Heteroatom parameters

Y .Nishimoto and Malaga (144) Rennsment of heteroatomic parameters from self- ... 

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). 

The most extensive correlation of ESR results for trimethylsilyl-substituted aromatic radical anions with Hiickel MO calculations to date is the work of Sipe and West (118, 119). The optimum heteroatom parameters for both the simple Hiickel treatment and the Hiickel-McLachlan treatment, using a Qm = 28 G, for Me3Si were as follows Hiickel, hsi = -2.00, kcS1 = 0.70 Hiickel-McLachlan, hsi = -1.80, kcsi = 0.70, X = 0.30 The values for Me3Ge were as follows Hiickel, hoe = -1.45, kCGe = 0.40 Hiickel-Me Lachlan, = -1.40, kCGe = 0.40 X =... 

Answer. This question pertains to the bond between a tricoordinated nitrogen atom and a zero-coordinated oxygen atom. It is in this chapter rather than the previous one because the student can use the SHMO heteroatom parameters in Table... 

The hetero atom parameters have been obtained in various ways, for example by striving for a best correlation of HOMO values with ionization energies, or of polarographic reduction potentials with LUMO values. The whole subject of SHM parameters and best heteroatom parameters is now of little practical importance, since much better quantitative molecular orbital methods are now readily available. 

Discussions of heteroatoms in the SHM written in the heyday of that method present the heteroatom parameters in a slightly more complicated way, in terms of the coulomb and resonance integrals a and p, rather than as simple numbers. 

The quantum mechanical interpretations of the electronic spectrum of I are not at aU satisfactory. This is partly due to the fact that there are no analogous compounds around to adjust the heteroatom parameters and there are more parameters to vary (model D) than observable ones. An improved all valence treatment for sulfur compounds is desirable. 

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