Polarization cross-relaxation

The first example of chemically induced multiplet polarization was observed on treatment of a solution of n-butyl bromide and n-butyl lithium in hexane with a little ether to initiate reaction by depolymerizing the organometallic compound (Ward and Lawler, 1967). Polarization (E/A) of the protons on carbon atoms 1 and 2 in the 1-butene produced was observed and taken as evidence of the correctness of an earlier suggestion (Bryce-Smith, 1956) that radical intermediates are involved in this elimination. Similar observations were made in the reaction of t-butyl lithium with n-butyl bromide when both 1-butene and isobutene were found to be polarized. The observations were particularly significant because multiplet polarization could not be explained by the electron-nuclear cross-relaxation theory of CIDNP then being advanced to explain net polarization (Lawler, 1967 Bargon and Fischer, 1967). 

Hence, provided that I g is known and that R has been determined by means of an independent experiment, provides the cross-relaxation rate ct. This enhancement is called nuclear Overhauser effect (nOe) (17,19) from Overhauser (20) who was the first to recognize that, by a related method, electron spin polarization could be transferred to nuclear spins (such a method can be worked out whenever EPR lines are relatively sharp it is presently known as DNP for Dynamic Nuclear Polarization). This effect is usually quantified by the so-called nOe factor p... 

Any CIDNP-based assignment of the sign and relative magnitude of hfcs is valid only if the radical pair mechanism (RPM) is operative they become invalid if an alternative process is the source of the observed effects. The triplet-Overhauser mechanism (TOM) is based on electron nuclear cross-relaxation. For effects induced via the TOM, the signal directions depend on the mechanism of cross-relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. 

A third source of misassignment has its roots in the existence of nuclear-nuclear cross- relaxation." Again, depending on the mechanism of cross-relaxation and on the polarization of the originally polarized nucleus, this may result in enhanced absorption or emission. This process induces nuclear spin polarization in nuclei without hfc, or alters the nuclear spin polarization of nuclei with weak hfcs. On the other hand, the magnitude of these effects may be quite small and fall below the threshold of chemical significance. 

Properties of the selective pulses are used therefore twofold in such experiments. Firstly, a selective pulse selectively perturbs the selected spin and the perturbation is distributed in the course of the experiment among the coupled spins, depending on the type of coupling (scalar, dipolar) and depending on the type of exchange mechanism (polarization transfer, cross polarization or cross relaxation). Secondly, the phase (selective 90° pulse) or the frequency (selective 180° pulse) of the selective pulse serve to label the response of both the selected and the residual coupled spins as positive or negative. 

In the cross-relaxation mechanism, the spin changes polarization (a (3) but not the site (i, j) ... 

The cross-polarization (CP), i.e. the transfer of I-spin polarization to the dilute spins (S), is a double resonance experiment in which the I and S spins are coupled by a certain interaction, determined by the cross relaxation time tb. From the dynamics of the CP process, usually described with the spin temperature concept, the following equation for the time dependence of S-spin polarization could be derived ... 

Up to now steady state NOEs have been considered, i.e. when one signal is saturated for a long time with respect to T of the nucleus on which NOE is going to be measured. Let s consider here what happens when the saturation time is short and variable. The resulting NOE is called truncated NOE [17] because not enough time is left for full polarization transfer. These experiments are of fundamental importance for the measurement of pi, for evaluating cross relaxation, and to avoid or to measure spin diffusion. 

The first step in the mechanism is the photoexcited triplet CIDEP process with the different quantum efficiencies of producing radicals in the upper and lower spin states being denoted as Q+ and Q, respectively. The corresponding nuclear spin states are n+ and n . The second step is the partial transfer of the electron spin polarization to the nuclear spin states by electron-nuclear cross relaxation, Wq and W2 Finally, the radicals with nuclear spin polarization must react to form diamagnetic products for CIDNP observation. These chemical reactions must compete... 

Under continuous uv irradiation, the observed steady-state polarization (whether by cw or by FT spectrometers) may be substantially modified by various nuclear relaxation processes. For example, Closs and Czeropski (35,36) have demonstrated that CIDNP can be transferred from a group of polarized nuclei to another group not originally polarized. Both the dipolar and the scalar relaxation mechanisms (of the nuclear Overhauser effects) can be operative. The extremely interesting case of intramolecular dipolar nuclear cross relaxation reported by Closs and Czeropski (35) involves the thermal reaction of... 

The polarization of the olefinic protons of the in-cage recombined product (1) was derived from the originally polarized cyclopropyl protons via the intramolecular dipolar cross relaxation. 

First attempts to explain the new NMR phenomena invoked electron-nuclear cross-relaxations in intermediate radicals and were based on a formalism similar to that of dynamic nuclear polarization or Overhauser effects ).Accord-... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

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