Semi-empirical molecular orbital calculations

These major trends in act can be qualitatively predicted using semi-empirical molecular orbital calculations. However, the methods fail to adequately predict some electronic effects, remote substituent effects and the influence of hydrogen bonding. Higher level ah initio or DFT calculations provide a better indication of trends in these circumstances. 

Semi-empirical molecular orbital calculations have been carried out on the model phosphorane HaPiCHa. Besides the expected transfer of charge, the inclusion of the phosphorus 2>d orbitals showed a significant hyperconjugative interaction between the CHg orbitals and a 2>d orbital of appropriate symmetry on phosphorus. Calculations on cyclopropylidene-phosphorane revealed a similar interaction between the Walsh orbitals of the ring and an in-plane phosphorus 2>d orbital. 

Semi-empirical molecular orbital calculations on the phosphorane (96)... 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

For more semi-empirical molecular orbital calculations and comparison with experimental data, see Section 4.01.2. For MO calculations on aromaticity and tautomerization, see Section 4.01.3. 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

X-Ray crystallographic analysis of 4-(2,4-dinitrophenyl)-2,5-dioxo-8-phenyl-l,2,3,4,5,6-hexahydropyrido[2,3-pyrida-zine 7 is in agreement with the semi-empirical molecular orbital calculation (AMI) <2000T2473>. 

The advent of semi-empirical molecular orbital calculations and density functional theory, which employ mathematical assumptions to simplify the application of quantum mechanics to drug molecules of intermediate to large size. 

AM 1 semi-empirical molecular-orbital calculations have been carried out on several sugars in order to establish energy minima and favored conformations.22 However, Tvaroska and Carter23 showed that this method does not provide the correct energy differences between anomers. The difficulty lies in the comparatively small energy differences (—1-2 kJ/mol) between anomers much-more refined calculations are necessary for this to emerge... 

AMI semi-empirical molecular orbital calculations on meso-ionic l,2,3,5-dithiadiazolium-4-thiolate 4 suggested it could be a viable local energy minima product for the reaction of A-benzyl-l,2,3,5-dithiadiazol-4-ylium chloride 3... 

The characteristic high carbonyl frequencies of cationic metal carbonyls are rationalized in terms of a reduced 77-back-donation from the metal (7, 56). Such a proposal implicitly assumes that the carbon-metal cr-bond is independent of charge on the metal, and has been supported by semi-empirical molecular orbital calculations. This assumption of invariance of carbon-metal and carbon-oxygen o--bonds with change in the metal s oxidation state has been invalidated by a Raman and infrared investigation of M(CO)6+, M(CO)6, and M(CO)6 species (3, 4). It appears that the carbon-metal cr-bond increases with rise of positive charge on the metal, with concomitant decrease of metal-carbon 7r-bonding. 

Semi-empirical molecular orbital calculations, which are based on the same or related quantum mechanical principles, but make approximations or assumptions to simplify the computations, or include some empirical parameters based on experimental data [254-259]. 

Louisnathan, S. J., and G. V. Gibbs (1972b). Variation of Si-0 distances in olivines, sodamelilite and sodium metasilicate as predicted by semi-empirical molecular orbital calculations. Amer. Mineral. 57, 1643-63. 

Cgo and the limiting value of 0.5743 for graphite). These values are in perfect agreement with those reported separately by Manolopoulos ". The HOMO-LUMO transition is allowed b, - a. A reasonably optimized structure has been found through molecular mechanics calculations, and used as the initial structure in semi-empirical molecular orbital calculations (AMl-Gaussian 90) within which the structure was further optimized. The important results are as follows. 

Large geometrical distortion in the lowest excited states of Pt(gly)2 is also suggested by spectroscopic studies, which have indicated that similar square planar complexes have pseudotetra-hedral excited states (93a), by semi-empirical molecular orbital calculations (56), and also by recent quenching and sensitization studies of the cis trans reaction (Section lII-D-1). ... 

Typically, electrostatic interactions are represented by some form of Coulomb s law-type function. In this type of calculation, the atoms in the molecule are assigned a charge distribution consisting of discrete point charge approximations centered on each atom. Other variations of these simple functions include exponential-type approximations (12) and variable dielectric approximations (3,11). The point charge approximation is often determined by ab-initio or semi-empirical molecular orbital calculations. The user may select the particular functional representation used in the calculation as well as the parametric form of the function. The variable dielectric function is represented by an approximation to the rigorously derived function (3,11) for computational efficiency. 

Semi-empirical molecular orbital calculations using the INDO/S method have been carried out for organoscandium complexes of the general formula Cp 2ScX (X = C1, I, Me, CH2Ph, NHPh).549 The utility of electrospray mass spectrometry for the characterization of air-sensitive organolanthanides and related species has been demonstrated.550 A wide variety of lanthanide complexes, including divalent and trivalent compounds, neutral and... 

In the period under review both ab initio and semi-empirical molecular orbital calculations of the electric field gradients at 14N nuclei have been reported. Pyrazole and imidazole have been the subjects of an ab initio approach using double zeta functions. (185) The calculated values of x an<3 7 are 111 g°°d agreement with experimental results obtained from microwave studies. The agreement is considerably better... 

The semi-empirical molecular orbital calculation software MOPAC in the CAChe Work System for Windows ver. 6.01 (Fujitsu, Inc.) was used in all of calculations for optimization of geometry by the Eigenvector Following method, for search of potential energies of various geometries of intermediates by use of the program with Optimized map, for search of the reaction path from the reactants to the products via the transition state by calculation of the intrinsic reaction coordinate (IRC) [10]. 

Miller, D. J., and L. C. Cusachs (1969), Semi-empirical Molecular Orbital Calculation) on the Bonding in Sulfur Compounds I. Elemental Sulfur, S6 and S8, Chem. Phys. Lett. 3, 501-503. 

Several theoretical calculations have already beai done for strained [n]para-cyclophanes by means of semi-empirical molecular orbital calculations and... 

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