Acidities under the Perspective of Energy Decomposition Analysis

The
rationalization of acid/base behavior is a central concern
for chemistry and related fields. In this work, we describe an alternative
approach toward the understanding of gas phase acidities based on
the localized molecular orbital energy decomposition analysis (LMOEDA)
method. Upon partitioning the molecules (and the corresponding anions)
over the X–OH (or X–O–) bond, we have
observed a perfect correlation between the interaction energy of the
two fragments and the acidity, as given by the energy difference between
the anion and the neutral molecule. On the basis of this correlation,
acidities could be interpreted according to the energy components
provided by LMOEDA, namely, electrostatic, exchange repulsion, polarization,
and dispersion. For example, alkyl groups increase the gas phase acidities
of alcohols mainly due to electrostatic and polarization interactions.
Carboxylic acids are stronger acids than alcohols through the ability
of oxygen to stabilize the extra charge formed in the anion (electrostatic
interactions) and also through a decrease of exchange repulsions between
the two fragments. Polarization interaction (orbital relaxation) also
plays an important role. Electrostatic and polarization interactions
dominate the enhanced acidity of sulfuric acid over ethanol. Electrostatic
and polarization interactions are also responsible for the higher
acidity of sulfuric over boric acid. The anomalous behavior of formic
acid compared to acetic, propionic, and butyric acids is also explained.
The examples worked in this report evince the still unexplored potential
of energy decomposition to the comprehension of acid/base phenomena.