Linear Free Energy Relationship and Kinetic Isotope Effects as Measures for the Transition State Variation. A Computational Study

2003 ◽  
Vol 68 (20) ◽  
pp. 7772-7778 ◽  
Author(s):  
Salai Cheettu Ammal ◽  
Masaaki Mishima ◽  
Hiroshi Yamataka
Keyword(s):  
Free Energy ◽  
Transition State ◽  
Computational Study ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Linear Free Energy Relationship ◽  
Linear Free Energy ◽  
Kinetic Isotope ◽  
State Variation

Linear free energy relationship and kinetic isotope effects as measures for the transition-state variation — A case of the neophyl system

2005 ◽  
Vol 83 (9) ◽  
pp. 1606-1614 ◽  
Author(s):  
Salai Cheettu Ammal ◽  
Hiroshi Yamataka
Keyword(s):  
Free Energy ◽  
Transition State ◽  
Density Functional ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Single Step ◽  
Linear Free Energy Relationship ◽  
Linear Free Energy ◽  
Kinetic Isotope ◽  
State Variation

Ab initio calculations at the MP2/6-31G* level and density functional theory (B3LYP/6-311+G**) calculations have been performed on acid-catalyzed ionizations of substituted neophyl alcohols to investigate whether a variation of the transition-state (TS) structure is reflected in the kinetic isotope effects (KIE) and linear free energy relationship. The effect of substituents on KIEs, TS structures, and activation and reaction energies was calculated. This study revealed that a curved Brønsted-type plot could arise for a single-step process from the variation of TS structure with the substituent, whereas the Hammett plots with a dual-parameter treatment can not detect such TS variation. The variation of KIEs at various positions of neophyl alcohol reflects the variation of TS structures in a manner consistent with the More O'Ferrall – Jencks type reaction diagram analyses.Key words: transition-state variation, substituent effect, kinetic isotope effect, linear free energy relationship.

The Importance of Anharmonicity of The Vibrational Excited States in Chemical Kinetics

1975 ◽  
Vol 53 (20) ◽  
pp. 3069-3074 ◽  
Author(s):  
Jan Bron
Keyword(s):  
Transition State ◽  
Excited States ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Vibrational States ◽  
Vibrational Anharmonicity ◽  
Kinetic Isotope ◽  
Large Correction ◽  
Lower Temperature ◽  
Lower Temperature Region

The corrections to rate constants for an harmonicity of vibrational excited states have been evaluated over the temperature range of 200–1100 K. The reaction O2 + X, where X is H or D, has been chosen as the model system. Only the influence of vibrational anharmonicity of the triatomic transition state has been determined. Two geometric shapes for the transition state, bent and isosceles configurations, have been investigated in detail by the bond order method.It is found that the correction can be large, depending upon the geometry and force field of the transition state and the temperature. The magnitude of the correction for anharmonicity of the vibrational excited states depends mainly, at a particular temperature, on the strength of the O—X bond in the transition state. In the case of a large correction, anharmonicity may lead to a nonlinear Arrhenius plot.Because of cancellation effects, the correction for anharmonicity of the excited vibrational states in kinetic isotope effects can be ignored in the lower temperature region. It has also been found that anharmonicity of the vibrational groundstate can explain unexpected large isotope effects.

Kinetics and Mechanism of the Aminolysis of S-Aryl O-Ethyl Dithiocarbonates in Acetonitrile

2004 ◽  
Vol 69 (12) ◽  
pp. 2174-2182 ◽  
Author(s):  
Hyuck Keun Oh ◽  
Ji Young Oh ◽  
Dae Dong Sung ◽  
Ikchoon Lee
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Interaction Constant ◽  
Kinetics And Mechanism ◽  
Cyclic Structure ◽  
Concerted Mechanism ◽  
Tetrahedral Intermediate ◽  
Kinetic Isotope ◽  
Hydrogen Bonded

The aminolysis of S-aryl O-ethyl dithiocarbonates with benzylamines are studied in acetonitrile at -25.0 °C. The βX (βnuc) values are in the range 0.67-0.77 with a negative cross-interaction constant, ρXZ = -0.24, which are interpreted to indicate a concerted mechanism. The kinetic isotope effects involving deuterated benzylamine nucleophiles (XC6H4CH2ND2) are large, kH/kD = 1.41-1.97, suggesting that the N-H(D) bond is partially broken in the transition state by forming a hydrogen-bonded four-center cyclic structure. The concerted mechanism is enforced by the strong push provided by the EtO group which enhances the nucleofugalities of both benzylamine and arenethiolate from the putative zwitterionic tetrahedral intermediate.

Kinetics of the reduction of nicotinonitrile cations by 1,4-dihydronicotinamides

1985 ◽  
Vol 63 (6) ◽  
pp. 1245-1249 ◽  
Author(s):  
John W. Bunting ◽  
John C. Brewer
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Spectral Studies ◽  
Close Correspondence ◽  
Charge Generation ◽  
Kinetic Isotope ◽  
Hammett Σ Constants ◽  
Kinetics Of

The rates of reduction of a series of 1-(Z-benzyl)nicotinonitrile cations by a series of 1-(X-benzyl)-1,4-dihydronicotinamides have been studied at 25 °C in 20% CH3CN – 80% H2O (pH 7.0 (5 mM phosphate), ionic strength 1.0 (KCl)). Spectral studies indicate the formation of 1,4-dihydronicotinonitrile products, without the formation of the isomeric 1,2-dihydro- or 1,6-dihydro-nicotinamide intermediates. Second-order rate constants (k2) for these reductions are closely correlated with the Hammett σ constants for X and Z. Thus, for X = H, log k2 = 0.63σz − 1.05, while for Z = 4-CN, log k2 = −0.64σx − 0.65. The close correspondence between these ρx and ρz values indicates that charge neutralization on the nicotinonitrile cation exactly balances charge generation on the nicotinamide cation product in the rate-determining transition state. Thus the migrating hydrogen species is electrically neutral in the rate-determining transition state, which contrasts with the hydridic transition states previously reported in the reduction of isoquinolinium cations by 1,4-dihydronicotinamides. When 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide is used as the reductant, primary kinetic isotope effects of 3.0 and 2.7 are observed for the reduction of the 1-methylnicotinonitrile and 1-(4-cyanobenzyl)-nicotinonitrile cations, respectively. These data are evaluated in terms of the various mechanistic possibilities for hydride transfer.

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