Magic Numbers in Formic Acid Water Clusters Revealed by VUV Photoionization Mass Spectrometry and Density Functional Calculations

The interaction between formic acid (FA) and water (W) holds significant importance in various chemical processes. Our study combines vacuum-ultraviolet photoionization mass spectrometry with density functional calculations to investigate formic acid water clusters generated in supersonic molecular beams. The mass spectra obtained reveal the formation of protonated clusters as the major product. Magic num- bers observed in the mass spectra include FA1W5H+, FA2W4H+, FA3W3H+, FA4W2H+, FA5W1H+ and FA6W2H+ clusters. Interestingly, adding water to these magic num- ber clusters leads to reduced stability. Our calculations shed light on the potentially stable structures, highlighting cyclic arrangements with molecules enclosed within the ring as the most stable structures, and demonstrate a decrease in the stability upon the addition of a water molecule. Comparing experimental appearance energies with calculated ionization energies suggests that the observed clusters are likely a result of fragmentation of larger clusters.