Stable Cluster Ions in Carboxylic Acid-Water Binary Systems and Their Reactivity with Ammonia Studied by Liquid Ionization Mass Spectrometry

Abstract

Intense peaks of cluster ions, (RCOOH)₅(H₂O)H⁺, have been observed for monocarboxylic acid-water binary systems and a stable ring-type structure has been suggested. The stability and the structure of such cluster ions in the acetic acid-water mixture were investigated by measuring reactions between binary cluster ions and ammonia by liquid ionization (LPI) mass spectrometry and MO calculation. The LPI mass spectra showed again the special stablity of (CH₃COOH)₅(H₃O)⁺ ion in the substitution reactions with ammonia. This substitution reaction seemed to require a certain threshold concentration of ammonia. The results of theoretical calculation with the MNDO-PM3 method revealed that the ring-type structure assumed was more stable than well known catamer-type structures. The results of calculation agreed well with the results obtained from LPI mass spectra. Differences between water and ammonia as a component of clusters are also described.