread. In the case of bands with higher half-width, it is thus necessary to use for their mathematical description, the weighted combination of the Lorentzian and Gauss functions. 

Figure 9 shows the Raman spectra of the i and 9 quenched to room temperature. In order to have spectra of all phases discussed here at similar conditions we present them at room temperature. The high-temperature spectra of t and 0 are very similar to ones measured upon quenching the sample to 300 K. All IR measurements were performed at 300 K. The Raman and IR spectra of the phase (obtained by compression at 300 K) from which these phases were formed are shovcn for comparison at 69 GPa (Fig. 9) and 97 GPa (Fig. 10). 

Ref. [122]. Unfortunately, the majority of inorganic and coordination compounds exist as solids at room temperature. Although some of these compounds can be vaporized at high temperatures without decomposition, it is rather difficult to measure their spectra by the conventional method. Furthermore, high-temperature spectra are difficult to interpret because of the increased importance of rotational and vibrational hot bands. 

Figure 7 Expanded views from a C in situ MAS study of methanol-to-gasoline chemistry showing the signals from methanol and dimethyl ether. In the high-temperature spectra two peaks are resolved for both species gas phase (48.0 ppm) and adsorbed (53.1 ppm) methanol, and gas phase (58.0 ppm) and adsorbed (61.0 ppm) dimethyl ether.

The thermochromic phenomenon is easily seen with polymer films, and the first studies [14,15,18-20] were only concerned with the optical spectroscopy of films and solutions. Among the first results was that the change of optical absorption occurs over a wide temperature range. The optical spectra taken at increasing temperatures show a blue-shift. The sequence of spectra may sometimes, but not always, have an isobestic point where all spectra share one common point. This shows that the sequence can be de-convoluted into two spectra, the low temperature and the high-temperature spectra, and that all intermediate... 

The assignment of the 12 methyl and meth jL ne resonances originating from the minor nucleosides in yeast tRNA was performed by Kan et 1] - These authors con red the high temperature spectra of... 

PROBLEM 20.19 The rapid Cope rearrangement of homotropilidene led to problems for W. R Roth, who first made the molecule. Can you explain in general terms the three perplexing NMR spectra for this molecule that are shown Note that both low- and high-temperature spectra have sharp peaks, whereas at an intermediate temperature the spectmm consists of two blobs. You are not required to analyze the spectra in detsdl—just give the general picture of what must be happening. 

Consider, for example, the induced spectra of liquid CS2 shown in figure lo( ), It is noticeable, particularly in the high temperature spectra which are almost exponential, that the forbidden bands have a common shape and the DID wings of the... 

Spinner assemblies have been constructed from three materials Kel-F, Delrin, and machinable boron nitride For observation of hydrocarbon materials at ambient and low temperature, the Kel-F assembly is used. It displays suitable mechanical properties and does not interfere with the carbon spectrum since the resonances of the carbons in the Kel-F are >10 KHz in width due to the unremoved C-F dipolar interactions. To observe fluorocarbon materials at ambient and low temperature by C-F dipolar decoupling/CP/MAS, the Delrin assembly is used since unremoved C-H dipolar interactions broaden the Delrin signal beyond detection. Finally, to obtain high temperature spectra, the BN system, which is transparent in C-NMR, has been employed. Samples in the form of powders are loaded into hollow rotors (volume ca. 65y ) which are fitted with threaded caps to confine the sample. In some cases, the polymer under study was itself machined in the form of a rotor and used as the analytical sample. The rotors have a cone diameter of 8mm and experience centripetal acceleration on the perimeter of ca. 0.4 million G when rotating at 5 KHz. ... 

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