Activation energies in nucleophilic displacement reactions measured at 296.deg. K in vacuo

Böhme, Diethard K.; Mackay, G. I.; Payzant, J. D.

doi:10.1021/ja00819a057

Cited by 99 publications

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“…Surprisingly, no thermochemical data for the formation of the F -(XCF 3 ) and X -(XCF 3 ) complexes (X ) Cl, Br, I) have been determined to date, either experimentally or computationally. In addition, no data are available on the thermochemistry of the transition states for either back-or front-side attack.…”

Section: Introductionmentioning

confidence: 99%

Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

Bogdanov

2006

J. Phys. Chem. A

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Density functional theory computations and pulsed-ionization high-pressure mass spectrometry experiments have been used to explore the potential energy surfaces for gas-phase S(N)2 reactions between halide ions and trifluoromethyl halides, X(-) + CF(3)Y --> Y(-) + CF(3)X. Structures of neutrals, ion-molecule complexes, and transition states show the possibility of two mechanisms: back- and front-side attack. From pulsed-ionization high-pressure mass spectrometry, enthalpy and entropy changes for the equilibrium clustering reactions for the formation of Cl(-)(BrCF(3)) (-16.5 +/- 0.2 kcal mol(-1) and -24.5 +/- 1 cal mol(-1) K(-1)), Cl(-)(ICF(3)) (-23.6 +/- 0.2 kcal mol(-1)), and Br(-)(BrCF(3)) (-13.9 +/- 0.2 kcal mol(-1) and -22.2 +/- 1 cal mol(-1) K(-1)) have been determined. These are in good to excellent agreement with computations at the B3LYP/6-311+G(3df)//B3LYP/6-311+G(d) level of theory. It is shown that complex formation takes place by a front-side attack complex, while the lowest energy S(N)2 reaction proceeds through a back-side attack transition state. This latter mechanism involves a potential energy profile which closely resembles a condensed phase S(N)2 reaction energy profile. It is also shown that the Cl(-) + CF(3)Br --> Br(-) + CF(3)Cl S(N)2 reaction can be interpreted using Marcus theory, in which case the reaction is described as being initiated by electron transfer. A potential energy surface at the B3LYP/6-311+G(d) level of theory confirms that the F(-) + CF(3)Br --> Br(-) + CF(4) S(N)2 reaction proceeds through a Walden inversion transition state.

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Section: Introductionmentioning

confidence: 99%

Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

Bogdanov

2006

J. Phys. Chem. A

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“…This traditional approach was first chal lenged by the discovery that typical organic reactions, in particular SN2 reactions of an ionic nucleophiles, can take place without activation in the gas phase [3,4], It appeared [5] that the barriers to such reactions in solu tion must be due entirely to some effect of the solvent, presumably to the energy needed to desolvate the ion so that the other reactant can approach. The idea that enzyme reac tions are analogs of gas phase processes was indeed first suggested on this basis [6].…”

Section: Introductionmentioning

confidence: 99%

New Ideas about Enzyme Reactions

Dewar¹

1986

Enzyme

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Since a proper substrate of an enzyme fits its active site closely, adsorption in the active site can occur only if all water is excluded from between them. Any subsequent reaction therefore takes place in the absence of solvent, i.e. as it would in the gas phase. The specificity and high rates of enzyme reactions can be explained immediately in terms of this analogy. Past experimental studies of enzyme mechanisms, based on analogy with reactions in solution, need to be reevaluated. Interpretation of enzyme reactions requires information concerning gas phase chemistry, which is usually lacking. The role of theoretical calculations in this connection is pointed out.

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“…While these prototypical condensed-phase and gasphase reactions are relatively well understood, significant differences in the reactivity of species in these phases have been observed [10][11][12][13][14][15]. For example, the acidities of aliphatic alcohols in the gas phase, observed by Brauman and Blair [16] to be t-C 4H90H > iso-C 3H70H > CzHsOH > CH 30H > HzO, have the reverse order of that measured in solution.…”

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confidence: 99%

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions: Methanol and ethanol as solvents

Eyet

Villano

Bierbaum

2008

J. Am. Soc. Mass Spectrom.

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The gas-phase reactions of F~(CH30H) and F~(CzH50H) with t-butyl bromide have been investigated to explore the effect of the solvent on the E2 transition state. Kinetic isotope effects (KIEs) were measured using a flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometer upon deuteration of both the alkyl halide and the alcohol. Kinetic isotope effects are significantly more pronounced than those previously observed for similar reactions of F~(HzO) with t-butyl halides. KIEs for the reaction of F~(CH30H) with t-butyl bromide are 2.10 upon deuteration of the neutral reagent and 0.74 upon deuteration of the solvent. KIEs for the reaction of F-(CzHsOH) with t-butyl bromide are 3.84 upon deuteration of the neutral reagent and 0.66 upon deuteration of the solvent. The magnitude of these effects is discussed in terms of transition-state looseness. Additionally, deuteration of the neutral regent and deuteration of the solvent do not produce completely separable isotope effects, which is likely due to a crowded transition state. These results are compared to our previous work on S N2 , and more recently have been extensively studied in the gas phase as well {3-9]. While these prototypical condensed-phase and gasphase reactions are relatively well understood, significant differences in the reactivity of species in these phases have been observed [10][11][12][13][14][15]. For example, the acidities of aliphatic alcohols in the gas phase, observed by Brauman and Blair [16] to be t-C 4H90H > iso-C 3H70H > CzHsOH > CH 30H > HzO, have the reverse order of that measured in solution. Additionally, the rate of a reaction usually varies significantly between the solution phase and the gas phase. For example, the reaction of hydroxide with methyl bromide is 16 orders of magnitude faster in the gas-phase compared with the reaction in aqueous medium [17]. This is due to the need for solvent reorganization as the reaction proceeds in solution, as well as the greater thermodynamic stability of the solvated ion. It is obvious that a single solvent molecule does not mimic the solution phase; however, a study of microsolvated ions will begin to shed light on the transition between these phases.Kinetic isotope effects (KIEs) are often used to study reaction mechanisms because they are a sensitive probe for transition-state structure. Deuterium KIEs are the Address reprint requests to Dr.

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Activation energies in nucleophilic displacement reactions measured at 296.deg. K in vacuo

Cited by 99 publications

References 0 publications

Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

New Ideas about Enzyme Reactions

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions: Methanol and ethanol as solvents

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Activation energies in nucleophilic displacement reactions measured at 296.deg. K in vacuo

Cited by 99 publications

References 0 publications

Gas Phase SN2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

Gas Phase SN2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

New Ideas about Enzyme Reactions

Deuterium kinetic isotope effects in microsolvated gas-phase E2 reactions: Methanol and ethanol as solvents

Contact Info

Product

Resources

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Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation

Gas Phase S_N2 Reactions of Halide Ions with Trifluoromethyl Halides: Front- and Back-Side Attack vs. Complex Formation