Cluster Ions in Monocarboxylic Acid-Water Binary Systems Studied by Liquid Ionization Mass Spectrometry

Abstract

Studies of clusters are important to understand the properties of clusters and chemical reactions in a condensed phase. Liquid ionization (LI) mass spectrometry provides informations about cluster ions produced at the surface of a liquid under atmospheric pressure and was applied to study of the cluster formation in carboxylic acid-water binary systems. Although the LI mass spectra were dependent to some extent on the experimental conditions, such as the acid concentration and the temperature of a sample, cluster ions represented as (R=CH, to CH3(CH02)3) acid-water binary systems. Some differences were observed in th e H+(RCOOH)77, (H20)(mostly m=1, ----8 and n=0-4) were observed commonly for mon ocarboxylic cluster ion distributions between m<4 and m>5. Besides, it is interesting to note that the anomaly abundant H+(RCOOF1)51-120 ions (referred to as 5-1 ion) were observed with very few 5-2 ions. These results strongly suggest that the 5-1 ions are stable and may have a ring structure composed of five acid molecules with one water molecule located inside the ring. Calculations were performed by the AM 1 method inplemented in MOPAC program packages. The optimized structure with the lowest heat of formation as far as examined, has the ring structure composed of five carboxyl groups connected with intermolecular hydrogen bonds. Alkyl groups are directed outward the ring and one oxonium ion is attached to slightly above the center of the ring. Hydrogen bonds play very important role to form clusters. Agreements between the observations and the calculations indicate that LI mass spectrometry is useful for studies of cluster ions and a liquid phase.