Htickel calculations

For C70 and higher fullerenes addition also occurs only at the [6,6] fusions. However, these sites are no longer all equivalent, and a variety of regioisomers can result. For C70, of the four possible products, as illustrated in Fig. 10, addition occurs exclusively at the polar fusion, A (,14). This is the sterically most accessible site, and energy calculations have shown there is the greatest release of steric strain on com-plexation (91). The equatorial [6,6] fusion, D, has lowest bond order, and correspondingly, no complexations to it have been observed. For C84, complexation with Vaska s compound occurs at the fusion with the highest bond order, as shown by Htickel calculations (28). 

Previous extended Htickel calculations on the bis(trifluoromethyl) derivative 6 show the favored isomer to be 6b, while the CNDO/2 method favors 6a [75JCS(P2)559], More recent ab initio calculations for different R substituents show that electron-releasing substituents favor the ring-opened dithione (6b), whereas electron-withdrawing substituents favor the cyclic structure (6a) (80JA6687). These conclusions are supported by electron diffraction (ED) (76JA899) and photoelectron (PE) spectral data... 

Table 1.2 Parameters for simple Htickel calculations for bonds with heteroatoms...

Alternatively, one might explore close-packed structures of boranes. Consider, for example, the difference between deltahedral and tetra-capped octahedral BioHio structures (Fig. 4). The former is stable as the dianion (11 cluster pairs) while, based on Extended Htickel calculations (60), the latter is slightly more stable for the +6 cation (7 cluster pairs). The capping principle predicts 7 cluster pairs for capped octahedra (61). A structurally characterized transition metal analogue of the tetra-... 

Because these are Htickel calculations, with the integrals simplified as in equation A2.5, the normalization and orthogonalization constraints can be written using the SUMSQO and SUMPRODUCTO functions, and so we find... 

There are two template EXCEL files for the orthogonality tests using the Extended Htickel approximation in the BonusPack. The first is a similar template to the one used for the Htickel calculations. It is applied to check the 2-dimensional basis functions of the groups, which are taken in pairs, with the totally symmetric basis function included in the calculation to make up the 5 X 5 structure of the matrix multiplications. In the second template, for the 3-dimensional basis functions, a 9 x 9 structure is employed so that a full set of, for example, Itiu, 2tiu and 3tiu functions can be checked at the same time. 

Since the diagonal elements depend only on the nature of the atom (i.e. the nuclear charge), this means that for example all carbon atoms have the same ability to attract lections. After having performed a Htickel calculation, the actual number of electrons -ttssQciated with atom A, p, can be calculated according to eq. (3.90) (see section 9.1, eqs. (9.5) and (9.4)). 

List the strengths and weaknesses of ab initio calculations compared to molecnlar mechanics and extended Htickel calculations. State the molecular features that can be calculated by each method. 

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

Extended Htickel calculations have been performed on the metal stabilised silylium cation [Co2(CO)6(HCCSiH2)]" and the synthesis and X-ray crystal structures of [Cp2Mo2(CO)4(Me3SiCCSiHPh2)] and [ Co2(CO)6(Me3SiCCSiHPh2) 201 have been... 

Extended Htickel calculations have been used to analyze the electronic structure of platinum complexes and also to rationalize their reactivity with various nucleophiles. Their inability to form four coordinate species of the type [Pt(S2CNR2)(ri -S2CNR2)2] has been ascribed to the lower charge on platinum (when compared with xanthate complexes), strong platinum-sulfur bonds and a repulsive interaction between the platinum 2>d electrons and the incoming ligand (1601). 

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