Spin multiplicity effect

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Such variation in the lifetimes of the ion pairs, which depends on the mode of activation, primarily arises from the difference in the spin multiplicities (see above). None the less, the long-lived ion-radical pair allows the in-cage proton transfer from the cation radical ArMe+ to the CA- anion radical to effectively compete with the back electron transfer,205 i.e.,... 

Spin multiplicity, permutational symmetry, dynamic Jahn-Teller and geometric phase effects, 706-711 Spin-orbit coupling conical intersections ... 

While our primary focus has been on stable closed-shell (or low-spin) coordination species in which covalency effects are pronounced, it is also useful to examine the opposite extreme of weak coordinate bonding and free-atom -like spin multiplicities, corresponding to the original assumptions of crystal-field theory. 

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) ...

Useful applications of the Evans proton NMR shift method have permitted the determination of effective magnetic moments peff (see Section 8) and spin multiplicities for several high-spin radical-cation species.This work makes available a simple procedure for such assignments when the spin carriers are stable in solution. 

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