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Extended Hiickel parameters

The extended Hiickel calculations follow the same mathematical form used earlier for the study of metalloporphyrins (30). The actual program used was a totally revised version of the one used at that time (31). The extended Hiickel parameters for sulfur and hydrogen are given in Table IV. We did not include the empty 3d orbitals. The form of the Hamiltonian is... [Pg.63]

HyperChem cannot perform a geometry optimization or molecular dynamics simulation using Extended Hiickel. Stable molecules can collapse, with nuclei piled on top of one another, or they can dissociate into atoms. With the commonly used parameters, the water molecule is predicted to be linear. [Pg.126]

Apart from speed, an appealing aspect of CNDO is its simplicity. It uses fewer parameters than any other method except for Extended Hiickel and, consequently, it is easier to understand the results of modifying a calculation. [Pg.149]

Theoreticians did little to improve their case by proposing yet more complicated and obviously unreUable parameter schemes. For example, it is usual to call the C2 axis of the water molecule the z-axis. The molecule doesn t care, it must have the same energy, electric dipole moment and enthalpy of formation no matter how we label the axes. I have to tell you that some of the more esoteric versions of extended Hiickel theory did not satisfy this simple criterion. It proved possible to calculate different physical properties depending on the arbitrary choice of coordinate system. [Pg.144]

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... [Pg.368]

To examine the effect of covalency on the QS, the values of the QS of some compounds have been calculated with the aid of the Extended Hiickel MO method (i 82, 61). In these calculations the values for the empirical parameters used were obtained from comparison with EPR experiments (see EPR studies). This method is suitable only for molecules with low symmetry, because effectsoof spin-orbit coupling and thermal mixing have been neglected. [Pg.116]

Extended Hiickel molecular-orbital (EHMO) calculations using structural parameters from the X-ray determination of Cp(CO)3Mo 3In (Fig. 29) and ideal symmetry Cih were carried out for this compound in order to investigate the extent of the indium-molybdenum ir-bonding.122 The HOMO of the compound is the 3e molecular orbital, which is In—Mo [Pg.54]

A detailed interpretation of the magnetic parameters of the ligand nuclei of Co(aca-cen) using an extended Hiickel-type calculation has been given by Rudin et al.12 219. ... [Pg.86]

The magnetic parameters of aquo-Mb obtained from Fig. 44 are collected in Table 15.1. The theoretical A values for the heme and histidine nitrogen, which are about 50% smaller than the observed values, have been determined by Mun et al.242), using an extended Hiickel-type calculation. According to these authors, the agreement between theoretical and experimental values could perhaps be improved further by considering electron core polarization effects. [Pg.90]

Exothermic reaction parameters of, 27 63 temperature oscillations, 27 65-67 Explosion, petovskite preparation, 36 250 Extended Hiickel treatment, 34 136, 147, 154, 156, 166, 173... [Pg.103]

Two groups have studied the bonding in pentadienyl-metal-tricar-bonyl complexes (119, 238) and are agreed that effective overlap between the pentadienyl nonbonding orbital and an orbital of suitable symmetry on the metal (Fig. 17) makes a major contribution to the stability of these complexes. However, the two types of molecular orbital calculation [one an extended Hiickel (119) and the other a parameter-free approximate Hartree-Fock calculation (255)] disagree about the precise ordering of energy levels in this type of complex. [Pg.30]

Which should require more empirical parameters a molecular mechanics force field (Chapter 3) or an extended Hiickel method program Explain. [Pg.173]

The electron distribution around an atom can be represented in several ways. Hydrogenlike functions based on solutions of the Schrodinger equation for the hydrogen atom, polynomial functions with adjustable parameters, Slater functions (Eq. 5.95), and Gaussian functions (Eq. 5.96) have all been used [34]. Of these, Slater and Gaussian functions are mathematically the simplest, and it is these that are currently used as the basis functions in molecular calculations. Slater functions are used in semiempirical calculations, like the extended Hiickel method (Section 4.4) and other semiempirical methods (Chapter 6). Modem molecular ab initio programs employ Gaussian functions. [Pg.233]

Structural Parameters and Multiplicities of Lowest Singlet (a2) and Triplet (ap) States of Carbenes RR C Predicted by Extended Hiickel Theory ... [Pg.159]

It is often assumed that resonance integrals are proportional to the overlap between the atomic orbitals which formally underlie the pi Hamiltonian [58]. If these are assumed to be ordinary 2p atomic orbitals for carbon, the distance dependence of the overlap can be calculated analytically, and the distance dependence of the t parameters is often taken to be of this form, as for example in Extended Hiickel Theory [59]. But there is no need to make this assumption since the parameters in the pi Hamiltonian should more properly be thought of as rescaled effective integrals, and there is evidence that the model performs better if the t values are allowed to vary more rapidly with distance. Accordingly, we have adopted the form... [Pg.555]


See other pages where Extended Hiickel parameters is mentioned: [Pg.118]    [Pg.14]    [Pg.118]    [Pg.14]    [Pg.199]    [Pg.25]    [Pg.82]    [Pg.114]    [Pg.471]    [Pg.4]    [Pg.142]    [Pg.148]    [Pg.29]    [Pg.224]    [Pg.217]    [Pg.134]    [Pg.81]    [Pg.71]    [Pg.103]    [Pg.120]    [Pg.227]    [Pg.217]    [Pg.712]    [Pg.715]    [Pg.393]    [Pg.359]    [Pg.372]    [Pg.94]    [Pg.84]    [Pg.165]    [Pg.32]    [Pg.104]    [Pg.395]    [Pg.400]    [Pg.465]    [Pg.318]   
See also in sourсe #XX -- [ Pg.109 ]




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