scholarly journals Deuterium Isotope Effects on Acid-Base Equilibrium of Organic Compounds

Molecules ◽  
2021 ◽  
Vol 26 (24) ◽  
pp. 7687
Author(s):  
Meiyi Liu ◽  
Jiali Gao
Keyword(s):  
Free Energy ◽  
Isotope Effects ◽  
Linear Free Energy Relationship ◽  
Acid Base ◽  
Exchange Effect ◽  
Linear Free Energy ◽  
Acid Base Equilibrium ◽  
Deuterium Isotope Effects ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

Deuterium isotope effects on acid–base equilibrium have been investigated using a combined path integral and free-energy perturbation simulation method. To understand the origin of the linear free-energy relationship of ΔpKa=pKaD2O−pKaH2O versus pKaH2O, we examined two theoretical models for computing the deuterium isotope effects. In Model 1, only the intrinsic isotope exchange effect of the acid itself in water was included by replacing the titratable protons with deuterons. Here, the dominant contribution is due to the difference in zero-point energy between the two isotopologues. In Model 2, the medium isotope effects are considered, in which the free energy change as a result of replacing H2O by D2O in solute–solvent hydrogen-bonding complexes is determined. Although the average ΔpKa change from Model 1 was found to be in reasonable agreement with the experimental average result, the pKaH2O dependence of the solvent isotope effects is absent. A linear free-energy relationship is obtained by including the medium effect in Model 2, and the main factor is due to solvent isotope effects in the anion–water complexes. The present study highlights the significant roles of both the intrinsic isotope exchange effect and the medium solvent isotope effect.

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.

On the displacement reactions of organic substances in water

2005 ◽  
Vol 83 (9) ◽  
pp. 1667-1719 ◽  
Author(s):  
J MW Scott
Keyword(s):  
Free Energy ◽  
Temperature Dependence ◽  
Isotope Effects ◽  
Linear Free Energy Relationship ◽  
Linear Free Energy Relationships ◽  
Organic Substances ◽  
Linear Free Energy ◽  
Organic Halides ◽  
Displacement Reactions ◽  
Energy Relationships

The hydrolyses of a series of organic halides, nitrates, sulphonates, and perchlorates are examined from the standpoint of thermodynamics, kinetics, temperature dependence, isotope effects, the Kurz equation, and the Guthrie linear free energy relationship. The thermodynamics of several definite bimolecular displacements are also examined. Some mechanistic revisions are suggested to reach an improved accommodation with the experimental observations.Key words: kinetics, thermodynamics, hydrolytic displacements, mechanism, ionic displacements, nucleofugality, linear free energy relationships.

The hydration of 1-aryl-3,3,3-trifluoropropynes and some other phenylacetylenes in sulfuric acid. An excess acidity analysis

1990 ◽  
Vol 68 (10) ◽  
pp. 1876-1881 ◽  
Author(s):  
Robin A. Cox ◽  
Ewart Grant ◽  
Todd Whitaker ◽  
Thomas T. Tidwell
Keyword(s):  
Sulfuric Acid ◽  
Isotope Effects ◽  
Activation Parameters ◽  
Linear Free Energy Relationship ◽  
Hydration Kinetics ◽  
Hydration Mechanism ◽  
Linear Free Energy ◽  
Vinyl Cation ◽  
Solvent Isotope ◽  
Kinetics Of

The excess acidity method has been used to analyse the hydration kinetics of the phenylacetylenes Y-C6H4-C≡C-Z in aqueous sulfuric acid mixtures; Z = CF3 (1), H (2), COC6H4-X (3), and CO2H (4). All substrates gave acetophenone-type products consistent with the normal hydration mechanism involving rate-determining vinyl cation formation. Standard-state log k0 intercepts, and m≠m* slopes, were both used in linear free energy relationship plots against the substituent σ+ values. Solvent isotope effects and activation parameters were obtained in some cases. The deactivating Z substituents in 1, 3, and 4 all cause reaction to be some 100 times slower than that of the parent phenylacetylene 2. Compounds 2,3, and 4 all have ρ+ values of about −3.8, but 1 is more substituent sensitive, with a ρ+ of −5.3. A σ+ value of 0.38 is calculated for the CF3C≡C substituent. The ρ+ values were found to be acidity independent for 1 and 2, and probably for 3, but not for 4. Proton transfer at the transition state was found to be most advanced for the fastest reaction, that of 2, contrary to intuition. Keywords: alkyne hydration, excess acidity, phenylacetylenes, vinyl cations, deactivated carbocations.

Kinetic deuterium isotope effects and kinetic solvent isotope effects for the solvolyses in water and in water + acetonitrile mixtures related to a series of substituted benzyl nitrates

1980 ◽  
Vol 58 (20) ◽  
pp. 2142-2145 ◽  
Author(s):  
Mohammed Ahsan ◽  
Ross Elmore Robertson ◽  
Michael Jesse Blandamer ◽  
John Marshall William Scott
Keyword(s):  
Aqueous Solution ◽  
Benzene Ring ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Bond Breaking ◽  
Relative Importance ◽  
Stage Process ◽  
Deuterium Isotope Effects ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

In aqueous solution, the α-deuterium isotope effects in the solvolyses of benzyl nitrates derivatives depend on the nature of the substituent in the benzene ring. In addition, the isotope effect for some derivatives depends on mole fraction of added acetonitrile while for others the isotope effect is insensitive to solvent composition. However, the kinetic solvent isotope effects for para-methyl and meta-trifluoromethyl derivatives remain unchanged when acetonitrile is added. These observations are accounted for in terms of a model which describes the solvolytic reaction as a two-stage process and contrasts the relative importance of bond-making and bond-breaking.

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