Lineshape changes, rate

The chemical shift lineshape changes observed for the crystalline fraction of melt-recrystallized PTFE from /L14°C to w28°C (Figure 6) are due to reorientation about an axis essentially parallel to Ozz (which is tilted VL2° from the molecular chain axis). The temperature dependence of the line-shape changes can be simulated as an increase in the rate of rotational diffusion about this axis (Figure 6) and this process has an activation energy of 48+11 kcal/mole. In addition, the... 

The lineshape changes observed for the amorphous fraction of the polymer between -128° and -60° (Figure 8) indicate some type of relaxation whose origin is similar to that just described. It appears that this process involves reorientation about the local chain axis and the rate of this process is 0.1 kHz at -80°C. The relaxation data of Figure 9 are consistent with this explanation and indicate that the rate of this motion is 30 kHz at -50°C. However, this process is coupled with a higher temperature process (see below) and does not give definitive lineshapes which are amenable to lineshape analysis. We can estimate that this relaxation has an activation energy of 16 5 kcal/mole. Therefore, we conclude that the process responsible for these results is the Y relaxation. 

The rate of the molecular motion responsible for the observed amorphous lineshape changes above -40° must be much less than or much greater than 1-10 kHz (T v) and much less or much greater than 100 MHz (T ) from -40 Zto 260°cl since no minimum was observed in the relaxation data. In addition, the rate must be >1-10 kHz to cause the observed lineshape changes. Therefore, the rate must be greater than 100 MHz. (At these temperatures, the rate of the a relaxation is <<1 Hz). 

These considerations lead us to conclude that the process responsible for the observed lineshape changes may be idealized as one in which the rate of the motion is fast ( 100 MHz) at all temperatures (-40°C to 260°C) and the amplitude of the motion grows as a function of temperature. Since the chemical shift lineshape is nearly axially symmetric at v -68°C (due to rapid reorientation about the local chain axis), we can describe this motion whose amplitude grows with temperature as reorientation of the local chain axis. 

Li EXSY spectra [13,64,94] yield important information with respect to the dynamic process responsible for the lineshape changes and the exchange mechanisms involved. On the basis of appropriate equations [173-176], rate constants can be obtained from the relative intensities of the crosspeaks and diagonal peaks. Alternatively, the saturation transfer experiment introduced by Hoffman and Forsen [177] can be applied using selective pulses. 

S Nonequivalent homonuclear spins. If two homonuclear spins that are not equivalent are coupled, then when other interactions are present the MAS lineshape will depend on the MAS rate (Wu and Wasylishen 1993). In the case of N in cis-azobenzene dioxide large changes in the lineshape were observed and simulations showed that the lineshape was dependent on the relative orientation of two chemical shift tensors and their orientation with respect to the intemuclear vector. 

The energy gap law can also be generalized to discuss the effects of inert rigid media upon radiationless transition rates and optical lineshapes of molecules in matrices. This generalization involves the inclusion of both the intra-and intermolecular vibrations in the rate expressions. The simplest type of inert medium is one that can only act as a heat bath, i.e., the intermolecular vibrations may have equilibrium displacements or frequency changes which... 

Among two-dimensional experiments, wideline exchange spectroscopy plays a prominent role in H NMR [1, 4, 61-64]. By correlating the frequency distributions at two different times, any changes in the resonance frequency due to reorientation can be detected in the off-diagonal intensity pattern. With the aid of lineshape simulation and by comparing different mixing times, detailed conclusions about the type and rate of motion can be drawn, as illustrated in Section 6.2.5 and Fig. 6.2.2. 

This type of experiment has been performed on Mg atoms by Karapanagioti et al. [479] the experimental data indicate a drastic change in the ionisation rate, while the coupling enables many different lineshapes to be produced by varying the detuning of the coupling laser, as shown in fig. 9.4 These results are interesting, because they demonstrate precisely the same kind of symmetry reversal effects as were described in chapter 8 (see fig. 8.6). 

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