Extended Huckel molecular orbital calculations

In this section we mention the paramagnetic monocationic complexes [Mo(CO)2(dppe)2]+ and [Mo(CO)2(bipy)2]+, both of which are produced by oxidation of the parent Mo° complexes. In the former case, the oxidants used include [MeCftKUNJfBFj,46 NO[PF6], I2 and AgIla-47 and a variety of analogues [Mo(CO)2L2]+ has been obtained (L2 - dppm, dmpe, diars, etc.).1 47 Electrochemical oxidation has also been used, with other measurements, to show that a rapid cis — trans isomerization follows oxidation and an explanation for this phenomenon has been proposed on the basis of extended Huckel molecular orbital calculations, the stereochemical change being dependent on the number of valence electrons and the nature of the coligand jr-donor or -acceptor capacity. Similar studies have been made upon the compounds [Mo(CO)2L2]+ (L = bipy or phen).47... 

Chemisorption state modeling by extended Huckel molecular orbital calculations were made in a collaborative study with R. M. Gavin, and full details of these calculations will be published elsewhere. 

Recent evidence favors Dg symmetry and a pentagonal bipyramidal structure for the heptafluoride. Claassen et al. (2) review the earlier debate about structure of lower symmetry. They provide convincing evidence of D symmetry from the first observation of Raman spectra of the vapor state and re-examination of the infrared spectra. Their data—including five fundamentals in Raman (two polarized), five fundamentals in infrared, no coincidences between Raman and infrared, and one fundamental inactive—are consistent only with symmetry. This is confirmed by Falconer et al. (4) who interpret their electric-deflection experiments as indicating a symmetry-forbidden dipole moment. Extended Huckel-molecular-orbital calculations (5) also predict Dg symmetry. The adopted structural parameters are from our approximate analysis of the electron-diffraction data of Thompson and Bartell (6). The authors gave a radial distribution curve and suggested only a gross (unrefined) structure because of the probable presence of... 

The methylidyne cubane [Cp Ti(/x-CH)]4 (Scheme 202) is obtained as a dark brown crystalline solid by thermolysis of Cp TiMe in toluene with methane elimination. This transformation was monitored by NMR and no intermediates are observed. The signals assignable to the methylidyne groups appear as singlets at 6 17.75 in the NMR and at 6 490.8 in the 13C NMR spectra. In order to analyze the interaction between the /i3-ligand and the titanium centers, extended Huckel molecular orbital calculations have been carried out. In contrast, the thermolysis of the trinuclear oxo alkyls [Cp Ti(/i-0)(CH2R)]3 (R = H, Me) affords the //3-alkylidyne derivatives [Cp Ti(//-0)]3-(/x3-CR).505-507... 

Extended Huckel molecular orbital calculations were used to obtain an initial, qualitative picture of the electronic structure changes involved in two-electron transfer. EHMO calculations were carried out in D2h symmetry on H2P" bridged analogs of the [M2(p-PR2)2(CO)8] complexes using the CACAO program (50). Structural information and computational details are provided in reference 18. 

Nevf Oostpounds (Miscellaneous) - Ihe tautoroerism and conformations of the benzothiadiaztoe antihypert sive agents were predicted by means of Extended Huckel Molecular Orbital calculations.3 speculations on the nature of the electronic molecular medianism of action and the structure of the receptor were also made.37... 

Synthetic and dynamic NMR studies of the ri3-triphenyl and Ti3-trimethylcyclopropenyl complexes [(ti -C5R5)Ru(ti3-C3R 3)X2] (R = H, Me R = Me, Ph X = Cl, Br, I) have been reported and the barrier to rotation of the ri3-cyclopropenyl ligands in isoelectronic molydenum and ruthenium complexes has been investigated using Extended Huckel molecular orbital calculations. 

Co, Rh and Ir The electronic structure of rhodium indenyl complexes of alkenes, dienes and carbon monoxide has been compared with that of the corresponding cyclopentadienyl complexes using photoelectron spectroscopy and Extended Huckel molecular orbital calculations. The synthesis, spectroscopic characterisation and reactions of [Rh(bipy)(C2H4)Cl] have been reported. ... 

The X-ray crystal structures of the (Tj -tetraphenylcyclobutadiene) complexes [CpM(Ti -C4Ph4)Cl2] (M = Nb, Mo) have been determined 5 and the structures compared with those of the bent sandwich compounds[Cp2MCl2] by means of Extended Huckel molecular orbital calculations. 

Extended Huckel Molecular Orbital calculations (EHMO) on the model compound with tetraazo porphin (TAP ) as the macrocycle and Fe as the central metal atom and the dianion of acetylene (C=C) as the bridging ligand indicate that the resulting 13 ai, crystal orbital of this polymer should be extremely concentrated along the central axis via the bridging ligand. The crystal orbital will be only partially filled and the polymer should exhibit metal-like conductivity along the one-dimensional chain [108,110]. 

Extended Huckel Molecular Orbital Calculations and Spectroscopic... 

Finally, the Ag-Ag distances in adjacent pairs of the photoluminescent ion [AgfCNlj]" decrease by as much as 0.58 A upon electronic excitation. Extended Huckel molecular orbital calculations indicate the formation of an exciplex, with a transition from a la la 2o 2a ground state with no formal bond (analogous to the orbital scheme shown in Eigure 11.4 for two Cu(l) ions), to a lo la 2o 2o ... 

Other approximate, more empirical methods are the extended Huckel 31> and hybrid-based Hiickel 32. 3> approaches. In these methods the electron repulsion is not taken into account explicitly. These are extensions of the early Huckel molecular orbitals 4> which have successfully been used in the n electron system of planar molecules. On account of the simplest feature of calculation, the Hiickel method has made possible the first quantum mechanical interpretation of the classical electronic theory of organic chemistry and has given a reasonable explanation for the chemical reactivity of sizable conjugated molecules. 

Haskins (Ref 9c) has compared molecular orbital ealens of some azides and tetrazoies with expl behavior, dipole moments, and binding energies. He used an iterative extended Hu eke i molecular orbital approach to calculate dipole moments of 1-(H)tetrazole, 2-(H)tetrazole, and 5-aminotetrazole and found good agreement with exptl measurements. For example, l-(H)tstra-zole was calculated as 5.269ju and measured as 5.1 l x. Binding energies ealed by the similar ex- tended Huckel molecular orbital (EHMO)... 

HOMO = highest occupied molecular orbital. 6LUMO = lowest unoccupied molecular orbital. CEH = extended Huckel (calculations). dCNDO = complete neglect of differential overlap. efcc = face-centered cubic. 

Molecular orbital calculations at the extended Huckel level indicate that the two disro-tatory modes of ring cleavage (but not the symmetry-forbidden conrotatory mode) require similar activation energies for a model methylenecyclopropane-PdCl2(HCN) complex. The disrotatory motion of the carbon-carbon bond breaking away from the metal (dis-... 

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