Other systems containing unpaired electrons

Normally the term radical (or free radical ) is confined to molecules or ions with one unpaired electron (called doublet states because the electron has two magnetic quantum numbers +1/2). In the non-metal field the most common paramagnetic species other than radicals are those with two unpaired electrons, called triplet states (magnetic quantum numbers 0, 1). Quite the most important triplet-state molecule is dioxygen and it is a great pity that ESR spectroscopy can, for various reasons, only be used to detect O2 in the gas-phase or certain crystalline solids. Other important triplet-states are sometimes obtained on photo-excitation of ordinary (singlet-state) molecules or ions, and these have reactions in some ways typical of di-radicals (i.e., 

Reactions [4] and [5] only occur under exceptional conditions. Reaction [6] is a typical electron-transfer reaction. 

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The relative sizes of the polarization and delocalization mechanisms will depend on the orbitals involved in the overlap and the occupancies of the metal orbitals. For example, the polarization mechanism appears to be relatively weak for Ni. in contrast to Mn + and Cr +. even when both mechanisms involve eg orbitals. This is most likely due to the smaller exchange interaction expected for the Ni + system, which contains fewer unpaired electrons. Some earlier studies neglected the polarization mechanism, assuming that it was much weaker than the other possible shift mechanisms, which lead to incorrect assignments. ... 

We note here that Eq. (2.8) holds for a single electron in an orbital which is well separated by any other excited level. In the case of multiple unpaired electrons in different molecular orbitals, Eq. (2.8) still may hold in the absence of strong spin-orbit coupling effects but the interpretation of the hyperfine constant becomes complicated the hyperfine coupling is the sum of that for each molecular orbital. Indeed, each metal orbital which contains an unpaired electron is involved in a molecular orbital and provides a contribution to the total p for the various nuclei. The experimental data, however, provides through Eq. (2.8) the sum of the A values and therefore the sum of p. In order to make the spin density or contact constants comparable for different systems independent of the value of 5, i.e. independent of the number of electrons, the value of p is normalized to one electron, i.e. it is divided by the number of electrons which is just 2S (in such a way that p, /2S =1). Eq. (2.2) becomes... 

The first consideration by looking at Eqs. (7.5), (7.6) and (7.7) is that NOE does not contain any structural information within the present scheme. Indeed, both / >/(/) and /(/). In practice, the two-spins scheme is never a good description of any real system. The nucleus I, on which NOE is going to be observed, is always coupled to other nuclei. When dealing with hyperfine coupled nuclei, often the major source of relaxation is not the coupling with J but the coupling with the unpaired electrons. The expression for t)i(j) then is (Appendix VII)... 

In certain cases, the number of carbon atoms which may contain the unpaired electron may, through the resonance with ionic structures, be increased. If this is so the free radical will be stabilized and its formation facilitated. Thus hexa-/ nitrophenylethane, (N02GgH4)3G—C(CgH4N02)3 dissociates completely into the radical (N02G6H4)3C under conditions in which hexaphenylethane only partially dissociates. In the first case the odd electron is not only shared between the ortho and para carbon atoms of the ring systems but also with the other carbon atoms, as for example, in the structure... 

Most organic molecules contain an even number of electrons and have zero spin in the ground state. On the other hand, radicals are systems with an odd number of electrons, and have at least one unpaired electron. The highest energy level (in the sense of the simplest IPM) is supposed to be occupied by the unpaired electron, and in most unsaturated planar radicals the corresponding orbital is a a orbital. 

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