Calculations Huckel molecular orbital

The logical order in which to present molecular orbital calculations is ab initio, with no approximations, through semiempirical calculations with a restricted number of approximations, to Huckel molecular orbital calculations in which the approximations are numerous and severe. Mathematically, however, the best order of presentation is just the reverse, with the progression from simple to difficult methods being from Huckel methods to ab initio calculations. We shall take this order in the following pages so that the mathematical steps can be presented in a graded way. 

For example, Horowitz (Revue Roumalne de Chlmle, 1978, 23, 603-611) performed Huckel molecular orbital calculations on a substituted, symmetrical trlazlne which demonstrated the alternating charge system In the ring. 

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... 

Data from the 31P NMR spectra of some [RhXL3] complexes are collected in Table 7. The l03Rh NMR spectrum of RhCl(PPh3)3 has also been obtained. In addition to the coupling constants recorded in Table 7 the additional coupling constant 3/[P(l)-Rh-P(2)] was evaluated as — 38 Hz.176 The NMR data have been used in Huckel molecular orbital calculations for some [RhCl(PR3)3] species.177... 

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. 

The mass spectra of various pyridazino[l,2-a]pyridazines have been reported (74JOC47). Huckel molecular orbital calculations have been reported for this ring system (66JSP(19)25). 

The ground state is assumed to be A. as suggested by Huckel-molecular-orbital calculation (IJ,). Singlet excited states (11. 13) are included near 28000 and 30000 cm", but the values are placed in brackets to emphasize the uncertainty in the analysis. Also tentatively adopted is the suggestion by Hastie et al. (23) that the continuous spectrum near 23000 cm" is due to a triplet excited state. 

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... 

Physical Properties of Isothiazoles and Isothiazolines. - HUckel molecular orbital calculations have been performed for a large number of 5-membered rings with 1 or 2 heteroatoms, including isothiazoles. The calculated relative order of stabilities... 

As with the 2-0 ring systems discussed in the previous chapter, there have been very few reports of the application of theoretical methods to the systems under discussion here. Huckel molecular orbital calculations have been carried out on the imidazo[l,2-i>]pyrazole (35). This model predicted the IH tautomer to be more stable than the 5H and this is in agreement with the observation that the N-1 glycoside (38) is thermodynamically more stable than the N-5 isomer (39) (see Table 2 for structures) <84JOC3534). The same model also predicted a n electron density of greater than 1.1 on C-7, which is in agreement with the upheld chemical shifts ( ca. 65 ppm) observed for this carbon in the C NMR spectra of (35), (38), and (39). 

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. 

J. Figueras, HMO A Graphics-Based Huckel Molecular Orbital Calculator, Trinity Software, P.O. Box 960, Campton, New Hampshire 03223. 

Although simple electronegativity issues suggested that 1,4-dithiocin might be expected to be the most aromatic of the 1,4-diheterocins <7772151 >, theoretical studies show that, as for 1,4-dioxocin, strain energy dominates its energetics. Thus, Huckel molecular orbital calculations do display the expected enhanced delocalization energy for 1,4-dithiocin relative to 1,4-dioxocin however. 

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... 

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