Linewidths asymmetry

The effect of a pressure of 80 and 150 MPa on the spin-state transition has been also studied [169], a series of spectra obtained at 150 MPa being shown in Fig. 32. The speetra show relaxation effects as line broadening and linewidth asymmetry. Calculated spectra were obtained in the same way as at ambient pressure. Rate constants for a number of temperatures are listed in Table 12, the parameter values resulting from an Arrhenius plot of the rate constants being listed in Table 13. In Fig. 33, the quantity 5g of Eq. (36) has been plotted as a... 

As seen in Figure 10a, the C CP/MAS NMR spectrum of the quinuclidine complex shows the expected asymmetry (for instance, different signals for C-8a and C-9a) due to the asymmetric position of the lithium cation relative to the carbon framework in accordance with the X-ray investigation . The observed linewidths are not exceptionally large, and only small effects on the linewidths were observed when Li was substituted by Li, as shown in Figure 10b. 

Nuclei with a spin number 7 of one or higher have a nonspherical charge distribution. This asymmetry is described by an electrical quadrupole moment, which, as we shall see later, affects the relaxation time and, consequently, the linewidth of the signal and coupling with neighboring nuclei. In quantum mechanical terms, the spin number 7 determines the number of orientations a nucleus may assume in an external uniform magnetic field in accordance with the formula 27+1. We are concerned with the proton whose spin number 7 is 1/2. 

The intermolecular distance between the hydrogen atoms and the chlorine atoms of adjacent molecules in crystalline chloroform has been obtained from a study of the relative linewidths of the 35C1 frequencies in the protonated and deuteriated species95. A Zeeman 35C1 NQR study96 of monocrystalline chloroform and methylene dichloride confirmed that the field-gradient principal z-axes coincide with the directions of the C—Cl bonds. The asymmetry parameters were small, as expected for aliphatic C—Cl bonds. 

Conventional X-ray diffraction measures a space and time average of the electronic density. Therefore, any dynamical disorder will be transformed into spatial disorder between positions whose probabilities are determined by the average time spent on each position. Certainly, one of the most tremendous advantages of NMR compared to X-ray diffraction is its ability to measure the occurrence of motion at different time-scales. Whether the motion correlation time is on the Larmor frequency scale, the linewidth scale or much slower (exchange NMR) will affect differently the NMR parameters like relaxation rates, apparent anisotropy and asymmetry of the interaction and ID or 2D lineshape. With suitable sequences, the motion correlation times and site probabilities as a function of an external parameter (temperature or pressure) can be explicitly measured. 

In a similar manner, the coalescence temperature for the methyl groups of the tetramer was determined as 4°C. AG was calculated as 12.7 kcal/mol which was 3.7 kcal/mol smaller than that for the pentamer. The hexamer showed a total of seven signals with narrow linewidth due to two methyl groups and six methine protons even at 70°C, indicating that the rate of helix sense reversal is much slower than the rate for the pentamer. This suggests the possibility that the symmetrical oligomers over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. This was confirmed by the chiral HPLC technique using (+)poly(triphenylmethyl methacrylate) as a stationary phase.282... 

Here, T2 is the spin-spin relaxation time Av is the linewidth eq is the electric held gradient at the nucleus eQ is the nuclear quadrupole moment and /(rR, rex) is some function of the correlation times rR and rex, for rotational motion and exchange of the ions, respectively. An asymmetry parameter of zero is assumed (18). 

Information about PPV chain motion could be obtained from a vinylene labelled material PPV-d2 [89] the presence of ring motion obscures data about chain motion from measurements of PPV-c/4. The interest here was to understand the ring-flip mechanism and determine whether there might exist a particular kink-propagation mechanism for the sp hybridized chain, analogous to the three-bond motions of vinyl polymers. At ambient temperature PPV-rf2 possesses a static Pake doublet spectrum. Figure 8.7(a) shows the spectrum at 220 "C the linewidth is reduced and there is a large apparent asymmetry parameter [t] = 0.14). 

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