Experimental techniques nuclear spin relaxation

Because we assume that our reader is most likely more of a theoretician, working in the area of computer simulations, rather than an NMR specialist, we will start with some background in nuclear spin relaxation. It gives us a good opportunity to discuss the relaxation models from a simulators point of view -as well as - to present the expressions to implement the method. Also, we believe that the material should be valuable to the reader from the NMR community, because it both shows how naturally the formalism is incorporated into the simulation techniques and demonstrates the benefits in employing MD simulations to evaluate the theoretical models and interpret experimental relaxation data. NMR relaxation [8,9] contains information of processes on molecular time scales, from nanoseconds to picoseconds, which perfectly coincides with the time scales of MD simulations (Figure 5). Since MD simulations are based on molecular interaction models, they can be used to elucidate and extract molecular information... 

By the same experimental technique, the temperature dependence of the nuclear spin relaxation rates was investigated for the radical cations of dimethoxy- and trimethoxybenzenes [89], The rates of these processes do not appear to be accessible by other methods. As was shown, l/Tfd of an aromatic proton in these radicals is proportional to the square of its hyperfine coupling constant. This result could be explained qualitatively by a simple MO model. Relaxation predominantly occurs by the dipolar interaction between the proton and the unpaired spin density in the pz orbital of the carbon atom the proton is attached to. Calculations on the basis of this model were performed with the density matrix formalism of MO theory and gave an agreement of experimental and predicted relaxation rates within a factor of 2. 

Experimental techniques for the measurement of Si nmr spectra (mainly for solutions) are briefly discussed, followed from considerations of the nuclear-spin relaxation mechanisms with emphasis on the Si nucleus. Chemical shifts 5 Si and indirect nuclear spin-spin coupling constants j( Si,x), the most prominent nmr parameters, are discussed in more detail The increasing reliability of quantum chemical methods for the calculation of these parameters is pointed out. The relationship between the electronic structure and the nmr parameters b Si and j( Si,x) is indicated... 

Among the several experimental techniques for studying colloid and sol-gel chemistry, perhaps the most powerful one is nuclear magnetic resonance (NMR) spectroscopy. NMR allows investigation of these systems by a combination of structural information, obtained through lineshape analysis and chemical shift data, and indirect information from dynamic measurements of spin relaxation and diffusion. 

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