Spin pairing, electrons

The ground state (0 kJ/mol) for the CL molecule is represented by the term symbol 3v . The first excited state (92 kJ/mol above the ground state) is a 1 singlet (electrons spin paired with both electrons in either the n x or the n y level). The 1 v state with paired spin electrons, one each in the 7i v and n y levels, is the next excited level 155 kJ/mol above the ground state. Reduction of 02 by one electron yields the superoxide ion (02), a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). Bond lengths and bond orders for these are given in Table 4.2. As noted in equation 4.2, the reduction potential for 02 in the presence of protons is thermodynamically favorable. Therefore, reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled.8... 

Fig. 19.4 Illustration of the effect of the number of fused benzene rings and edge functionality on the electron spin pairing processes (M = spin multiplicity) in graphene (a) C14H10 (M = 1) ...

Eclipsed conformation, 7, 254 Electrocyclic reactions, 341, 344-348 Electrol ic oxidation, 307 Electrolytic reduction, 307 Electromeric effect, 24 Electron configuration, 3 Electron density, 21, 26, 29, 393 Electron-donating groups, 23, 26 addition to 0=C and, 183 addition to 0=0 and, 205, 206 aromatic substitution and, 153, 158 pinacol change and, 115 Electronegativity, 21, 22, 95 Electrons, lone pair, 10, 72 Electron spin, paired, 2, 308 Electron-withdrawing groups, 23 acidity and, 59, 61, 62, 272... 

When appropriate numbers of electrons are added to occupy the molecular orbitals, the results shown in Figure 7.20 are obtained. Both N2 and F2 have all their electrons spin-paired, but 02 has two unpaired electrons in the degenerate ir 2p orbitals. Both N2 and F2 are therefore diamagnetic, while 02 is paramagnetic. 

In the weak-interaction model (85) developed in the previous section to explain ion-pairing in metal-ammonia solutions, aggregation interactions involving Ms+ and es are relatively weak, and leave the isolated solvated electron properties virtually intact. However, a major difficulty (29,54,134) arises with the type of model when one considers the precise nature of the corresponding electron spin-pairing interaction in ammonia solutions. It is worth expanding on this issue because it probably remains one of the fundamental dilemmas of metal-ammonia solutions in the dilute range (54). 

Fig. 5. Electron spin pairing in metal-ammonia solutions at 25°C (298 K), 0°C (273 K), and -33°C (240 K). Paramagnetic spin concentrations for sodium-ammonia and potassium-ammonia solutions. [Experimental data from Refs. 47, 76, 98, 114,115, and 159. Adapted from Harris (88). Used with permission.] The solid line indicates the expected spin-pairing behavior for noninteracting electrons (88).

The phenomenon of four singly-occupied active-space orbitals (AO or MO) arises in many seemingly-unrelated molecular situations. The singlet spin (S = 0) Rumer diagrams for these orbitals indicate that there are two linearly-independent or canonical spin-pairings schemes. These spin-pairings are well-exemplified by the 7i-electrons of butadiene, for which two canonical Lewis VB structures with different 7i-electron spin pairings are those of 1 and 2. 

As an aside, it is interesting to note that the tetragonal compound FcNiN can be prepared. From the model, it was predicted that the compound should be ordered into alternate Fe and Ni layers perpendicular to the c axis with the N atoms filling all the octahedral interstices of the Fc layers. (The filled d - orbitals at the Ni atoms prevent occupation of the octahedral sites in a Ni layer.) This atomic ordering was indeed found. Further, with all localized electrons spin-paired, the compound is paramagnetic (224). 

A radical pair (two interacting radicals) can exist in either the singlet state (electronic spins paired) or triplet state (with electronic spins parallel). 

Boon, W. and Vangerven, L. (1992) Magnetization of electron-spin pairs in a free-radical at low-temperatures in a high magnetic held. Physica B, 177,527-30. [203] Boone, C. D. G, Derissen J. L. and Schoone, J. C. (1977). Anthranilic acid II (o-aminobenzoic acid). Acta Crystallogr. B, 33, 3205-6. [59]... 

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