Organic molecules closed/open shell

In Figure 4.20(b), the encounter of a closed-shell species M with the open-shell excited species M shows that the occupation of the very same molecular orbitals leads to a net stabilizing situation two electrons sink to the lower Pb orbital, with an energy gain — 2A E, while one electron is raised by +AE to I, but the excited electron now sinks into P so that the energy balance is approximately E = — 2AE + AE — AE = — 2AE. This simple picture of excimer formation applies to closed-shell organic molecules as well as to He atoms. The excimer is indeed an electronically excited dimer which is stable only so long as its electrons follow the distribution of M. ... 

Free radicals and radical ions usually have excited states of low energy, and this can be understood from a simple orbital picture (Figure 4.89). In a closed-shell organic molecule the energy spacing between the HOMO and LUMO is quite large, but in the open-shell species the lowest excitations involve transitions of an electron between closely spaced orbitals. Many free radicals and radical ions of organic molecules absorb in the VIS or NIR, while the closed-shell precursors absorb only in the UV. 

For anti-aromatic, non-aromatic, and aromatic systems NICS takes values from positive, to zero, to negative, respectively. Both closed- and open-shell systems can be treated [56], In case of planar rings the out-of-plane component of the NICS tensor (NICSzz) has advantage over the total NICS [57], Recent studies of small aromatic, anti-aromatic, and non-aromatic organic molecules demonstrated that NICSzz is equal to the z-component of the induced magnetic field [58],... 

Even with no unpaired electrons present, even-electron ions can undergo reactions thought to be characteristic of a radical site, even at sites remote from the charge (Gross 1988). Note that similar reactions occur in organic chemistry y-H/)9-cleavage rearrangements of neutral molecules take place in both closed-shell (retro-ene reaction) and open-shell (Norrish II process) systems, while these and p-H decompositions occur thermally in esters, xanthates, sulfoxides, and carbamates. For the EE" ions of Equation 8.35 (Kraft and Spiteller 1967),... 

The open state of YFj/PAH capsules for dextran 2000 kDa was observed at pH = 5, but at pH = 10 aU inorganic/organic capsules were closed and the dextran molecules captured inside. Changing the pH again to an acidic medium resulted in an opening of the capsule shell and release of dextran molecules into the surrounding solution (Fig. 3.10). A similar capsule permeability versus pH dependence was pre-... 

---