Preassociation, Free-Ion, and Ion-Pair Pathways in the Electrophilic Bromination of Substitutedcis- andtrans-Stilbenes in Protic Solvents

1997 ◽  
Vol 119 (51) ◽  
pp. 12492-12502 ◽  
Author(s):  
Marie-Françoise Ruasse ◽  
Giacomo Lo Moro ◽  
Bernard Galland ◽  
Roberto Bianchini ◽  
Cinzia Chiappe ◽  
...  
Keyword(s):  
Ion Pair ◽  
Protic Solvents ◽  
Free Ion ◽  
Electrophilic Bromination

scholarly journals The Ion Pair Thermal Model of MALDI MS

2021 ◽  
Author(s):  
Shiyue Fang
Keyword(s):  
Gas Phase ◽  
Thermal Model ◽  
Ion Pair ◽  
Pair Formation ◽  
Maldi Ms ◽  
Ion Extraction ◽  
Free Ion ◽  
Pair Separation ◽  
Ion Yields ◽  
Ion Pair Separation

The ion pair thermal model for MALDI MS is described. Key elements of the model include thermal desorption and ionization, strong tendency to neutralization via ion pair formation and proton transfer in the gas phase, thermal equilibrium, overall charge neutral plume, and thermal energy assisted free ion generation via ion pair separation by ion extraction potential. The quantities of ions in the solid sample and in the gaseous plume are estimated. Ion yields of different classes of molecules including peptides, nucleic acids, permanent salts and neutral molecules are estimated at the macroscale and single ion pair levels. The estimated ion yields are close to experimentally observed values under certain assumptions. Explanations of several observations in MALDI MS such as mostly single-charged peaks, improvement of spectra by ammonium cation, and ion suppression are provided. We expect that the model can give insights for the design of new conditions and systems for improving the sensitivity and resolution of MALDI MS and improving its capability and reliability to analyze large biomolecules.

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

Tautomerism of 5-nitro-2-furylnitromethane

1981 ◽  
Vol 46 (12) ◽  
pp. 3122-3127
Author(s):  
Iva Sroková ◽  
Pavel Vetešník ◽  
Adolf Jurášek ◽  
Jaroslav Kováč
Keyword(s):  
Kinetic Study ◽  
Temperature Dependence ◽  
Thermodynamic Parameters ◽  
Rate Constants ◽  
Strong Acid ◽  
Ion Pair ◽  
Aprotic Solvents ◽  
Protic Solvents ◽  
Parallel Reactions ◽  
Experimental Values

UV study of properties of 5-nitro-2-furylnitromethane (I) in various solvents has shown that this compound is a very strong acid (pKa 3.98) forming an ion pair in protic solvents (water, alcohols) and existing exclusively as nitro-form in aprotic solvents (tetrachloromethane, chloroform, n-hexane, diethyl ether, dioxane). Kinetic study of tautomerism in water has given experimental values of combined rate constants of parallel reactions. Thermodynamic parameters of the tautomeric transformations have been calculated from temperature dependence of the rate constants.

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

1989 ◽  
Vol 67 (1) ◽  
pp. 21-26 ◽  
Author(s):  
Zhu-Gen Lai ◽  
Kenneth Charles Westaway
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Transition States ◽  
Substituent Effects ◽  
Ion Pairing ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Free Ion

The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.

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