Download Chemically Induced Dynamic Nuclear and Electron by Claude Richard, Pierre Granger (auth.) PDF

By Claude Richard, Pierre Granger (auth.)

Anomalous electron-spin country populations within the Electron Paramagnetic Re­ sonance (EPR) spectra of radicals shaped in the course of radio lysis experiments have been saw in 1963 by way of FESSENDEN and SCHULER [170a]. This phenomenon didn't obtain a lot recognition on the time. In 1967, BARGON, FISCHER, and JOHNSEN [5] and independently WARD and LAWLER [7,8] stated the same phenomenon for Nuclear Magnetic Resonance (NMR) spectra taken in the course of radical reactions: emission or stronger absorption, or either. The earliest makes an attempt to provide an explanation for this new NMR phenomenon taken care of those results in a manner just like that of Dynamic Nuclear Polarization (DNP) or the Overhauser influence. even supposing the polarization has a very various starting place, DNP gave its identify to this impression: Chemically caused Dynamic Nuclear Polariza­ tion (CIDNP). [The identify Chemically precipitated Dynamic Electron Polarization (CIDEP) used to be brought later through analogy with CIDNP]. After the preliminary publica­ tions, all of the new info proven that the 1st thought couldn't be right. In 1969, a brand new thought used to be proposed through CLOSS [18] and independently by means of KAPTEIN and OOSTERHOFF [23] and known as the radical-pair conception. This mechanism used to be proposed to account for the observations of polarization in either NMR and EPR. The radical-pair concept is predicated on vulnerable interactions in a couple of radicals: the energy of interplay among the digital states of the radicals relies specifically at the nuclear-spin states.

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The rate equations for the population of singlet states and triplet states are: dN;/dt = r; - (kc + ke + L L k;ju) N; + L L kju;Nju " J " J (40 a) Application 59 where Ni or N j " is the population of the corresponding state i or ja and ri, rj " are the pair production rates in states i or ja. As reaction products are formed with S states, the number of molecules produced in nuclear-spin state i are: (41) Free radicals in nuclear-spin state i which escape the radical pair are produced at a rate: (42) The enhancement factor between states i and i' is: Mi-Mi' V;i'=---- Mi+Mi'

B) The Effective Hamiltonian. We are concerned with the study of the value of each energy which appears in Eq. (18) and to leave out the terms which are too small. i) Jt ex' This term is very important for short inter-radical distances. It explains the big difference between Sand T state energies which appear in Fig. 4. If we assume that the time spent outside region II is very short, then we are only concerned with the value of J in region II. It is then assumed that J remains constant. In this case J is of the order of the hyperfine coupling constant A.

N e (j) (i) r 12 where the two unpaired electrons of the radical pair are labelled 1 and 2; e2 /r12 is the electrostatic repUlsion energy of these two electrons, U~i denotes the attraction between the core of radical a and electron j of radical b, uld has the same meaning for radical b and electrons i of radical a, and u(ab) is the repulsion between the entire core of radical a and the entire core of radical b. If couplings between electrons and between electrons and orbital angular momentum are neglected, we can use, as basic space wave functions to calculate the energy Eo from ~, the two functions: where lPa and lPb are the space wave functions of the unpaired electrons on radicals lPsym and lPAnti are space functions but not spin functions.

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