Radicals have singly occupied molecular orbitals

Radicals have singly occupied molecular orbitals... 

Bock et al. studied the cation radicals of various organosilanes having a -system in the / position extensively and found that the charge is delocalized considerably into the silyl groups [9,10]. Kira and Sakurai et al. also carried out an ESR study of cation radicals of allylsilanes and revealed the large polarization of the SOMO (singly occupied molecular orbital) as a consequence of the a-% interaction [11],... 

Radicals with nucleophilic character add to electrophilic alkenes more rapidly than to nucleophilic alkenes whilst, conversely, the rate of addition of electrophilic radicals to electron-rich alkenes is greater than addition to electron-deficient alkenes. To be more sophisticated, we should refer to a high-SOMO (singly occupied molecular orbital nucleophilic) radical interacting favourably with a low-LUMO (lowest unoccupied molecular orbital electrophilic) alkene but, for simplicity and brevity, we will continue to use these short-hand terms [177]. For instance, compare rates of addition of perfluor-oalkyl radicals to ethene and various fluorinated derivatives with their rates of H-atom abstraction from heptane (Table 7.9) [186]. Broadly, reactivity of the alkene decreases with fluorine content, with trifluoromethyl having a large effect. 

In contrast to the HOMO and LUMO, the singly occupied molecular orbital (SOMO) can be correlated both qualitatively and quantitatively with experimentally measurable EPR hyperfine couplings (hfcs). As a result, host-guest systems that have redox-active guests that are stable as radicals provide excellent tools for studying the effects of noncovalent interactions on redox properties. 

As a last open shell system, we have chosen the H2C0 radical, which has been well characterized both at the theoretical and experimental level [77-80]. In this radical the symmetry of the singly occupied molecular orbital (a rt-in plane orbital located mainly on the oxygen center) [79] determines that only spin polarization effects contribute to isotropic hcc s of C and O, whereas spin densities at hydrogens have also a direct delocalization contribution. The results of table 5 show that, as expected, isotropic hcc s at the H atoms are well reproduced by aU the functionals, whereas the results for heavy atoms are much more scattered. In particular, the hyperfine constant of the carbon atom is -35 G at the PBEO level, in better agreement with the experimental value (-39 G)[80] than the B3LYP prediction (-34 G). It is noteworthy that the PBEO functional provides values for the H and C atoms which are sufficiently accurate. Unfortunately, no experimental value is available for the oxygen atom. 

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