Does the most stable formic acid tetramer have pi stacking or C-H...O interactions?

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Physical Chemistry

Background:

• Understanding molecular clusters is crucial for chemical physics.
• Formic acid tetramer ((HCOOH)4) exhibits complex hydrogen bonding and structural arrangements.
• Accurate prediction of low-energy isomers is essential for interpreting experimental data.

Purpose of the Study:

• To investigate the low-energy minima of the formic acid tetramer potential energy surface.
• To identify the global minimum structure of (HCOOH)4.
• To characterize the relative stability of different proposed structures.

Main Methods:

• Utilizing advanced computational chemistry techniques.
• Employing Density Functional Theory (DFT) for electronic structure calculations.
• Applying Moller-Plesset (MP) perturbation theory and coupled-cluster (CC) methods for high-accuracy energy evaluations.

Main Results:

• The potential energy surface of (HCOOH)4 is relatively flat, allowing for multiple stable configurations.
• An aligned pi-pi stack of two dimers is predicted as the most probable global minimum.
• Other pi-pi stacks, a planar C-H...O bonded structure, and a bowl-shaped isomer are found within 1 kcal/mol of the global minimum.

Conclusions:

• The formic acid tetramer exists in several low-energy configurations.
• Accurate theoretical calculations are vital for determining the preferred structure of molecular clusters.
• The interplay of pi-pi stacking and hydrogen bonding dictates the stability of (HCOOH)4 isomers.