Proton affinities and ion enthalpies.

Area of Science:

• Physical Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Proton affinities are crucial for understanding chemical reactivity and molecular interactions.
• Isomeric organic esters present a unique case for studying substituent effects on protonation sites.
• Previous studies suggested localized charge distribution upon protonation in esters.

Purpose of the Study:

• To investigate and compare the proton affinities of isomeric alkyl acetates and methyl alkanoates.
• To understand the influence of alkyl substitution on the protonation energetics of esters.
• To elucidate the charge distribution and relaxation phenomena in protonated and ionized ester molecules.

Main Methods:

• Compilation of existing literature data on proton affinities.
• Experimental measurement of proton affinities using the kinetic method.
• Analysis of ionization energies from core-electron and lone pair ionization experiments.

Main Results:

• Isomeric alkyl acetates and methyl alkanoates exhibit nearly identical proton affinities.
• Protonation of esters leads to extensive charge relaxation or polarization, primarily at the keto carbon.
• Enthalpies of protonated esters correlate more closely with neutral molecule enthalpies than radical cations.

Conclusions:

• The unexpected similarity in proton affinities of isomeric esters is explained by significant charge redistribution post-protonation.
• Charge relaxation processes in protonated esters are more extensive than in radical cations formed by electron detachment.
• The observed phenomenon in organic esters may represent a broader principle applicable to other molecular systems.