Huckel-type calculations

The NMR spectrum in CDCI3 at ambient temperature displays two Mo—CO ( 222.2 and 228.79 ppm) and two characteristic high-field coordinated C=C d 58.64 and 71.91 ppm) resonances, indicating that the solid state structure of the Mo-chelate complex is maintained in solution. Conformational rigidity is generally observed in many if-com-plexes. Rotational barriers about the metal-olefin axes and conformational preferences in these complexes have been estimated using extended Huckel-type calculations. ... 

It is in the definition of the integrals or the lack of it, that most m.o. methods differ. The overlap integrals are easy to compute and were, indeed, tabulated for Slater-type atomic orbitals a long time ago (ref. 79). In some simple calculations, namely extended Huckel-type calculations a rough estimation of Hmn is as follows (see also page 79). 

A more advanced extended Huckel-type calculation for nickel glyoxime was performed by Ingraham 128i but he did not perform a population analysis 12to show the extent of -orbital participation. 

A detailed interpretation of the magnetic parameters of the ligand nuclei of Co(aca-cen) using an extended Huckel-type calculation has been given by Rudin et al. ... 

The optimised interlayer distance of a concentric bilayered CNT by density-functional theory treatment was calculated to be 3.39 A [23] compared with the experimental value of 3.4 A [24]. Modification of the electronic structure (especially metallic state) due to the inner tube has been examined for two kinds of models of concentric bilayered CNT, (5, 5)-(10, 10) and (9, 0)-(18, 0), in the framework of the Huckel-type treatment [25]. The stacked layer patterns considered are illustrated in Fig. 8. It has been predicted that metallic property would not change within this stacking mode due to symmetry reason, which is almost similar to the case in the interlayer interaction of two graphene sheets [26]. Moreover, in the three-dimensional graphite, the interlayer distance of which is 3.35 A [27], there is only a slight overlapping (0.03-0.04 eV) of the HO and the LU bands at the Fermi level of a sheet of graphite plane [28,29],... 

Considering only the interaction between HOMO of R and LVMO of S, elementary perturbation theory shows that the result of the orbital interaction is a repulsion of the levels, the occupied level becomes more stable, the unoccupied level less stable. The simplest Huckel-type formulation of PMO theory gives equations for the intermolecular perturbation energy change A that are quite simple in form, Eqs. 3—6 18,20,22,26-28) Q is a first-order Coulombic energy that can be calculated in terms of... 

In the crystal the atoms of this molecule are found to be distributed in three planes Hie two tolyl rings are rotated by 24° (trans to Br) and 68° (cis to Br) with respect to the ethylene plane. Semiempirical Huckel molecular orbital (MO)-type calculations give essentially the same angles in the crystal, but predict 35° and 45° for these angles in the Bee molecule. 

The elusive radical cation of pyridine (140) has been obtained by irradiation of pyridine in CFCb at 4 K (79MI20403) and g values and hyperfine coupling constants have been measured for the parent molecule and deuterated derivatives. This species is of cr-type, the odd electron spending most of its time in the N sp2 lone pair orbital. Radical cations and anions of pyridinium bis(alkoxycarbonyl)methylides have been produced in the former case (78CC817) as a cyclopropenone complex, and in the latter by reduction of pyridinium bis(methoxycarbonyl)methylide with sodium (79JMR(35)l7l). The coupling constants in the ESR spectrum of both the radical cation and the anion agree to some extent with simple Huckel MO calculations. 

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>. 

TT-Electron calculations. Huckel-type molecular orbital (HMO) method Bases—first calculations Base pairs—first calculations 56CR(243)380 59BBA(36)343... 

The barrier to pyramidal inversion of methyl-substituted structure (12) is calculated to be high, relative to the barrier for thiophene dioxide, and this has been interpreted in terms of the energetically unfavored planar 4n Huckel-type system <7iMl 105-03,72MI 105-02). 

Simple Huckel-type molecular orbital calculations indicate that cyclopropenone (la) should be a resonance-stabilized aromatic compound. In 1959, 2,3-diphenylcyclopropenone was synthesized for the first time. Since then, numerous derivatives of cyclopropenone and its heteroanalogs, cyclopropenethione (lb), cyclopropeneselone (Ic), cyclopropenimine (Id), and phosphatriafulvene (le) have been prepared. 

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