Htickel molecular orbital calculation

Htickel molecular orbital calculations have been performed for anthranil the n electron densities and mobile bond orders of Del Re80 and Berthier and Del Re81 are given in Fig. 2. 

Extensive research activities have been directed towards elucidating the actual site of activation and reduction of dinitrogen. Extended Htickel molecular orbital calculations carried out on idealised models (Figure 4.28, 45a) of the cofactors FeMco (M = Mo, V, Fe) of the three nitrogenases suggest, based on the similarity of the electronic structures. 

In contrast to the useful conceptual framework provided by the approximate approach just described, the results of more detailed molecular orbital calculations have on the whole been rather disappointing. Thus, although some semi-empirical SCF treatments were attempted, most of the earlier MO calculations for metallocene systems (18, 161, 162, 163, 164,165) suffered from such deficiencies as the neglect of the a-framework, or the use of various one-electron Hamiltonians, for example the various Wolfsberg-Helmholz techniques. Of late, Drago and his coworkers have carried out further Extended Htickel type computations for a wide range of both metallocene and bis-arene species (153, 154), and similar... 

First all it should be noted that for some types of the matrix S (see below) the eigenvectors c, of the topological matrix of the molecule coincide precisely with those of the Htickel method, i.e. with the series of coefficients defining the molecular orbitals calculated by this method. 

That is why the molecular orbitals calculated by the Htickel method are called topological,... 

The Htickel method predicts that in the case of a long polyene chain the longitudinal polarizability of n electrons varies as the cube of the molecular length, while the o polarizability should increase only linearly 18>. Refined molecular orbital calculations (taking into account bond alternation etc.) do not exhibit this abnormal asymptotic behaviour. 

In the simple and extended Htickel methods, the molecular orbitals are calculated and then filled from the bottom up with the available electrons. However, in ab initio calculations the occupancy of the orbitals is taken into account as they are being calculated. Explain. (Hint look at the expression for the Fock matrix elements in terms of the density matrix.)... 

Fig. 21. A molecular orbital diagram for M6Yi2 " based on an extended Htickel calculation, adapted from reference (108).

A formalized analysis of the EFG was also employed by Bagin et al. (1980) their model effectively explained the large quadrupole splitting that is associated with octahedral Fe, again using vacancies as part of their model. More recently, an elegant paper by Aldridge et al. (1986) used molecular orbital methods to model the electronic structure of biotite. They showed that iterative extended Htickel theory calculations of... 

Extended Htickel gives a qualitative view of the valence orbitals. The formulation of extended Htickel is such that it is only applicable to the valence orbitals. The method reproduces the correct symmetry properties for the valence orbitals. Energetics, such as band gaps, are sometimes reasonable and other times reproduce trends better than absolute values. Extended Htickel tends to be more useful for examining orbital symmetry and energy than for predicting molecular geometries. It is the method of choice for many band structure calculations due to the very computation-intensive nature of those calculations. 

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