Spin-lattice relaxation efficiency

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

Using a simple kinetic model, Solomon demonstrated that the spin-lattice relaxation of the I and S spins was described by a system of coupled differential equations, with bi-exponential functions as general solutions. A single exponential relaxation for the I spin, corresponding to a well-defined Tu, could only be obtained in certain limiting situations, e.g., if the other spin, S, was different from I and had an independent and highly efficient relaxation pathway. This limit is normally fulfilled if S represents an electron spin. The spin-lattice relaxation rate, for the nuclear spin, I, is in such a situation given by ... 

The most important relaxation processes in NMR involve interactions with other nuclear spins that are in the state of random thermal motion. This is called spin-lattice relaxation and results in a simple exponential recovery process after the spins are disturbed in an NMR experiment. The exponential recovery is characterised by a time constant Tj that can be measured for different types of nuclei. For organic liquids and samples in solution, Tj is typically of the order of several seconds. In the presence of paramagnetic impurities or in very viscous solvents, relaxation of the spins can be very efficient and NMR spectra obtained become broad. 

The longitudinal relaxation time (often called the spin-lattice relaxation time), Tj, is concerned with the rate at which nuclei in a molecule exchange energy with their surroundings (the lattice). This time constant can vary from 10 to 10 s and is directly related to the efficiency of the coupling between the nuclear spin and the lattice [142, 143, 192]. 

The efficiency of spin-lattice relaxation of a given nucleus is also dependent on the number of nearby nuclear dipoles. In fortunate cases this can be used to obtain structural information. For example, there are more protons spatially close to the methine carbons in the c/.v-configurated tricyclic compound 3 than in the trans-isomer220. Thus, the 7 value of the methine carbons in the (Tv-isomer is considerably smaller. 

In this mechanistic scheme, the CIDNP intensities of reactant and product are determined by the competition of key steps at each stage of the reaction. For the system discussed here, the qualitative features of the observed polarization suggest that nuclear spin lattice relaxation during the lifetime of the olefin triplet state is negligible, that singlet and triplet pairs recombine with similar efficiencies, and that the triplet state decays to each of the isomers with equal efficiency. 

In addition to the direct interaction of magnetic dipoles, spin-lattice relaxation can proceed by way of interactions between the magnetic dipole of the target nucleus and fluctuating electric fields in the lattice. This is why neighboring quadrupolar nuclei (those with / >, Section 2.1) can bring about very efficient spin-lattice relaxation (short T values). 

To summarize the features of solid-state NMR spectroscopy, spin-lattice relaxation is generally inefficient (long 7, s), spin-spin relaxation is extremely efficient (short T2 s), and spectral lines are quite broad due to the anisotropic nature of the interactions. 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

With temperature increase from T = 1.2 K to T > 5 K, the decay behavior changes drastically. At T = 5 K, the decay is already monoexponential with a decay time of r(5 K) = (230 10) ps (Plot (b) of Fig. 6). Within limits of experimental error this value is constant at least up to T = 40 K [57]. Obviously, temperature increase induces an efficient spin-lattice relaxation between the three triplet substates. This leads to a fast thermalization. The observed monoexponential decay demonstrates that the sir is much faster than the shortest emission decay component. 

Si spin-lattice relaxation times for organosilicon compounds are generally greater than 20 s. Even in cases where there are directly attached protons which contribute to a very efficient DD mechanism in... 

Spin-lattice relaxation is a process by which the excited spins give up energy to the surroundings (the lattice). This type of relaxation is most efficient when the molecule tumbles at a rate that is very close to the resonance frequency of the nucleus being studied. The rate of tumbling of a molecule is described by the correlation time The correlation time can be approximated by... 

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