Hiickel calculations substitution

Both HMO calculations and more elaborate MO methods can be applied to the issue of the position of electrophilic substitution in aromatic molecules. The most direct approach is to calculate the localization energy. This is the energy difference between the aromatic molecule and the n-complex intermediate. In simple Hiickel calculations, the localization energy is just the difference between the energy calculated for the initial n system and that remaining after two electrons and the carbon atom at the site of substitution have been removed from the conjugated system ... 

The manifestation of a Mills-Nixon effect in benzocyclopropene (1) was examined by a variety of methods. Thus examination of the PE spectra of cyclo-proparenes and appropriate model compounds provided no information on the question.Similarly, NMR investigations using vicinal coupling constants or orthobenzylic coupling constants, involving methyl-substituted cydoproparenes revealed no evidence for sterically induced bond fixation. Theoretical calculations led to contradictory results. Early extended Hiickel calculations supported the concept in indanes but in the opposite sense than that suggested by Mills and Nixon. Semiempirical calculations for benzocyclopropene favored structure 258 over A substantial Mills-Nixon effect in the same direction resulted from... 

The complexes with the coordination mode (5) have a characteristic IR frequency in the region 1000-1200cm-1 (v(C—S, exocyclic)). They are generally prepared by ligand substitution. A few X-ray structures confirm the coordination mode of these complexes,9 e.g. compound (6) (C—S 147.8, C—S 172, Pt—S 232.8, Pt—C 206.3 pm, S—C—S 136.2° C—Pt—S 45.5°.10 Extended Hiickel calculations show that a reaction path starting from an end-on approach of the CS2 molecule is energetically most accessible.10b... 

Do not use Hiickel calculations. MOs are given on p. 275 It is known that dienolates react preferentially at position a. Also, despite increasing steric hindrance, substitution tends to boost the nucleophilicity of the reactive site. Suggest a justification. 

For benzo[b]furan and indole no such precise data are available, but it is possible to adduce some information from the various reactions described below. The positional reactivity orders for these molecules and also for benzo[b]thiophene, which have been calculated by various methods, are given in Table 8.1. In principle the ab initio calculations should be the more reliable, but neither the tt nor the (a + it) order is correct for benzo[6]thiophene, suggesting that these are incorrect for the other molecules also. The calculations using the STO-3G basis set certainly wrongly predict the site of most rapid protonation. Notably, only the Hiickel calculations give the correct order for benzo[b]thiophene and indeed they are usually the most reliable indicators for electrophilic aromatic substitution. 

In the photoaddition of acetone and other ketones to 1-, 2- and 1,2-di-methylimidazoles the products sire a-hydroxyalkylimidazoles (153) which are derived from the selective attack of excited carbonyl oxygen at C-5. In the case of 2-methylimidazole the products are the 4-mono- (8%) and 4,5-di- (14.5%) substituted compounds, but imidazole itself does not react. The suggestion that it is not a sufficiently electron-rich substrate is not particularly convincing. The reaction mechanism (Scheme 72) may reflect the greater radicd reactivity at C-5, and the comparative stabilities of the radical intermediates derived from carbonyl attack at this position. Hiickel calculations of radical reactivity indices show that, indeed, C-5 is more reactive, and the radical intermediate at C-5 is more stable than that at C-4, but a concerted cycloaddition could also give rise to the oxetane (152). Such an oxetane can be isolated in the photochemical addition of benzophenone to 1-acetylimidazole. 

Hiickel calculations of the electron affinities of these substituted compounds have been carried out. The heteroatom inductive and resonance parameters, ht and ku have been established for the previously mentioned substituents, and were determined by a procedure different from normal. The Hiickel calculations were made on the difference in electron affinity from the parent compound, and the parameters were adjusted to agree with the experimental change in electron affinity. In general, Hiickel calculations have severe limitations however, when the calculations are made only on the AEA, the inaccuracy of the Hiickel estimate of the aromatic framework is minimized. As a result good agreement was obtained between the Hiickel estimates and the experimental A EA. 

Extended Hiickel calculations on 10 are in substantial agreement with the structure derived from microwave spectroscopic studies Extensive restricted Hartree-Fock calculations have been carried out on the inversion of substituted bicyclo[ 1.1.0] butanes and on the energetics of variation of the flap angle The results indicate that the ring system is surprisingly flexible, with little energy required for variations of up to 20° in the flap angle. These calculations also reveal that inversion of the system involves non-least-motion movement of the substituents at C(d and C ). 

The reaction between the allene complex trows-ReCl(T 2-CH2=C=CHPhXDPPE)2 and dry HCl in THF gives the T 2-vinyl trons-ReCl( n2-CBz=CH2XDPPE)2- A kinetic analysis shows that there is a complex rate law inconsistent with a simple direct protonation at the Ph-substituted carbon atom. Instead it appears that the initial site of attack is the metal and indeed this is consistent with the results of an extended Hiickel calculation analysis. 

OBu In] with metal hexacarbonyls gives the unusual cmnplexes [(CO)jMSn(p.-OBu ),InM(CO)3] (M=Mo,Cr) via initial coordination of the metal at the indium and tin sites . Two papers include both indium and thallium compounds. Thus, the preparation of [ CpM(CX>)3 3E] (MssMo,Cr,Es In,Tl) is described, extended Hiickel calculations indicating that the In-Mo bond is polar, and tire ic-bonding interaction is only weak . The synthesis and electrochemistry of the porphyrin (P) complexes [(P)ERe((X))s] is also described . Hnally one report describes the synthesis of phosphine substituted crown complexes containing a thallium-iridium bond. The nature of this interaction has been studied in some detail . 

A first attempt toward this aim came with a theoretical work, based on extended Hiickel calculations, considering the response of the electronic levels to a progressive substitution of hydrogens in an all-trans polyethylene. The series of linear fluoropolymers (-CH2-CH2-, -CHF-CH2-, -CF2-CH2-, -CHF-CHF-, -CHF-CFH-, -CP.2-CHF-, -CF2-CF2-) so obtained was artificially considered in a zig-zag planar conformation. Theoretical conclusions were that calculated changes should be observable in XPS measurements as it was confirmed soon after >. A more systematic study of polymer primary structures by XPS and... 

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