ChemInform Abstract: INTRINSIC ACIDITIES OF CARBON AND NITROGEN ACIDS MEASURED BY GAS PHASE PROTON TRANSFER EQUILIBRIUMS. SUBSTITUENT EFFECTS ON THE STABILITIES OF GAS PHASE CARBANIONS AND NITROGEN ANIONS

1976 ◽  
Vol 7 (35) ◽  
Author(s):  
T. B. MCMAHON ◽  
P. KEBARLE
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Substituent Effects ◽  
Carbon And Nitrogen

Stabilities in the gas phase of the hydrogen bonded complexes, YC6H4OH-X−, of substituted phenols, YC6E4OH, with the halide anions X− (Cl−, Br−, I−)

1990 ◽  
Vol 68 (11) ◽  
pp. 2070-2077 ◽  
Author(s):  
Gary J. C. Paul ◽  
Paul Kebarle
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Proton Transfer ◽  
Gas Phase ◽  
Equilibrium Constants ◽  
Substituent Effects ◽  
Closed Shell ◽  
Bond Energies ◽  
Substituted Phenols ◽  
Hydrogen Bonded

The equilibria, YPhOH + Br− = YPhOH-Br−, involving 26 differently substituted phenols, were determined with a pulsed high pressure mass spectrometer. The −ΔG0 evaluated from the equilibrium constants represent the hydrogen bond free energies in YPhOH-Br−. These data and data for X− = Cl− and I−, determined previously in this laboratory, are used to examine the substituent effects on the hydrogen bonding. It was found that the hydrogen bond energies in YPhOH-X− increase approximately linearly with the gas phase acidities of the phenols, YPhOH. This is in agreement with earlier observations that showed the bond energies in AH-B−, where AH were oxygen and nitrogen acids and B− closed shell anions, increase with increasing acidity of AH.A detailed analysis of the substituent effects, which is possible for YPhOH-X−, shows that the relationship with the acidity of AH can be divided into two parts. One is the increasing extent of actual proton transfer from AH on formation of the hydrogen bonded complex. Such proton transfer occurs in YPhOH-X− only for the series X− = Cl−. The second effect, which occurs for Cl− and is dominant for Br− and I−, is not directly related to the acidity of the phenols (or AH in general) but depends on a similarity of the substituent effects on the acidity and the stabilization of YPhOH-X− (or AH-B− in general). The dominant contribution to YPhOH-X− stabilization in this case is due to the field effects of the substituents, i.e., π delocalization plays only a small part. Therefore, the correlation with the acidity of YPhOH, where π delocalization is important, is not very close. Keywords: hydrogen bonding, substituent effects, ion–molecule equilibria, stability constants, thermochemistry.

Electrostatic effects on ionization equilibria: MNDO study of proton and hydrogen transfer reactions of 5-fluoropentanoic acid

1985 ◽  
Vol 63 (5) ◽  
pp. 1068-1072 ◽  
Author(s):  
Zdenek Friedl
Keyword(s):  
Proton Transfer ◽  
Gas Phase ◽  
Carboxylic Acids ◽  
Hydrogen Transfer ◽  
Opposite Sign ◽  
Substituent Effects ◽  
Transfer Reactions ◽  
Small Contribution ◽  
Bond Strengths ◽  
Ionization Equilibria

Relative proton transfer enthalpies δΔH0 and homolytic bond strengths δΔH0 (A—H) of sp and ap conformers of 5-fluoropentanoic acid, as well as some related aliphatic and 4-X-bicyclo[2.2.2]octane-1-carboxylic halogenoacids, have been calculated by the MNDO method. The results, together with literature data concerning the gas phase acidities, were compared with the prediction of the electrostatic theory. It is shown that the substituent effects on acidities of carboxylic acids are largely due to substituent interactions in the anions and only to a smaller extent to interactions in the neutral acids. A small contribution (about one-eighth to one-fourth as large in magnitude but of opposite sign) is possibly also made by substituent effects on homolytic bond strengths DH0 (A—H).

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