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A theoretical study that addresses the question of whether nucleophilic addition to the carbonyl groups of acid chlorides, esters, and anhydrides involves an addition elimination pathway or proceeds by a concerted S(sub N)2-like mechanism in the absence of the generally assumed tetrahedral intermediate is presented.

Research has been conducted on the dimethyl sulfide oxidation. The use of theoretical calculations in investigating this oxidation is described.

Research has been conducted on the dioxirane dimethyldioxirane. The strain energies of these dioxiranes have been investigated via the use of the high-level computational schemes and the dioxirane reactivity towards alkenes has been predicted.

Research has been conducted on the cyclic molecules' strain energy. The effect of substituents on this energy has been examined and the results are presented.

Quantitative theoretical evidence suggests that the neighboring silicon group is silyl peroxides effectively stabilizes the developing negative charge on oxygen in a concerted transfer of an oxygen atom to nucleophiles. The bond dissociation energy of a dialkyl peroxide is 11-12 kcal/mol less than that of the corresponding silyl peroxide.

Research reveals that the diastereoselectivity in the epoxidation of chiral allylic alcohols is dictated by a hydrogen-bonding between the peroxy acids and the allylic alcohols. Computational analysis of the structural implications of the findings are presented.

The effect of trifluoroacetic acid catalyst in the rate of epoxidation of (Z)-cyclooctene with m-chloroperbenzoic acid was determined through utilization of kinetic and theoretical studies. Data indicated that the increase in the rate was due to complexation of the peroxy acid with the unddissociated acid catalyst (HA) rather than the protonation of the peroxy acid.

The mechanisms that mediate aromatic nucleophilic substitutions during the reaction of halide anions with halobenzenes and halonitrobenzenes were analyzed based on ab initio computations. Ab initio analysis of the identity of the halide-exchange reaction indicated the presence of multistep SNAr reactions that involved the formation of an intermediate sigma-complex. Furthermore, the first stage of the gas phase aromatic nucleophilic substitution reaction induced the formation of a loose ion-molecule prereaction complex.

The decarboxylation of beta-keto acids involves a complete proton transfer from the carboxylic acid to the beta-carbonyl group which leads to the formation of a dipolar ion in the transition state. The dipolar intermediate of the beta-keto acid exhibits an increase in the rate of decarboxylation when the dielectric constant of the solvent decreases. The rate of enzymatic decarboxylation is also enhanced by the polarizable sulfur atom and a positively charged ammonium group.

The calculated values of kinetic isotope effect of intramolecular elimination reaction are in accordance with the experimentally determined values. The primary and secondary isotope effects can be evaluated by using intramolecular Cope elimination, involving thermal decomposition of an amine oxide. The intramolecular Cope elimination reaction is an intramolecular syn-elimination. The transition state geometries for Cope and ylide reactions can thus be evaluated by the calculations.

Methyl substituents on N,N-dimethyl-alpha-ketopropanamide reveal steric reactions. N, N-dimethyl-alpha-ketopropanamide moves away from planarity and causes the lack of a syn-conformer as a local minimum due to the interaction. Crystal geometry of FK506 reveals orthogonal orientation which shows a torsional profile and the size of the obstacle for C-C bond rotation. These features are similar in the compound due to the reaction. Easy exchange of the cis and trans structures for the amide bond in FK506 at ambient temperatures is revealed by the size of the C-N rotational obstacle in the compound.

The complexation of mercuric chloride and silver trifluoromethanesulfonate with several alkenes was studied using nuclear magnetic resonance spectroscopic analyses. The results were based on the formation constants calculated from the spectra of the observed complexes. Moreover, the data revealed that the rate-determining step in the bromination reaction was the formation of the bromonium ion while for oxymercuration, the solvent attack on the mercuronium ion was the rate-determining step.

The asymmetric oxidation of enolates with N-sulfonyloxaziridines results in the production of highly enantioselective alpha-hydroxy carbonyl compounds. A molecular orbital system serves as a model for the transition state of oxygen atom transfer that takes place in the synthesis. The transition state involves the positioning of lithium cation in the oxygen atoms of both enolate and oxaziridine compounds. The binding of sulfonyl oxygen atom and lithium cation affects stereoselectivity of the resulting alpha-hydroxy carbonyl compound.

A free radical cation chain reaction allows the conversion of quadricyclane (Q) to norbornadiene (N), despite a high thermal barrier for reversion. One-electron oxidations start the chain reactions and the conversion of Q to N releases energy. The electronic structures of the radical cations of Q and N, namely Q(super .+)and N(super .+), are symmetrical. A study predicts a half life of 9.14 X 10 to the -6th power at 298 K for valence isomerization of Q(super .+) to N(super .+).

Quantum mechanical calculations (DFT) have described a mechanism for the oxidative C-H bond cleavage step in Fenton-like hydrocarbon hydroxylation. The transition structure for the oxidation step in anionic iron(III) hydroperoxides has displayed a resemblance to model porphyrin calculations on cytochrome P450 hydroxylation.

A [super 1]H NMR spectroscopy is used to study the kinetics of the Curtius rearrangements of methyl 1-azidocarbonyl cycloprop-2-enel-carboxylate and methyl 1-azidocarbonyl cyclopropane-1-carboxylate. The results from the comparison of reaction pathways at the CBS-QB3 level for the Curtius rearrangement versus the loss of [N.sub.2] to form a nitrene intermediate has provided strong evidence for the dominant process of the concerted Curtius rearrangement.

Density functional theory and calculations are used to investigate the mechanism of thiolate-disulfide exchange reactions, which proceed through nucleophilic substitution process. The resulting trisulfur intermediate formed has an activation barrier of 10.7 kcal/mol, which indicates the formation of a new disulfide bond.

The characterization of transition structures of gas-phase [S.sub.N]2 displacement reaction of a tertiary phosphine nucleophile on a dimethyl disulfide involving mechanism of S-S bond is reported. The reaction proceeds through phosphonium cation intermediate with formation of a salt-like complex involving the displaced thiolate anion that provides sufficient H-bonding stabilization.