Isotopic substitution impurities

Equation (5.44) makes it possible to analyze the dependence of the intensity of impurity absorption on the polarization of the incident light. Rashba has treated this problem within the framework of the theory of small-radius excitons (2), (12) for the isotopic substitutional impurities. 

Local biphonons in crystals with isotopic substitution impurities... 

We begin, in this section, a discussion of features of the biphonon spectrum in imperfect crystals. For the sake of simplicity we shall assume that the crystal contains only the simplest point defects isotopic substitution impurities. Before going over to a theory of local biphonons, we shall make several qualitative remarks concerning the effect of anharmonicity on the spectrum of local vibrations... 

Local biphonon in an 1ANHiBr crystal containing the isotopic substitution impurity 15N... 

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

The presence of isotopic impurities causes clear effects in the vibrational spectra. Almost all modes studied so far show frequency shifts on S/ S substitution [81, 107]. The average shift of the internal modes is ca. 0.6 cm , and of the external modes it is 0.1-0.3 cm (Tables 3, 4 and 5). Furthermore, the isotopomers which are statistically distributed in crystals of natural composition can act as additional scattering centers for the phonon propagation. Therefore, in such crystals the lifetime of the phonons is shortened in comparison with isotopically pure crystals and, as a conse-... 

MS delivers both information about the mass and the isotope pattern of a compound and can be used for the structural analysis upon performance of MS/MS experiments. Therefore, it is a valuable tool for the identification and characterization of an analyte as well as for the identification of impurities. Potential applications are the identification of IL in fhe quality control or in environmental studies. Unwanted by-products formed during the s)mthe-sis or by the hydrolysis of components of the ILs can be identified by this method. The analysis of fhe IL itself is also a prerequisite for the analysis of compounds dissolved in fhese media, as will be ouflined in the section 14.4. Beside the identification of fhe ILs, a characterization of different properties like water miscibility and the formation of ion clusfers, providing valuable information abouf fhe molecular structure of the IL, can be performed by means of MS techniques. The majority of studies reported up to now have dealt with ILs encompassing substituted imidazolium or pyridinium cations, therefore fhe following discussion concentrates on these compounds unless otherwise stated. 

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