Semiempirical PM3 calculations

Conqjutational methods can also be used to describe enolate stmcture. Most of the stmctural features of enolates are correctly modeled by B3LYP computations with dimethyl ether as the solvent molecule. Although semiempirical PM3 calculations give adequate representations of the geometries of the aggregates, the energy values are not accurate. 

Similarly, other cycloadducts of nitrile oxides with C6o were synthesized. The cycloadducts were characterized by 13C NMR spectroscopy and high-resolution fast atom bombardment (FAB) mass spectrometry. It should be mentioned that X-ray structure determination of the 3-(9-anthryl)-4,5-dihydroisoxazole derivative of C6o, with CS2 included in the crystals, was achieved at 173 K (255). Cycloaddition of fullerene C60 with the stable 2-(phenylsulfonyl)benzonitrile oxide was also studied (256). Fullerene formed with 2-PhSC>2C6H4CNO 1 1 and 1 2 adducts. The IR, NMR, and mass spectra of the adducts were examined. Di(isopropoxy)phosphorylformonitrile oxide gives mono- and diadducts with C60 (257). Structures of the adducts were studied using a combination of high performance liquid chromatography (HPLC), semiempirical PM3 calculations, and the dipole moments. 

The acyclic precursor is an oc, 3-unsaturated amido aldehyde that was condensed with iV-methylhydroxylamine to generate the nitrone ( )-48, which then underwent a spontaneous cycloaddition with the alkene to afford the 5,5-ring system of the isoxazolidinyl lactam 47. The observed product arises via the ( )-nitrone transition state A [or the (Z)-nitrone equivalent] in which the position of the benzyl group ot to the nitrone effectively controls the two adjacent stereocenters while a third stereocenter is predicted from the alkene geometry. Both transition states maintain the benzyl auxiliary in an equatorial position and thus avoid the unfavorable 1,3-diaxial interaction with the nitrone methyl or oxygen found in transition state B. Semiempirical PM3 calculations confirm the extra stability, predicting exclusive formation of the observed product 47. Related cycloadducts from the intramolecular reaction of nitrones containing ester- rather than amide-tethered alkene functionality are also known (83-85). 

Figure 24 Isosurfaces at +0.02 of molecular orbitals of the model compounds of polysilanes obtained by semiempirical PM3 calculation.

The stereoselective enolization of 3-pentanone by LiTMP mixed aggregates with butyl-lithium was studied by Pratt and coworkers. The mixed aggregate resulted in a slightly higher stereoselectivity, which increased with decreasing amount of the lithium base. Semiempirical PM3 calculations were used in an attempt to determine the mechanism of ketone deprotonation by the mixed aggregate. Equations 20 and 21 show two alternative mechanisms for the formation of lithium acetone enolate in thf solution, involving... 

Concentration-dependent isotope effects (lEs) have been measured in the competitive photocyanation of naphthalene and perdeuterionaphthalene, and have been shown to be influenced by the concentration of the reagents naphthalene, cyanide and oxygen. Together with semiempirical PM3 calculations and other measurements, the variation of the IE with naphthalene concentration is suggested to be ascribable to excited state equilibration a detailed mechanism to explain the observed lEs is proposed. 

G calculations on 2,3,7,8-tetrachlorooxanthrene have been carried out and the authors found that a near planar structure is favored for this compound <88JOC547l). Thianthrene has been the subject of many theoretical studies. Molecular mechanics calculations predict an inversion barrier of 4.6 kcal mol by the MM method <85JA5323), and a dihedral angle of 150° or 125° by the semiempirical PM3 calculational method <930M77S) or hy the 4-31G basis set <90JST(204)4i), respectively. 

Semiempirical PM3 calculations have been performed on a novel alkaloid possessing the pyraz-ino[2,l-/)]quinazoline skeleton to assist proton assignment <92JCS(Pl)1495>. 

Although these deprotonations are kinetically controlled we were surprised to learn that simple semiempirical (PM3) calculations on the diastereomeric lithium intermediates are consistent with the observed selectivities [80]. Apparently, similar structural features determine the relative energies of the diastereomeric transition states and diastereomeric ground states in these internal chelate-directed lithiations. A further conclusion can be drawn (with more uncertainty) due to the favorable complexation of the lithium cation by four donor ligands, most intermediates (if not all) are monomeric and have a very low tendency for oligomerization. 

Atom-atom nonbonded potential energy calculations were carried out with the ROTEN program because the variations of the side groups are very important to steric hindrance. A semiempirical PM3 calculation was done with the same condition as for color developers. 

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