Unpaired electron spin distribution radicals

Other molecular properties and phenomena that can benefit from the aid of visualization are the distribution of unpaired electron spin in radicals and the changes in orbitals and charge distribution as a reaction progresses. These and many other... 

For closed-shell molecules (in which all electrons are paired), the spin density is zero everywhere. For open-shell molecules (in which one or more electrons are unpaired), the spin density indicates the distribution of unpaired electrons. Spin density is an obvious indicator of reactivity of radicals (in which there is a single unpaired electron). Bonds will be made to centers for which the spin density is greatest. For example, the spin density isosurface for allyl radical suggests that reaction will occur on one of the terminal carbons and not on the central carbon. 

Electron spin resonance (esr) 10 2 to 1 Excitation of unpaired electron-spin orientations in a magnetic field Electron distribution in radicals, electron-transfer reactions (Section 27-9)... 

Preliminary results for Li salts are shown in Table XII.It is seen that for the neutral radical TTBP the Li enhancement is negative, whereas for the radical anion DBSQ it is positive. Both these radicals have a similar electronic distribution, so the different effects of the radicals may be due to the stronger interaction between the positively charged lithium ion and the negative DBSQ ion in forming a transient ion pair, and thus allowing a transfer of some unpaired electron spin density. Although for WBPC the unpaired electron density resides on... 

The process of establishing a geometry for and assigning a ground-state electronic symmetry toa free radical is one of comparison of the measured hyperfine interactions with those (a) predicted theoretically, and (b) determined experimentally for iso-electronic species. Great reliance is placed on the isoelectronic principle free radicals having the same numbers of electrons and nuclei will have similar structures and unpaired electron spin density distributions. In cases where the isoelectronic principle cannot be invoked for lack of examples one must resort to the theoretical estimation of hyperfine interactions inorder to ascertain that a proposed electronic structure is consistent with the experimental parameters. 

A major limitation for NMR spectroscopy is the intrinsically low sensitivity due to the rather unfavorable Boltzmann distribution for nuclear spins at thermal equilibrium. Thus, considerable effort in magnetic resonance spectroscopy is made towards sensitivity enhancement by hyperpolarization techniques, such as optical polarization, para-hydrogen-induced polarization enhancement, and dynamic nuclear polarization (DNP), a method which exploits the magnetization of unpaired electrons in stable radicals or transition metals to enhance nuclear polarization beyond the Boltzmann limit. In the chapter Dynamic Nuclear Hyperpolarization in Liquids, the fundamental theory for different polarization transfer... 

In PMD radicals, the bond orders are the same as those in the polymethines with the closed electron shell, insofar as the single occupied MO with its modes near atoms does not contribute to the bond orders. Also, an unpaired electron leads the electron density distribution to equalize. PMD radicals are characterized by a considerable alternation of spin density, which is confirmed by epr spectroscopy data (3,19,20). 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

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