Variable-temperature absorption spectra

Figure 1. Variable temperature absorption spectra of PDN6S (top) and PDN6G (bottom).

Relationship to Electronic Properties. As a result of the close connection between bond conformation and electronic properties (4), the analysis of chain conformation in the polysilylenes has been of interest to researchers in this field, both from the experimental and theoretical viewpoints. As reported by Trefonas et al. (5), most asymmetrically substituted alkyl polysilylenes in solution at room temperature display an electronic absorption with ranging from 303 to 309 nm. The variable-temperature absorption spectrum of PMHS is shown in Figure 4 (4). At room temperature, max is 308 nm, and as the solution is cooled, there is a continuous red shift with the X x reaching 328 nm at -95 °C. Some workers 4, 6) suggest that this observation is a reflection of an increasing population of trans rotational states in the silicon backbone as the temperature is lowered. This suggestion is supported by the finding that these spectra can be adequately modeled by a rotational isomeric-state treatment (4). 

Lithium intercalation in VeOis has been studied by Stallworth et al. ° Variable-temperature Li NMR indicated considerable mobility for Li+ in the intercalated materials. The Li NMR data were compared with ESR spectra and near-edge X-ray absorption fine structure (NEXAFS) data on the same materials, and a correlation between vanadium oxidation state (from NEXAFS data) and NMR shift was observed. The authors explained the shifts in terms of different coupling mechanisms between the and shifts. The shifts were, however, extracted from static NMR experiments, and it is possible that some of the different local environments, typically revealed in a MAS spectrum, were not seen in this study. 

Variables That Influence Absorbance Common variables that influence the absorption spectrum of a substance include the nature of the solvent, the pH of the solution, the temperature, high electrolyte concentrations, and the presence of interfering substances. The effects of these variables must be known and conditions for the analysis chosen such that the absorbance will not be materially affected by small, uncontrolled variations in their magnitudes. 

The UV absorption spectrum of 2,6-diaminopyridine in ethanol/isooctane mixed solvent with variable temperature or ethanol concentration is consistent with the formation of the hydrogen-bonded complex of (/i)-6-amino-2( l//)-pyridinimine with two molecules of ethanol. The formation of the imino form may be restricted to the 2,6-diaminopyridine and its methyl derivatives, in which each amino group at the 2- and 6-positions has at least one hydrogen atom. Ab initio STO-3G calculations... 

Staying with this example, what if the absorption spectrum of a component varies with some other experimental variable, such as temperature This is a situation in which the application of a multilinear model is problematic. Either of two adjustments in the situation might allow both wavelength and temperature to be used as independent variables in a multilinear model (1) If the dependence of its absorption spectrum on temperature is because a component is shifting between discrete low-temperature and high-temperature forms, then each form can properly be regarded as a separate component whose absorption spectrum is independent of temperature. (2) Alternatively, it may be necessary to restrict the temperatures used to a sufficiently narrow range that the absorption spectrum is essentially constant. 

Whether the relaxation is fast or slow at 4.2 K can be checked experimentally, using the following arguments. Figures 2.6b shows a 4.2 K Mossbauer spectrum recorded for B = 8.0 T. The solid line outlines the contribution of Z. The remaining absorption pattern, well understood, originates from the [Feni(H20)6]3 + contaminant. The sharpness of the absorption lines of Z shows that the intermediate has nearly axial symmetry thus, we can set EID = 0 in Equation 2.2 and Ax = Ay in Equation 2.3. We have recorded, and published, spectra in variable applied fields and at different temperatures.11 Since the spectra are a bit noisy, it will aid the reader if we... 

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