Hyperfine modulation model

For main-chain acrylic radicals, created in solution at room temperature and above, the presence of a superposition of conformations or Gaussian distributions is unlikely. Polymers undergo conformational jumps on the submicrosecond timescale, even in bulk at room temperamre. ° The first two theories above require that the radicals be fairly rigid with little (Gaussian distribution) or no (superposition of static conformations) movement around the Cp bond. The main-chain radical is sterically hindered but still quite flexible, and a dramatic change in the hybridization at Ca is unlikely. We have approached our simulations with the hyperfine modulation model. 

The third and most recent theory comes from Matsumoto and Giese, who suggested that the observed steady-state (SSEPR) spectrum of radical c is due to a superposition of two conformations of the same radical, and that one of these structures has a pyramidalized center. Iwasaki et al. invoked a fourth model, hyperfine modulation, to explain the spectra of the propagating radical. They were able to simulate the observed 9- and 13-line SSEPR spectra using a set of modified Bloch equations for a two-site exchange model between two conformations. 

Hyperfine coupling constants provide a direct experimental measure of the distribution of unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for electronic wave functions [1,2], Conversely, given an accurate theoretical model, one can obtain considerable information on the equilibrium stmcture of a free radical from the computed hyperfine coupling constants and from their dependenee on temperature. In this scenario, proper account of vibrational modulation effects is not less important than the use of a high quality electronic wave function. 

The F.SF.FM due to to hyperfine couplings of specifically deuterated samples were more easily analysed than those from in early applications because of the deeper modulations. Examples of experimental results, leading to a suggested interpretation by a geometric model, are shown in Fig. 3.31. 

A simple demonstration of tau-suppression is illuslrated in Figure 17, which again uses the idealized model of a weak hyperfine spectrum. The top spectrum corresponds to data recorded with typical stimulated echo pulse parameters, specifically, tau equal to 150 ns, the starting value of the interpulse spacing between pulses 2 and 3 set to 70 ns, and toe latter incremented by 10 ns. The complete spectrum is observed because tau does not correspond to any harmonic of the modulation frequencies extant But, if one records toe modulation series with tau set to 126 us, which correspouds to the period of toe Amj=2 line at 7.94 MHz, there is... 

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