Orbital ordering, electronic

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

In order to apply the theory, one first draws a valence bond formula with the correct constitution, including all lone electron pairs. This formula shows how many valence electron pairs are to be considered at an atom. Every electron pair is taken as one unit (orbital). The electron pairs are being attracted by the corresponding atomic nucleus, but they exercise a mutual repulsion. A function proportional to 1 /rn can be used to approximate the... 

A drawback associated with Eqn. IV.3 is that this expression is more concerned with counting orbitals than electrons. In order to improve this situation, a further description of the bonds originating from Bl is introduced through Defs. III.2, III.3, and III.7, thus permitting a reformulation of Eqn. IV.2... 

As was mentioned previously, simple orbital products (electron configurations) must be converted into antisymmetrized orbital products (Slater determinants) in order to satisfy the Pauli principle. Thus, proper many-electron wavefunctions satisfy constraints of exchange antisymmetry that have no counterpart in pre-quantum theories. 

The electrical conductivity of TTF TCNQ is of the order of 10 S m at room temperature and increases with decreasing temperature until around 80 K when the conductivity drops as the temperature is lowered. TCNQ is a good electron acceptor and, for example, accepts electrons from alkali metal atoms to form ionic salts. In TTF-TCNQ, the columns of each type of molecule interact to form delocalised orbitals. Some electrons from the highest energy filled band of TTF move across to partly fill a band of TCNQ, so that both types of columns have partially occupied bands. The number of electrons transferred corresponds to about 0.69 electrons per molecule. This partial transfer only occurs with molecules such as tetrathiafulvalene whose electron donor ability is neither too small nor too large. With poor electron donors, no charge transfer... 

The presence or nodes and small subnodal maxima" docs have a profound effect on the energy of electrons in different orbitals. An electron in an orbital with these subnodal maxima (particularly s orbitals with higher values of m are said to be penetrating, that is. they have considerable electron density in the region cl the nucleus. This is the fundamental reason for the ordering of the energy levels in poly-electronic atoms lj. 2s, 2p. 3. ip, etc. (see pages 20-22). 

We found that these more sophisticated spin-coupled calculations, which used larger basis sets with polarization functions on all of the atoms and which allowed the a orbitals to relax, produced a picture of bonding in the 7t-electron system of benzene which is practically identical to that described earlier. As before, we found six equivalent spin-coupled orbitals which are transformed into one another by successive C6 rotations. The overlaps between the orbitals, ordered cpa to cp6 around the ring, are reported in Table 1. In this case, the electron correlation effects incorporated in the spin-coupled model provide an energy improvement over the SCF description of 170 kJ mol - with a further lowering of 20 kJ mol -1 on including spin-coupled ionic structures. 

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