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Inversion Mechanism (inversion + mechanism)
Selected AbstractsTheoretical investigation of ion pair SN2 reactions of alkali isothiocyanates with alkyl halides.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005Part 1. Abstract The gas-phase ionic SN2 reactions NCS - + CH3F and ion pair SN2 reaction LiNCS + CH3F with inversion mechanism were investigated at the level of MP2(full)/6-311+G**//HF/6-311+G**. Both of them involve the reactants complex, inversion transition state, and products complex. There are two possible reaction pathways in the ionic SN2 reaction but four reaction pathways in the ion pair SN2 reaction. Our results indicate that the introduction of lithium significantly lower the reaction barrier and make the ion pair displacement reaction more facile. For both ionic and ion pair reaction, methyl thiocyanate is predicted to be the major product, but the latter is more selective. More-stable methyl isothiocyanate can be prepared by thermal rearrangement of methyl thiocyanate. The theoretical predictions are consistent with the known experimental results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanismsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004Yi Ren Abstract Identity ion-pair SN2 reactions LiX + CH3X , XCH3 + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [,H(inv)] are found to be much higher than the corresponding values for the gas phase anionic SN2 reactions, decreasing in the following order: F (263.6 kJ mol,1) > Cl (203.3 kJ mol,1) > Br (174.7 kJ mol,1) > I (150.7 kJ mol,1). The barrier gaps between the two mechanisms [,H (ret) , ,H (inv)] increase in the order F (,62.7 kJ mol,1) < Cl (4.4 kJ mol,1) < Br (24.9 kJ mol,1) < I (45.1 kJ mol,1). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic SN2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole,dipole complexes CH3X · · · LiX (,Hcomp) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol,1 for I up to 58.9 kJ mol,1 for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair SN2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair SN2 reactions in the solution, but the potential profile for reaction LiI + CH3I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [,H (inv)] with the geometric looseness of the inversion transition state %C,X,, the dissociation energies of the C,X bond (DC,X) and Li,X bond (DLi,X) are observed, respectively. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 461,467, 2004 [source] Effect of solvent on the inversion of pyramidal sulfonium and selenonium compoundsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2001Andreas Wiegrefe Abstract The effects of temperature and solvent on the cis,trans equilibrium and isomerization rates of different cyclic sulfonium and selenonium compounds were investigated by 1H NMR spectroscopy. A non-dissociative process was considered to be the most probable mechanism for inversion of configuration. While the temperature had no apparent effect on the equilibrium, in which the trans -diastereoisomer dominated in all cases, changing the solvent from dimethylformamide to acetonitrile and to water led to increasing amounts of the cis -diastereoisomer. Additionally, the rate of stereomutation of the thiolanium compound was slowed by a factor of 2 and that of the selenolanium compound by a factor of 85. While the pyramidal (vertex) inversion is the most probable mechanism for the sulfonium compounds investigated, some evidence is presented that indicates that the selenonium compound could isomerize via an edge inversion mechanism. Copyright © 2001 John Wiley & Sons, Ltd. [source] Influence of the processing conditions on a two-phase reactive blend system: EVA/PP thermoplastic vulcanizatePOLYMER ENGINEERING & SCIENCE, Issue 11 2002Catherine Joubert The elaboration of a TPV based on copolymer of ethylene and vinyl acetate (EVA) and polypropylene (PP) as thermoplastic phase was investigated in a batch mixer. The crosslinking reaction is carried out through a transesterification reaction between ester groups of EVA and alcoxysilane groups of the crosslinker agent tetrapropoxyorthosilicate (TPOS). The main advantage of this crosslinking reaction is that it can be well controlled and suitable for different processing conditions. The aim of the present study is to get a better understanding of the dispersion mechanism and of the phase inversion of the EVA major phase during its dynamic vulcanization into the PP minor