Photoionization, Structures, and Energetics of Na-Doped Formic Acid-Water Clusters

The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na-doped clusters formed in the supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na-FA(H₂ O)_n clusters show an enlarging of mass distribution toward heavier clusters with respect to the Na-(H₂ O)_n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. Density functional theory and coupled-cluster type (DLPNO-CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na-FA(H₂ O)_n as well as neutral FA(H₂ O)_n . Na-doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg-type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculations on neutral FA(H₂ O)_n clusters show the possibility of similarly stable structures in small clusters containing up to n=4-5 water molecules, where FA interacts significantly with waters. This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.