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.