Elimination mechanisms. Deuteroxide/hydroxide isotope effects as a measure of proton transfer in the transition states for E2 elimination of 2-(p-trimethylammoniophenyl)ethyl 'Onium ions and halides. Mapping of the reaction-coordinate motion

1975 ◽  
Vol 97 (11) ◽  
pp. 3102-3108 ◽  
Author(s):  
Donald A. Winey ◽  
Edward R. Thornton
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Transition States ◽  
Reaction Coordinate

Hydration of acetone in the gas phase and in water solvent

2010 ◽  
Vol 88 (1) ◽  
pp. 56-64 ◽  
Author(s):  
Kiyull Yang ◽  
Yih-Huang Hsieh ◽  
Chan-Kyung Kim ◽  
Hui Zhang ◽  
Saul Wolfe
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Isotope Effects ◽  
Bond Formation ◽  
Transition States ◽  
Water Solvent ◽  
Cooperative Process

In water solvent, the hydration of acetone proceeds by a cyclic (cooperative) process in which concurrent C–O bond formation and proton transfer to oxygen take place through a solvent and (or) catalyst bridge. Reactivity is determined primarily by the concentration of a reactant complex and not the barrier from this complex. This situation is reversed in the gas phase; although the concentrations of reactive complexes are much higher than in solution, the barriers are also higher and dominant in determining reactivity. Calculations of isotope effects suggest that multiple hydron transfers are synchronous in the gas phase to avoid zwitterionic transition states. In solution, such transition states are stabilized by solvation and hydron transfers can be asynchronous.

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.

An MNDO SCF-MO study of proton transfer from fluoroethanols

1986 ◽  
Vol 64 (6) ◽  
pp. 1173-1178 ◽  
Author(s):  
Bo Anhede ◽  
Nils-Åke Bergman ◽  
A. Jerry Kresge
Keyword(s):  
Proton Transfer ◽  
Isotope Effects ◽  
Transition States ◽  
Kinetic Isotope Effects ◽  
Proton Exchange ◽  
Transfer Reactions ◽  
Correction Factors ◽  
Kinetic Isotope ◽  
Proton Transfer Reactions ◽  
Marcus Equation

Proton exchange between β-fluorinated ethanols and ethoxide ions has been studied using the MNDO SCF-MO method. Calculations were performed on reactions of ethoxide ion with ethanols substituted in the β-position with 0, 1, 2, and 3 fluorine atoms as well as on reactions where both the ethanol and the ethoxide ion were substituted with the same number (1, 2, 3) of fluorine atoms in the β-position. The energies obtained for the ion–molecule reactant complexes and the transition states from these reactions have been analyzed using the Marcus equation. Through the calculated force-constant matrices of reactants and transition states we also calculated the kinetic isotope effects for the proton-transfer reactions. The semiclassical isotopic rate constant ratios (kH/kD)s were found to be of rather normal magnitude and showed a variation with the energy of reaction. The calculated ratios of tunnel correction factors, QtH/QtD, proved to be unrealistically high. These factors were also calculated with the frequencies scaled down by 10% and this was found to reduce the QtH/QtD ratios to more realistic values.

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.

scholarly journals On the 3D → 2D Isomerization of Hexaborane(12)

Chemistry ◽  
2021 ◽  
Vol 3 (1) ◽  
pp. 28-38
Author(s):  
Josep M. Oliva-Enrich ◽  
Ibon Alkorta ◽  
José Elguero ◽  
Maxime Ferrer ◽  
José I. Burgos
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Energy Surface ◽  
Transition States ◽  
Three Dimensional ◽  
Reaction Coordinate ◽  
Intrinsic Reaction Coordinate ◽  
Limiting Factor ◽  
Two Dimensional ◽  
Axis Of Rotation

By following the intrinsic reaction coordinate connecting transition states with energy minima on the potential energy surface, we have determined the reaction steps connecting three-dimensional hexaborane(12) with unknown planar two-dimensional hexaborane(12). In an effort to predict the potential synthesis of finite planar borane molecules, we found that the reaction limiting factor stems from the breaking of the central boron-boron bond perpendicular to the C2 axis of rotation in three-dimensional hexaborane(12).

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