Ground state electronic symmetry

The process of establishing a geometry for and assigning a ground-state electronic symmetry toa free radical is one of comparison of the measured hyperfine interactions with those (a) predicted theoretically, and (b) determined experimentally for iso-electronic species. Great reliance is placed on the isoelectronic principle free radicals having the same numbers of electrons and nuclei will have similar structures and unpaired electron spin density distributions. In cases where the isoelectronic principle cannot be invoked for lack of examples one must resort to the theoretical estimation of hyperfine interactions inorder to ascertain that a proposed electronic structure is consistent with the experimental parameters. 

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

Thus four of the seven lowest H20 MOs are linear combinations of the four a, symmetry orbitals listed above, and are a, MOs similarly, the two lowest b2 MOs are linear combinations of 02p and H,1j — H21.s, and the lowest bx MO is (in this minimal-basis calculation) identical with 02px. The coefficients in the linear combinations and the orbital energies are found by iterative solution of the Hartree-Fock-Roothaan equations. One finds the ground-state electronic configuration of H20 to be... 

In D6h symmetry the dipole moment operator forms a basis for the representations Aiu Ei . The ground-state electronic state function belongs to A, and... 

Optimized structure of the F3Si-0-0 radical in ground state (Cs symmetry), modeling the site in silica, is shown in the Figure 7.16d. The unpaired electron occupies the 2px atomic orbital of the terminal oxygen atom (the X-axis is perpendicular to the plane of symmetry). The first electronically excited state (2A ) of the radical is related to the electronic transition of the lone pair of the terminal O atom to the 2px atomic orbital of the same atom. The energy of this vertical transition was found to be equal to AE — 0.5 eV. 

The benzene and azabenzenes form iso-electronic series of molecules, as naphthalene and the azanaphthalenes also do. The ground state electronic and geometric structures are therefore quite similar within one series. The substitution of CH groups with nitrogens introduces lone-pair to 7r transitions, and lowers the benzene and naphthalene symmetries. Small and systematic trends are found for linear response properties of the azabenzenes [189]. Each molecule is, however, very specific with respect to phosphorescence due to the delicate nature of the SOC and electric dipole activity interactions. 

The point group symmetry of thiophene is C v According to MO theory, the ground-state electronic configuration can be written as... 

To sum up this second part of our contribution, we propose to describe the behaviour of the cyclic delocalized molecules in all the magnetic properties of the matter (i.e. N.M.R., diamagnetism, Faraday effect) by the concept of potential strobilism. This depends directly upon the electronic and geometrical characteristics of the ground state electron (o+ir) distribution, symmetry point group, etc. 

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