Huckel calculations heteroatoms

Using a similar approach, Evans and co-workers obtained values of hsi = -1.50, kcsi = 0.55, 8 = 0.15 for a series of trimethylsilyl-substituted naphthalene radical anions based on a Q value of -26.1 49). For phenyltrimethylsilylacetylene radical anion, the values of the heteroatom parameters that gave the best fit of the Huckel calculated spin densities with experimental values, using a Q value of 28, were hs, = -1.3, and kCSi = 0.65 when a 8 value of 0.1 was assumed. A resulting C—Si 7r-bond order of about 0.3 is obtained 43). 

Calculations of the Huckel type are too approximate to usefully calculate individual molecular properties and have been superceded by more sophisticated methods. However, because they can give general analytical expressions that show how properties vary as the nature of heteroatoms changes, they can still give useful qualitative insights into trends in molecular properties, e.g., <2004JA11202>. 

The acidity of the hydrogen atoms at C-2 and C-4, as measured by hydrogen-deuterium exchange data, could not be correlated with Huckel MO, Pariser-Parr-Pople (PPP), or variable / ,y-PPP localization energies calculations. The deficiency of /S-electron methods was probably because of the field effects of the heteroatoms and their lone pairs of electrons. ... 

Physical Properties of Isothiazoles and Isothiazolines. - HUckel molecular orbital calculations have been performed for a large number of 5-membered rings with 1 or 2 heteroatoms, including isothiazoles. The calculated relative order of stabilities... 

Huckel theory was extended to cover various other systems, including those with heteroatoms, but it was not particularly successful and has largely been superseded by other semi-empirical methods. Nevertheless, for appropriate problems Huckel theory can be very useful. One example is the calculations of P W Fowler and colleagues, who studied the relationship between geometry and electronic structure for a range of buckminster-fullerenes (the parent molecule of which, C50, was discovered in 1985) [Fowler 1993] The fullerenes (or buckyballs ) are excellent candidates for Hiickel theory as they are composed of carbon and have extensive tt systems three examples are shown in Figure 2.22. 

While Huckel s 4n + 2 rule applies only to monocyclic systems, HMO theory is applicable to many other systems. HMO calculations of fused-ring systems are carried out in much the same way as for monocyclic species and provide energy levels and atomic coefficients for the systems. The incorporation of heteroatoms is also possible. Because of the underlying assumption of orthogonality of the a and 77 systems of electrons, HMO theory is restricted to planar molecules. 

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