Semiempirical molecular orbital calculations

Semiempirical molecular orbital calculations have gone through many stages of refinement and elaboration since Pople s 1965 papers on CNDO. Programs like PM3, which is widely used in contemporary research, are the cumulative achievement of numerous authors including Michael Dewar (1977), Walter Thiel (1998), James Stewart (1990), and their coworkers. 

Dixon SL, Merz KM Jr (1996) Semiempirical molecular orbital calculations with linear system size scaling. J Chem Phys 104(17) 6643-6649... 

Dixon SL, Merz KM Jr (1997) Fast, accurate semiempirical molecular orbital calculations for macromolecules. J Chem Phys 107(3) 879-893... 

An alkyl radical and a nitroxide radical exist in an equlibrium with the corresponding alkoxyamine as their coupling product (Eq. 57). Moad and Rizzardo [213] and Kazmaier et al. [214] independently estimated the effects of the structure of the alkyl group and the nitroxide on the dissociation energy of various alkoxyamines into the radicals by semiempirical molecular orbital calculations. The bond dissociation energies determined are summarized in Table 5 ... 

Fig. 8 AMI semiempirical molecular orbital calculations of esfenvalerate (9a) and 7t-cation radical of oxoferryl(IV) porphyrin...

Hitherto, thio ether formation has clearly been proved only in the case of the ji-donor substituted 4-nitrosophenetol and the electron-rich l-methyl-2-nitrosoimidazole. The low yields of this adduct (about 2% at 1 1- and about 10% at 1 5-stoichiometry for 4-nitrosophenetol reacting with GSH56) may be the reason for its rare discovery. However, other nitrosoarenes should yield this family, too. Semiempirical molecular orbital calculations (MNDO) indicate a similar positive charge at the exposition of the N-(methylthiol-S -yl)-aniline cation and -4-anisole cation as well (Scheme 6). Furthermore, formation of l-(glutathion-S -yl)-2-naphthylamine was reported to occur in mixtures of 2-nitrosonaphthalene and GSH12. 

Katritzky (Katritzky et al., 1986) has recently advanced the idea that captodative-substituted radicals should be stabilized significantly by polar solvents. This hypothesis, which is qualitatively derived from the polar resonance structures for these radicals, was supported by semiempirical molecular orbital calculations. An experimental test was carried out by Beckhaus and Riichardt (1987). For the dissociation of [24] and [25] into the radicals [21] and [28], they were unable to confirm Katritzky s hypothesis. The rate of thermolysis of [24] and [25] is not affected by a change in solvent polarity. If the stabilization were of the order of Katritzky s prediction, it should, however, have become evident in the rate measurements. The experiments thus suggest that the contribution of polar resonance structures to the ground state of the radicals is not appreciable. See, however, the results obtained by Koch (1986) on the dl meso isomerization of [47]. 

Semiempirical molecular orbital calculations performed for a set of pyrrolo-benzodiazepines using MNDO and AMI were used for the interpretation of their mass-spectroscopic data (1996MI653). 

Ab initio and semiempirical molecular orbital calculations have been used, together with charge-transfer theories, to investigate the structures of organodioxide anions and related charge-transfer complexes between carbanions and molecular oxygen.219... 

This reaction was extended to prepare 7-nitro- and 9-nitro derivatives of pyrido[l,2-a]pyrimidinone 119 (R = N02, R1 = H R = H, R1 = N02) when the appropriate starting material 118 was heated in boiling dichloro-methane in the presence of triethylamine or when it was stirred in aqueous sodium hydroxide at 20°C (92AJC1825) or treated with sodium azide in aqueous tetrahydrofuran (92AJC2037). Semiempirical molecular orbital calculations (AMI) indicated that the 2-hydroxy-4-oxo tautomeric forms 119 are more stable than the alternative 4-hydroxy-2-oxo tautomeric forms. 

Another search trial [54] for the above two molecules was done by using additional information on the charges of the atoms in each molecule, charge in this case means the net charge obtained by a semiempirical molecular orbital calculation. Two modes of charge designation can be used in the system. 

The nature of the bonding in heteronuclear gold cluster compounds has been investigated primarily by means of semiempirical molecular orbital calculations and the results of these studies have allowed a number of structural generalizations to be made. These theoretical developments have been discussed in detail in a series of papers by Mingos et al. (210, 256, 279-282) and only a brief description of the results of this work will be given in this section. 

Semiempirical molecular orbital calculations on this model [309] suggest that, in the case of propylene polymerisation, equatorial 2,4-substitution of the metallacyclopentane ring is the most stable form this would lead to regiose-lective head-to-tail propagation during the polymerisation of propylene and, moreover, to the formation of isotactic polypropylene [51]. Such calculations concern a case, however, that has not been confirmed by experiments a coordination of propylene at Ti(II) species and subsequent reaction according to the above scheme is not as obvious as that of ethylene. 

Ohkura K, Hori H (1998) Analysis of structure-permeability correlation of nitrophe-nol analogs in newborn rat abdominal skin using semiempirical molecular orbital calculation. Bioorg Med Chem 7 309-314... 

AMI semiempirical molecular orbital calculations have been useful at predicting the nature of the LUMO level, i.e., a or it, and PET bond cleavage propensity [92]. In a series of arylmethyl-p-cyanobenzylsulfonium salts where aryl is phenyl, 1-naphthyl, 9-anthryl and 5-naphthacenyl the LUMO level was calculated to be ct in each case except for the 5-naphthacenyl derivative. The lack of photochemistry in the 5-naphthacenyl derivative as well as fluorescent quantum yields, singlet lifetimes, and reduction potential behavior are consistent with the conclusions from the calculations [92],... 

Energy Parameters in Polypeptides. II. Semiempirical Molecular Orbital Calculations for Model Peptides. 

Tel. 612-683-3688, fax 612-683-3099, e-mail mcole cray.com DGauss for density functional theory calculations with nonlocal, SCF corrections, and geometry optimization. Cadpac 5.0 for ab initio calculations. MNDO90 for semiempirical molecular orbital calculations. A package with a graphics front end for structure input and visualizations of electron density, electrostatic potentials, and molecular orbitals. Silicon Graphics and Macintosh (under X-Windows) networked to a Cray. 

The difference in reactivity between the a-stannyl sulfide 16A and the a-silyl sulfide 16B can be explained by comparing the two-center energies of their carbon-metal bonds. Semiempirical molecular orbital calculation revealed that the bond energies decrease in the order of 2-silyl, 2-germyl, and 2-stannyl-1,3-dithiane cation radicals. As the silyl dithiane was completely consumed by the oxidation under the... 

A search for Oxford Molecular Ltd., which produces a variety of software for sequence alignment, molecular modeling of antibodies, QSAR, semiempirical molecular orbital calculations, and so on, yielded five hits in CJACS through 1993. A search for CompuDrug produced a total of two hits, one in 1990 and one in 1992. 

---