Electron-nuclear 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). 

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. 

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... 

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. 

As stated above, CIDNP denotes the transient occurrence of anomalous line intensities in NMR spectra recorded during chemical reactions or shortly after their completion. The phenomenon was first observed in 1967 by Bargon, Fischer and Johnsen [35a] in thermal decompositions of peroxides and azo compounds, and, independently, by Ward and Lawler [35b] in the reactions of alkyl lithium with alkyl halides. It was immediately realized that the line anomalities are caused by populations of the nuclear spin states in the reaction products that deviate from the Boltzmann populations. After initial attempts of interpreting CIDNP by electron-nuclear cross-relaxation, the radical pair mechanism was developed in 1969 by Kaptein and Oosterhoff [36a], and independently by Closs [36b],... 

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... 

When the electron spins are coupled with nuclear spins, the cross relaxation accompanying the change of the nuclear spin state can occur. In this case the apparent spectral overlap of the A and the B spins is not necessary. The spectral averaging of Eq. (3) is therefore a difficult task. Instead, we assume that the spectral overlap function in Eq. (2) is given by a constant F. Then, the spatial averaging of Eq. (3) is necessary for correlating the observed relaxation kinetics with the theory. The result of the spatial averaging will be shown for the two extreme cases of the spatial distribution of radicals in solids. 

In this section, I review the work on nuclear multi-spin relaxation phenomena - the work where one of the involved spins belongs to electron will be covered in section 2.7. We begin with the cross-relaxation (nuclear Overhauser enhancement, NOE) measurements and continue with experiments designed for saturation transfer difference (STD) measurements. Next, we turn to investigations of more complicated multispin relaxation phenomena such as cross-correlated relaxation. Finally, papers devoted to relaxation-optimized methods and to large spin systems are also included in this section. 

Two ongoing experiments are described. In both experiments optical pumping is employed as a means of creating non-equilibrium spin distributions. In both cases the spins couple with one another via either nuclear or electron spin-spin interaction. The first experiment is to measure spatial transport of spin orientation in order to determine the mean spin-spin cross-relaxation rate between Tm" ions in SrF2. In the second experiment transient changes in spin population are observed as Pr" "3 spin levels come into resonance with neighboring F spin levels. This monitors the cross-relaxation rate between Pr" "3 and F and provides a means for verifying the spectroscopy of the Pr" "3 ions. 

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