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.

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. VII. Deprotonation of 2-nitropropane by oxy anion bases

1983 ◽  
Vol 36 (8) ◽  
pp. 1503
Author(s):  
DJ McLennan
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Isotope Effects ◽  
Experimental Results ◽  
Electron Delocalization ◽  
Model Calculations ◽  
Charge Delocalization ◽  
Successful Model ◽  
Deuterium Isotope Effects ◽  
State Models

Model calculations of primary and secondary deuterium isotope effects for the hydroxide-induced deprotonation of 2-nitropropane are reported. Various transition-state models have been examined in an effort to reproduce experimental results. A purely pyramidal transition state in which proton transfer has run far ahead of carbon rehybridization and charge delocalization is a successful model as far as isotope effects are concerned, but may fail on other counts. Three incipient trigonal models for the transition state have been tested, and, although none can be firmly eliminated by the resultant isotope effects, those involving the proton transfer's running ahead of electron delocalization and perhaps carbon rehybridization are favoured.

Model Calculations of Isotope Effects. III. Isotope Effects in Hydrogen Transfer Reactions, and Transition State Force Fields

1979 ◽  
Vol 32 (9) ◽  
pp. 1869
Author(s):  
DJ McLennan
Keyword(s):  
Transition State ◽  
Hydrogen Transfer ◽  
Force Fields ◽  
Isotope Effects ◽  
Semiempirical Method ◽  
Model Calculations ◽  
Surface Hydrogen ◽  
Kinetic Isotope ◽  
State Force ◽  
Better Than

Model calculations of kinetic isotope effects for the reactions: C2H6+·CH3 → ·C2H5+CH4 CH4 + ·CF3 → ·CH3+CHF3 are reported. Transition state geometries were those calculated by Dewar and coworkers using the MNDO semiempirical method. Transition state force fields were formulated from empirical expressions for stretching, bending, linear bending and interaction valence force constants by using bond orders as disposable parameters. Although it proved impossible to assign a best force field to either reaction, the calculated isotope effects generally were in satisfactory agreement with experiment, and were better than those calculated from the MNDO potential energy surface. Hydrogen tunnelling is apparently implicated in the CH4+ CF3 reaction.

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 Isotope Effects. IV. Transition State Structures and the Question of Tunnelling in Proton Transfer Reactions

1979 ◽  
Vol 32 (9) ◽  
pp. 1883 ◽  
Author(s):  
DJ McLennan
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Normal Values ◽  
Isotope Effects ◽  
Transition States ◽  
Transfer Reactions ◽  
Model Calculations ◽  
Proton Transfer Reactions ◽  
Donor Acceptor ◽  
State Models

Model calculations of primary hydrogen isotope effects in proton transfer reactions are reported. The geometries and force fields of transition state models have been systematically varied with respect to both reactant-like and product-like character and to tight against loose character. The models include both hypothetical cut-off molecules and 2-nitropropane. Values of kH/kD greater than 17 are calculated for loose, symmetrical transition states in which the sum of the bond orders pertaining to the transferring proton is set at 0.6, and higher than normal values of (ED-EH) and ADIAH are also associated with such transition states. It is suggested that transition state looseness is a consequence of repulsive donor-acceptor steric interactions, and that several sets of experimental results which have hitherto been rationalized by the invocation of proton tunnelling may equally well be explained by postulating loose transition states.

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.

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