Semiempirical study

Whereas oxaziridine and diaziridine were partial subjects of comprehensive theoretical studies on cyclic compounds (73MI50800), diazirine and some of its simple derivatives were the special target of quantum chemical investigations. Since diazirine, the lowest molecular weight heterocycle, has only five atoms and is of high symmetry, there was a chance for ab initio calculations, which followed some semiempirical studies. 

A theory of two-phase bubble flow has been developed by Nicklin (N1), who shows that the motion of bubbles arises partly from buoyancy and partly from the nominal velocity caused by the entry of the two phases into the tube. Theoretical and semiempirical studies of bubble flow have also been presented by Azbel (A2) and by Azizyan and Smirnov (A3), and further experimental data on holdup have been recently reported by Yoshida and Akita (Yl), by Braulick et al. (B9) and by Towell et al. (T3). 

Na3F2 cluster, direct molecular dynamics, semiempirical studies, 415 Near-adiabatic limit, molecular systems,... 

Maran, U., Pakkanen, T. A. and Karelson, M. A semiempirical study of the solvent effect on the Menshutkin reaction, J.Chem.Soc.PerkinII, submitted (1995). 

Numerous semiempirical studies have been made of semibullvalene and a wide range of derivatives (see Williams and Kurtz, 1988 Dewar and Jie, 1988, and references cited therein). Agreement with experimental data, where available, is excellent. Several target molecules are predicted to exhibit the elusive goal of neutral homoaromaticity which provides an enticing challenge for synthetic efforts. 

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

An extensive AMI semiempirical study on a wide range of anomerically substituted acetamides was published in the first major paper describing this reaction. For rearrangement of 221, enthalpies of activation were clearly lowered in the sequence... 

The limited knowledge of thermal behaviour of halogenated acids has been extended significantly by a pyrolysis (infrared laser-powered) and semiempirical study which has established that mono-, di- and tri-chloroacetic, trifluoroacetic, and bromoacetic acid eliminate HX and that both bromo- and iodo-acetic acid undergo C—X bond homolysis acetic acid undergoes decarboxylation and dehydration under the same conditions.46 The semiempirical calculations of corresponding activation energies are consistent with these conclusions. 

Results of a PM3 semiempirical study of the quaternary benzenesulfonamide salt of /raw.v-3-(hydroxymcthyl)-2-phcnyl-l -mcthylpyrrolidinc indicate that it fragments in a stepwise manner via an intermediate benzylic cation. The unexpected formation of a ring-opened sulfonamide rather than the expected tosylate ester on reaction of the 2-aryl-3-hydroxymethylpyrrolidine with R"S02C1 is thereby explained.95... 

Ab initio and semiempirical studies on the transition structure of the Baeyer-Villiger rearrangement of acetone with performic acid have been undertaken,510 and a theoretical investigation has been made511 of the mechanism of the Baeyer-Villiger reaction in non-polar solvents. The effect of fluoromethyl groups on the Baeyer-... 

A semiempirical study of rotational barriers of various thioamides and thioureas was published by Feigel and Strassncr111" and a careful, high level ab-initio study of formamide and thioformamide has been carried out by Chu and coworkers1116. 

Diels-Alder reactivity of thiophene and benzothiophene remains poorly understood. AMI semiempirical studies examining the activation of thiophene for this thermally allowed [4+2] cycloaddition process have shown that the usual synthesis approaches (use of highly reactive dienophiles, substitution on thiophene, increased reaction pressures) have only small effects on rate enhancement. However, use of the corresponding S-methylthiophenium salts, which have little aromaticity, should provide excellent activation for Diels-Alder reactions of thiophenes even with poor dienophiles such as ethylene <95JHC483>. This AMI approach has been applied to examine Diels-Alder reactions of benzo[6] and benzo[c]thiophenes the theoretical data agree with experimental results <95JCS(P1)1217>. 

A systematic semiempirical study of the core-level photoemission spectra of a wide range of 3d transition-metal compounds has been carried out (Bocquet et al., 1992,1996). The values for U and A obtained from a simplified Cl cluster model analysis are demonstrated in Figure 7.2. As can be inferred from the graphs, the heavier 3d transition metal compounds shown in the figure are expected to be charge-transfer insulators, whereas the compounds of the fighter metals are generally expected to be of the Mott-Hubbard type. 

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