phase. It was proved that the initial viscosity ratio, , = ,pp/,EVA, between EVA and PP plays an important part in the morphology development of the reactive blend. The viscosity ratio must be close to the critical ratio expressed by Utracki's model of phase inversion mechanism. Furthermore, the influence of different processing parameters on the variation of the morphology and on the mechanical properties of the ultimate TPV was investigated. The main conclusion of this study is that the characteristic time of crosslinking must be of the same order than the time of mixing. Indeed, better mechanical properties are obtained when a progressive phase inversion occurred and when it is controlled by rheological aspects and transient morphology equilibrium of the two phases and not by the mechanical fragmentation of the crosslinked EVA. For example, in our experimental conditions (concerning the amounts of catalyst and crosslinker reagents), high shear rates can be avoided ( < 80 s,1) as the self-heating of the blend under shear considerably increases (,T , 50°C for = 225 s,1), leading to faster kinetics and consequently to a phase inversion controlled by the fragmentation of the crosslinked EVA phase. [source] Structure of jack bean chitinaseACTA CRYSTALLOGRAPHICA SECTION D, Issue 9 2000M. Hahn The structure of jack bean chitinase was solved at 1.8,Å resolution by molecular replacement. It is an ,-helical protein with three disulfide bridges. The active site is related in structure to animal and viral lysozymes. However, unlike in lysozyme, the architecture of the active site suggests a single-step cleavage. According to this mechanism, Glu68 is the proton donor and Glu90 assists in the reaction by moving towards the substrate and recruiting a water molecule that acts as the nucleophile. In this model, a water molecule was found in contact with Glu90 O,1 and Thr119,O, at a distance of 3.0 and 2.8,Å, respectively. The model is in accordance with the observed inversion mechanism. [source] Kinetic Study of Thermal Z to E Isomerization Reactions of Azobenzene and 4-Dimethylamino-4,-nitroazobenzene in Ionic Liquids [1-R-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide with R=Butyl, Pentyl, and Hexyl]CHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2006Keita Baba Abstract Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4,-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R=butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4,-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers,Grote,Hynes model or by the Agmon,Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 104,107 s,1. [source] Racemization of the gastrointestinal antisecretory chiral drug esomeprazole magnesium via the pyramidal inversion mechanism: A theoretical studyCHIRALITY, Issue 9 2010Hili Marom Abstract The pyramidal inversion mechanisms of the 6-methoxy and the 5-methoxy tautomers of (S)-omeprazole were studied, employing ab initio and DFT methods. The conformational space of the model molecule (S)-2-[(3-methyl-2-pyridinyl)methyl]sulfinyl-1H -benzimidazole was calculated, with respect to rotations around single bonds, at the B3LYP/6-311G(d,p) level. All of the resulting conformations were used as starting points for full optimizations of (S)-omeprazole, at B3LYP/6-31G(d), B3LYP/6-311G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-311G(2df,2pd), MP2/6-31G(d), and MP2/6-311G(d,p) levels. Four distinct pathways were found for enantiomerization via the pyramidal inversion mechanism for each of the tautomers of (S)-omeprazole. Each transition state, in which the sulfur, the oxygen and the two carbon atoms connected directly to the sulfur are in one plane, connects two diastereomeric minima. The enantiomerization is completed by free rotation around the sulfur,methylene bond, and around the methylene,pyridine ring bond. The effective Gibbs' free energy barrier for racemization ,G of the two tautomers of (S)-omeprazole are 39.8 kcal/mol (5-methoxy tautomer) and 40.0 kcal/mol (6-methoxy tautomer), indicating that the enantiomers of omeprazole are stable at room temperature (in the gas phase). The 5-methoxy tautomer of (S)-omeprazole was found to be slightly more stable than the 6-methoxy tautomer, in the gas phase. The energy barrier (,G,) for the(S,M) (S,P) diastereomerization of (S)-omeprazole due to the rotation around the pyridine chiral axis was very low, 5.8 kcal/mole at B3LYP/6-311G(d,p). Chirality 2010. © 2010 Wiley-Liss, Inc. [source] | ||||||||||