SOMO singly occupied molecular orbital

It is important to note here that both of the 5-exo radical cyclizations (133—>132—>131, Scheme 27) must proceed in a cis fashion the transition state leading to a strained mms-fused bicy-clo[3.3.0]octane does not permit efficient overlap between the singly occupied molecular orbital (SOMO) of the radical and the lowest unoccupied molecular orbital (LUMO) of the alkene. The relative orientation of the two side chains in the monocyclic radical precursor 134 is thus very significant because it dictates the relationship between the two outer rings (i. e. syn or anti) in the tricyclic product. The cis-anti-cis ring fusion stereochemistry of hirsutene would arise naturally from a cyclization precursor with trans-disposed side chain appendages (see 134). 

A radical has a singly occupied molecular orbital (SOMO). This is the frontier orbital. The SOMO interacts with HOMO and the LUMO of closed-shell molecules to stabilize the transition state (Scheme 27). The radical can be a donor toward a monomer with low LUMO or an acceptor toward one with high HOMO. 

It is well known that Hund s rule is applicable to atoms, but hardly so to the exchange coupling between two singly occupied molecular orbitals (SOMOs) of a diradical with small overlap integrals. Several MO-based approaches were then developed. Diradicals were featured by a pair of non-bonding molecular orbitals (NBMOs), which are occupied by two electrons [65-67]. Within the framework of Hiickel MO approximation, the relationship between the number of NBMOs,... 

Thus, the apparent paradox lies in the fact that radical and radical-ion electrocyclic reactions are all forbidden in the Woodward-Hoffinann sense because the symmetry of the singly occupied molecular orbital (SOMO) changes... 

Figure 4 (a) Singly occupied molecular orbital (SOMO) of dimer radical anion of acetonitrile... 

Figure 6 Singly occupied molecular orbital (SOMO) of a propeller-like trimer radical anion of acetonitrile obtained using density functional theory. The structure was immersed in a polarizable dielectric continuum with the properties of liquid acetonitrile. Several surfaces (on the right) and midplane cuts (on the left) are shown. The SOMO has a diffuse halo that envelops the whole cluster within this halo, there is a more compact kernel that has nodes at the cavity center and on the molecules.

For Eq. (9.35) to be useful the density matrix employed must be accurate. In particular, localization of excess spin must be well predicted. ROHF methods leave something to be desired in this regard. Since all doubly occupied orbitals at the ROHF level are spatially identical, they make no contribution to P only singly occupied orbitals contribute. As discussed in Section 6.3.3, this can lead to the incorrect prediction of a zero h.f.s. for all atoms in the nodal plane(s) of the singly occupied orbital(s), since their interaction with the unpaired spin(s) arises from spin polarization. In metal complexes as well, the importance of spin polarization compared to tire simple analysis of orbital amplitude for singly occupied molecular orbitals (SOMOs) has been emphasized (Braden and Tyler 1998). 

The increasing s-character of the singly occupied molecular orbital (SOMO) necessary for deviation of geometry of the radical from planarity is evident in the hyperfine splitting observed. There is an increase in the hyperfine splitting constant ( ) value which accompanies the increase in s-character and the concomitant decrease in planarity of the radical95 (see Table 9). 

Radicals can be either reduced (to anions or organometallics) or oxidized to cations by formal single electron transfer (Scheme 11).50 Such redox reactions can be conducted either chemically or electro-chemically51 and the rates of electron transfer are usually analyzed by the Marcus theory and related treatments.50 These rates depend (in part) on the difference in reduction potential between the radical and the reductant (or oxidant). Thus a species such as an a-amino radical with high-lying singly occupied molecular orbital (SOMO) is more readily oxidized, while a species such as the malonyl radical with a low-lying SOMO is more readily reduced. The inherent difference in reduction potential of substituted radicals is an important control element in several kinds of reactions. 

The anion diradical (BQNN)- has two nondegenerated single occupied molecular orbitals (SOMOs). One is delocalized over the entire molecule and the other (SOMO ) is localized within the NN group. 

HMO) formalism, because its singly occupied molecular orbital (SOMO) is constrained by symmetry to be on those atoms. More sophisticated molecular orbital analysis finds not only equal, positive spin densities on the end carbons but also a small negative 7T-spin density on the central carbon due to spin polarization. At the UB3LYP/6-31G level, the spin density p ) - p(C3) = +0.700, mostly from 7T-spin contributions, and p(C2) = -0.275.27 The experimental numbers estimated for TT-spin density (not overall spin density) are p(C ) = p(C3) = +0.582 and p(C2) = -0.164 from electron paramagnetic resonance (EPR) studies of 13C hyperfine coupling (hfc).28... 

The shape of the singly occupied molecular orbital (SOMO) supports the theory of head-to-tail interactions between two 1,3,2,4-dithiadiazolyl monomers to give the 1,2,3,5-dimer 11 via transition state 10 (Equation 2). 

Another type of transitions where OVGF can be readily applied corresponds to excitations in open-shell systems with electron transferring from doubly occupied orbitals to the singly occupied molecular orbital (SOMO). The excitation energies for such transitions can be evaluated using IPs of the corresponding anion ... 

The most interesting feature about the Jahn-Teller-distorted Qq ion is the electron density of the singly occupied molecular orbital (SOMO). It turns out that the SOMO is localized mainly around the equator and has only very small electron density around the poles, through which the principal symmetry axis... 

Fig. 11.10. Three-dimensional representation of the singly occupied molecular orbital (SOMO) for the quinquepyrrole. (Reprinted with permission from D. J. Fermin, M. Mostany, and B. Scharifker, Electronically Conducting Polymers Synthesis and Electrochemical Properties of Polypyrrole, Curr. Topics Electrochem. 2 132-136, 1993.)...

In Chapter 4 we talked about the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of organic molecules. CH3 (like all radicals) has an orbital containing one electron, which we call a Singly Occupied Molecular Orbital (SOMO). 

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