Theoretical investigation of the structures and spectroscopic properties of (H2O4)n (n = 1-4) clusters

Density functional theory and ab initio calculations were performed to elucidate the hydrogen interactions in (H2O4)n (n = 1–4) clusters. The optimized geometries, binding energies, and harmonic vibrational frequencies were predicted at various levels of theory. The trans conformer of the H2O4 monomer was predicted to be the most stable structure at the CCSD(T)/aug‐cc‐pVTZ level of theory. The binding energies per H2O4 monomer increased in absolute value by 9.0, 10.1, and 11.8 kcal/mol from n = 2 to n = 4 at the MP2/cc‐pVTZ level of theory (after the zero‐point vibrational energy and basis set superposition error corrections). This result implies that the intermolecular hydrogen bonds were stronger in the long‐chain clusters, that is, the formation of the longer chain in the (H2O4)n clusters was more energetically favorable. H2O4 (as the HOO radical dimer) has been proposed as an important intermediate in the formation of H2O2 and molecular oxygen from the HOO radical. The concentration of (H2O4)n clusters under typical atmospheric conditions should be very low. Theoretical results suggest that the formation of the longer chain in the (H2O4)n clusters is more energetically favorable. This result implies that the thermodynamic stability increased for larger values of n in the H2On dimers.