scholarly journals Global cyclic transition – 2020: theory and methodology

2020 ◽  
Vol 132 (2) ◽  
Author(s):  
M. Tulegenova
Keyword(s):  
Cyclic Transition

Kinetic Studies of [4n+2]π-Thermal Cyclodimerization of 1-(3-Pyridazinyl)-3-oxidopyridinium Betaines

1992 ◽  
Vol 57 (9) ◽  
pp. 1951-1959 ◽  
Author(s):  
Madlene L. Iskander ◽  
Samia A. El-Abbady ◽  
Alyaa A. Shalaby ◽  
Ahmed H. Moustafa
Keyword(s):  
Energy Gap ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
First Order ◽  
Thermodynamic Activation Parameters ◽  
Cyclic Transition State ◽  
Complete Dissociation ◽  
Homo Lumo

The reactivity of the base induced cyclodimerization of 1-(6-arylpyridazin-3-yl)-3-oxidopyridinium chlorides in a pericyclic process have been investigated kinetically at λ 380 nm. The reaction was found to be second order with respect to the liberated betaine and zero order with respect to the base. On the other hand dedimerization (monomer formation) was found to be first order. It was shown that dimerization is favoured at low temperature, whereas dedimerization process is favoured at relatively high temperature (ca 70 °C). Solvent effects on the reaction rate have been found to follow the order ethanol > chloroform ≈ 1,2-dichloroethane. Complete dissociation was accomplished only in 1,2-dichloroethane at ca 70 °C. The thermodynamic activation parameters have been calculated by a standard method. Thus, ∆G# has been found to be independent on substituents and solvents. The high negative values of ∆S# supports the cyclic transition state which is in favour with the concerted mechanism. MO calculations using SCF-PPP approximation method indicated low HOMO-LUMO energy gap of the investigated betaines.

STUDY OF IRON–CARBON MIXED CLUSTERS FeCn (n=2−5): A POSSIBLE LINEAR-TO-CYCLIC TRANSITION FROM FeC3 TO FeC4

1996 ◽  
Vol 03 (01) ◽  
pp. 423-427 ◽  
Author(s):  
LAI-SHENG WANG
Keyword(s):  
Photoelectron Spectroscopy ◽  
Cluster Size ◽  
Structural Transition ◽  
Carbon Clusters ◽  
Cyclic Structure ◽  
Cyclic Transition ◽  
Pure Carbon ◽  
Cyclic Isomer ◽  
Anion Photoelectron Spectroscopy ◽  
Mixed Clusters

Small Fe/C mixed clusters, [Formula: see text], are studied by anion photoelectron spectroscopy at 3.49-eV photon energy. The spectra of [Formula: see text] and [Formula: see text] show well-resolved features and are consistent with linear-to-linear detachment transitions. The [Formula: see text] spectrum is quite broad, suggesting large geometry change from the anion to the neutral. Since [Formula: see text] is known to be linear, our data suggest that neutral FeC4 may have a cyclic structure. The spectrum of [Formula: see text] exhibits both sharp and broad features, consistent with the existence of both a linear and a cyclic isomer. A linear-to-cyclic structural transition is thus suggested to take place for the neutral FeCn clusters from n=3−4; for n<4 they are linear and for n≥4 they can be both linear and cyclic. This is similar to the linear-to-cyclic transition occurring for pure carbon clusters from n=9−10 (Ref. 10) but at a much smaller cluster size due to the presence of the Fe atom.

Divergent Reactivity of Stannane and Silane in the Trifluoromethylation of PdII: Cyclic Transition State versus Difluorocarbene Release

2018 ◽  
Vol 130 (46) ◽  
pp. 15301-15305 ◽  
Author(s):  
Maoping Pu ◽  
Italo A. Sanhueza ◽  
Erdem Senol ◽  
Franziska Schoenebeck
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State

scholarly journals Kinetics and Correlation Analysis of Reactivity in the Oxidation of Some α-Hydroxy Acids by Benzimidazolium Dichromate

2018 ◽  
Vol 43 (3-4) ◽  
pp. 300-314
Author(s):  
Dinesh Panday ◽  
Teena Kachawa ◽  
Seema Kothari
Keyword(s):  
Solvent Effect ◽  
Mandelic Acid ◽  
Bond Cleavage ◽  
Hydroxy Acids ◽  
Cyclic Transition ◽  
Hydride Ion ◽  
Rate Determining Step ◽  
Reaction Constant ◽  
Cyclic Transition State ◽  
Hydride Ion Transfer

Kinetic and mechanistic studies of the oxidation of mandelic acid and nine monosubstituted mandelic acids by benzimidazolium dichromate (BIDC) in dimethyl sulfoxide are discussed with an emphasis on correlation of structure and reactivity. The reactions were of first order with respect to BIDC. However, Michaelis-Menten type kinetics were observed with respect to hydroxy acids. The reactions are catalysed by protons. The deuterium isotope effect for the oxidation of mandelic acid ( kH/ kD = 5.91 at 298 K) indicated an α-C-H bond cleavage in the rate-determining step. An analysis of the solvent effect showed that the role of cationsolvation is major. The reaction showed an excellent correlation with the Hammett σ values, the reaction constant being negative. Based on the kinetic data, analysis of the solvent effect and results of structure-reactivity correlation along with some non-kinetic parameters, a mechanism involving rate-determining oxidative decomposition of the complex through hydride-ion transfer via a cyclic transition state to give the corresponding oxoacid is suggested.

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