Model calculations of isotope effects. IX. Origin of large hydrogen and carbon isotope effects in the deprotonation of 2-nitropropane by pyridine bases

1983 ◽  
Vol 36 (8) ◽  
pp. 1521
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Bond Orders ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Pyridine Bases ◽  
State Models ◽  
Experimental Findings ◽  
Large Primary

The abnormally large primary hydrogen and carbon kinetic isotope effects found in the deprotonation of 2-nitropropane by hindered pyridine bases are investigated by means of model calculations. Transition-state models have been varied between tight and loose extremes, and between carbanion-like and nitronate-like structures. The only models that reproduce the experimental findings are those in which the sum of the bond orders to the transferring proton is less than unity (loose transition states) and which are subject to tunnelling corrections.

Model calculations of isotope effects. VIII. Deprotonation of nitroethane

1983 ◽  
Vol 36 (8) ◽  
pp. 1513
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Partial Negative Charge ◽  
Hydrogen Deuterium ◽  
State Models ◽  
Experimental Values ◽  
Comparison Of The Results

Transition-state models for the base-promoted deprotonation of nitroethane have been designed, and primary and secondary hydrogen-deuterium kinetic isotope effects have been calculated. Comparison of the results with experimental values of the primary isotope effects allows no firm conclusions to be reached concerning probable transition-state structures. However, the secondary α-deuterium isotope effect comparison disqualifies from consideration those transition states in which rehybridization of Cα and delocalization of the partial negative charge by the nitro group keep pace with the extent of deprotonation. Transition-state models wherein Cα is carbanionic and essentially pyramidal yield theoretical isotope effects lying within the experimental range.

Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

1974 ◽  
Vol 52 (6) ◽  
pp. 903-909 ◽  
Author(s):  
Jan Bron
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Benzyl Bromide ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Model Calculations ◽  
Kinetic Isotope ◽  
Proposed Model ◽  
Standard Reaction ◽  
Deuterium Isotope Effects

The results of calculations indicate that a previously proposed model for the transition state in "borderline" substitution reactions can be generalized and, as a result, the observed differences in the carbon-13 and deuterium isotope effects of SN1, SN2, and "borderline" reactions rationalized. Although the conclusions may apply more generally, the standard reaction investigated is the solvolysis of benzyl bromide. The importance of resonance interaction with the phenyl ring, the significance of the product- or reactant-like character of the transition state, and the influence of the magnitude of force constants in determining isotope effects are examined. The temperature dependence of kinetic isotope effects in solvolysis is also investigated.

13C kinetic isotope effects and reaction coordinate motions in transition states for SN2 displacement reactions

1976 ◽  
Vol 54 (7) ◽  
pp. 1146-1161 ◽  
Author(s):  
Warren Edward Buddenbaum ◽  
Vernon Jack Shiner Jr.
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Methyl Iodide ◽  
Isotope Effects ◽  
Transition States ◽  
Kinetic Isotope Effects ◽  
Chloride Ions ◽  
Reaction Coordinate ◽  
Kinetic Isotope ◽  
State Models

Reaction coordinate motions and 13C kinetic isotope effects at 25 °C have been calculated for the SN2 reactions of methyl iodide with iodide, cyanide, and chloride ions and for the SN2 reaction of benzyl bromide with hydroxide ion using transition state models characterized by single interaction force constant, F12, between the bond being formed and the bond being broken. The isotope effect calculations show that the dependence of calculated 13C isotope effects on transition state symmetry found by Willi and Sims etal. holds true for reaction barriers corresponding to small values of νL, while the symmetry dependence observed by Bron holds true for barriers corresponding to large values of νL.νL was also found to have a strong influence on the reaction coordinate motions of the transition states. In particular, for the methyl iodide reactions an increase in νL increases the distortion of the methyl group in the direction expected for a classical SN2 reaction. Finally, reaction coordinate motions were used to show that the model proposed by Bron for the borderline region between SN1 and SN2 reaction mechanisms predicts an increase in the 13C kinetic isotope effect with decreasing total bond order and not the decrease suggested by Bron.

Model calculations of isotope effects. VI. The structure of the transition state for hydrogen atom transfer from ethane to methyl radicals

1982 ◽  
Vol 35 (5) ◽  
pp. 1045 ◽  
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Hydrogen Atom Transfer ◽  
Methyl Radicals ◽  
Bond Orders ◽  
Empirical Relationships ◽  
Model Calculations ◽  
Ab Initio Structure ◽  
State Models ◽  
State Force

Kinetic hydrogen isotope effects for the reaction C2H6 + CDB → C2H5 + CHD3 have been calculated for a large number of transition state models, bond orders being based on an ab initio structure for the ethyl radical. Various empirical relationships for transition state force fields in terms of partial bond orders were examined for each model structure. No transition state model reproduced the experimental intermolecular and intramolecular isotope effects over the temperature range, but when an Eckart tunnel correction was applied a single model gave satisfactory agreement.

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.

Sign in / Sign up

Export Citation Format

Share Document