Extended Hiickel molecular orbital calculation

Were the isoelectronic anion (i.e., C2B3Hg ) to be prepared, the most stable configuration should have adjacent basal carbons to accommodate the basal bridge hydrogen as well as the carbon. Either cis or trans basal carbons should be anticipated in the isoelectronic dianion (C2B3H5 ) which would have no bridge hydrogens. Rudolphs s Extended Hiickel Molecular Orbital calculations would favor cis basal carbons in the latter case. 

Extended Hiickel molecular orbital calculations have been made for the icosahedral platinaboranes and carbaboranes [Bn Pt(PH3)2 Hu]2-, [Bi0C Pt(PH3)2 Hu]2, and... 

It has been found that whereas Cu11 porphyrins luminesce, the Ag" complexes do not. By an examination of electronic absorption spectra, emission spectra, redox potentials and near-IR absorption data, it was proposed that this could be rationalized on the basis of the energy of the big (dx2 yi) orbital. Extended Hiickel molecular orbital calculations predicted that the d-n transition would be above the lowest (n, jt ) levels for Cu11 but below them in the Agn complexes. The near-IR absorptions found for Ag11 were attributed to CT transitions.554... 

Studies of the tautomeric state of 1,2,4-thiadiazines have been more or less confined to the biologically active 1,2,4-benzothiadiazines. Ultraviolet spectroscopic studies suggest that the 4//-tautomer of 1,2,4-benzothiadiazines, e.g. 152, is preferred in ethanol (60JOC970) in alkali, the anion dominates. Extended Hiickel molecular orbital calculations (70MI2) and 13C-NMR studies (79T2151) confirm this view (Scheme 6). 

Chapter 1 of Volume 15 deals with density functional theory (DFT). As with many quantum mechanical calculations, it is easy to become wrapped up in the theory and lose sight of the chemical phenomena we are trying to explain with the calculations. Equally important to how the numerical calculations are done is how the results can be interpreted to gain chemical insight. Dr. F. Matthias Bickelhaupt and Professor Evert Jan Baerends show how the results of DFT calculations can be analyzed to open up chemical understanding. This chapter illustrates that the plain numbers from a quantum mechanical calculation can be interpreted to be conceptually useful to chemists. In many ways, this chapter evokes memories of the famous way Professor Roald Hoffmann has extracted information from extended Hiickel molecular orbital calculations. 

Silvestre and Hoffmann have considered the conversion of ruthenium acetylide complexes to the corresponding vinylidene species using extended Hiickel molecular orbital calculations (69). Although the rearrangement of free acetylene to its vinylidene isomer is thermodynamically disfavored, their results indicate that the transformation becomes thermodynamically... 

This is, in principle, in agreement with extended Hiickel molecular-orbital calculations. The calculated binding energy and difference between the energies of the LUMO and HOMO for the four possible isomers indicate the experimentally isolated structure to be the most stable one, although the extended Hiickel calculation does not show a substantial relative stability (Table I). 

If the above generalization is correct, then simple extended Hiickel molecular orbital calculations for the surface chemisorption states of simple hydrocarbons or hydrocarbon fragments should sense this tendency for multicenter C-H-metal bonds to form (these... 

Concerted two-electron transfer and reversible metal-metal bond cleavage in phosphine-bridged dimers have been investigated, and extended Hiickel molecular orbital calculations have shown that the redox-active orbital is a metal-metal antibonding orbital. A Ru-Ru-bonded dimeric cation [Ru(Cp)2]2++ has been prepared and characterized electrochemically. The electrochemistry of these dimers may give insight into more complex clusters and polymeric metals. 

Whereas empirical force-field calculations predict a ring A boat conformation for lanost-8-en-3-one, combined empirical force-field-extended Hiickel molecular orbital calculations favour a ring A chair conformation. Europium-shift n.m.r. results indicate that the molecule adopts the latter conformation. Studies on the peracetic acid-boron trifluoride etherate Baeyer-Villiger oxidation of 4,4-dimethyl-3-keto-triterpenoids to 5-lactones (18) and their subsequent ring contraction to y-lactones have been reported. Dehydrogenation of lanost-8-en-3j8-ol with 2,3-dichloro-5,6-dicyanobenzoquinone afforded, in addition to the corresponding 7,9(1 l)-diene, the aromatic seco-lanostane derivatives (19)... 

A nonempirical molecular orbital calculation reported recently for COS by dementi" has indicated, however, that the first excited state is a 7T—IT type. Only s and p type atomic orbitals were used in these calculations. To examine the effect of the 3d orbitals of the sulfur atom on the molecular orbital energy values, an extended Hiickel molecular orbital calculation has also been carried out with and without the inclusion of the 3d orbitals. The resulting energy level diagram for the ground and excited molecular orbitals, in comparison with the atomic... 

The molecular and electronic structures of (CpM)4(//2-E)6 and (CpM)4(/x2-E)3(//3-E)3 (E = O, Se) for titanium as well as group 5 and 6 transition metals have been compared using extended Hiickel molecular orbital calculations. The conclusions indicate that the M4(/x2-0)3(/x3-0)3 structure cannot exist unless the M4-tetrahedron is severely distorted by lengthening of the M(apical)-M(basal) distance. The M4(/x2-Se)3(/x3-Se)3 structure can exist with a small distortion. The M4(/x2-E)6 structure is preferred over M4(//2-E)3(//3-E)3 when M-E multiple bonding is important, that is, when E = O. There is little M-M interaction in any of the cluster orbitals.830... 

Historically, there have been two major efforts in the development of semiempiri-cal calculations under the direction of John Pople (Nobel Laureate 1998) and M.J.S. Dewar. The earliest calculations of this nature are, however, the Hiickel (HMO) and extended Hiickel molecular orbital calculations (EHMO). While not mathematically rigorous, or used extensively at the present time, these calculations can provide considerable insight into the methods and in specific applications can be quite accurate. 

Structure building from library of fragments and molecules, manipulation, MM2 energy minimization, stick, ball-and-stick, or space-filling display, extended Hiickel molecular orbital calculation of orbital, electron density, and electrostatic maps. Applicable to chemical reactivity modeling. Tektronix enhanced Mac-II workstation with stereoscopic graphics. 

Puddephatt etal. [41] have studied the C-H or C-C bond activation in the alkane complexes [PtMe(CH4)L2] or [PtMe(CHjCH3)L2] (L = NH3 or PH3) as well as the reductive elimination of methane or ethane from the five-coordinate model complexes [PtHMe2L2] or [PtMesLi], respectively, by carrying out extended Hiickel molecular orbital calculations and density functional theory. The oxidative addition and reductive elimination reactions occur by a concerted mechanism, probably with a pinched trigonal-bipyramidal complex on the... 

Efficient 1,3-asymmetric induction occurs during the reaction of chiral vinylrhenium complexes of type (51) in equation (20). Extended-Hiickel molecular orbital calculations and stereochemical results are consistent with a mechanism... 

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