The gas-phase hydrogen bond complexes between formic acid with hydroxyl radical: a theoretical study

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• Radical hydrogen bonding plays a crucial role in chemical reactions and molecular interactions.
• Understanding the complexes formed between formic acid (HCOOH) and hydroxyl radicals (OH) is essential for atmospheric chemistry and reaction mechanisms.

Purpose of the Study:

• To theoretically investigate radical hydrogen bond complexes between syn- and anti-formic acid (HCOOH) and hydroxyl radicals (OH).
• To analyze the bonding features, stability, and cooperative effects within these complexes.
• To discuss the implications of these complexes in the HCOOH plus OH reaction and report vibrational frequency shifts.

Main Methods:

• Utilizing advanced theoretical methods: B3LYP, MP2, QCISD, and CCSD(T).
• Employing the 6-311 + G(2df,2p) basis set for high accuracy.
• Analyzing bonding characteristics with Atoms in Molecules (AIM) theory and Natural Bond Orbital (NBO) partitioning.

Main Results:

• Identified twelve complexes with single hydrogen bonds and three complexes with cyclic, multi-bond structures exhibiting cooperative effects.
• Observed a stabilizing O...O interaction in one complex.
• Calculated complex stabilities ranging from -0.81 to -5.96 kcal mol⁻¹.
• Reported harmonic vibrational frequencies for O-H stretching and HOC wagging modes, including frequency red-shifts and intensity ratios.

Conclusions:

• The study elucidates the nature and stability of radical hydrogen bond complexes between HCOOH and OH.
• Cooperative effects and stabilizing interactions are significant in these systems.
• The findings provide insights into the HCOOH + OH reaction mechanism and molecular interactions.