Field dependence peak position

A. Previous models of water (see 1-6 in Section V.A.l) and also the hat-curved model itself cannot describe properly the R-band arising in water and therefore cannot explain a small isotope shift of the center frequency vR. Indeed, in these models the R-band arises due to free rotors. Since the moment of inertia I of D20 molecule is about twice that of H20, the estimated center of the R-band for D20 would be placed at y/2 lower frequency than for H20. This result would contradict the recorded experimental data, since vR(D20) vR(H20) 200 cm-1. The first attempt to overcome this difficulty was made in GT, p. 549, where the cosine-squared (CS) potential model was formally (i.e., irrespective of a physical origin of such potential) applied for description of dielectric response of rotators moving above the CS well (in this work the librators were assumed to move in the rectangular well). The nonuniform CS potential yields a rather narrow absorption band this property agrees with the experimental data [17, 42, 54]. The absorption-peak position Vcs depends on the field parameter p of the model given by... 

The assumption that the polymer spectra and structure correspond to those of the model compounds rests essentially on the approximate correspondence of peak positions. It is important that the spectra be compared at approximately the same temperature, as there is a marked dependence of the peak positions upon temperature. For the polymer at 150°, the peak positions, in -values with respect to the solvent as + 63.8 , are 104.2, 106.0, 125.6, and 127.8. At 25°, the solvent peak position was found to be +63.2 if>, and the model compound peak positions were 106.3, 108.8, 126.6, and 128.9, deviating substantially from the polymer positions. When the model compound spectra are observed at 150°, there is a marked down-field" shift, which brings the - values into much closer correspondence with those of the polymer 104.1, 106.2, 126.2, and 127.7. There appear to be some small relative changes as well, particularly for the meso compound, for the appearance of its AB-type CFa resonance is distinctly temperature dependent. The most likely explanation for this behavior is that it arises from changes in the relative populations of the internal-rotation energy levels, which correspond to changes in the time-averaged conformational structures of the molecules. 

There are however problems with such a blanket assignment of peaks at ca. 30 ppm toAF >, since they generally show only a very small magnetic field dependence (Meinhold et al. 1993a), by contrast with the strong field dependence of the crystalline AT compounds (Jansen et al. 1998). Furthermore, a consequence of such an assignment is that the position of these peaks coincides with the isotropic chemical shift, implying... 

The interpretation of these observations is as follows (3) The changes in the vibrational frequency of the -S03 stretching vibration are due to the polarization of the S-0 dipole by the electrostatic field of adjacent counterions. The magnitude of this polarization depends on the radius of the bare cation, being the largest for Li, and practically negligible for Rb and Cs. In the fully hydrated membrane, the peak position is independent of... 

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