Clusters of betaine with positive and negative ions: Evidence for the betaine tetramer being magic

Betaine (Bet) is a pure zwitterion with an extraordinarily large dipole moment, which allows it to form stable clusters in the gas phase of the form X^±Bet_N, where X^± is a positive or negative ion. We show here that such clusters have a prominent magic number at N = 4 for all X^± ions used in this work. Nevertheless, we observe a marked difference in the fragmentation pattern of anionic and cationic clusters: While cationic clusters fragment by evaporating one betaine monomer at a time, fragmentation of anionic clusters is through fission resulting in the emission of one or several betaine molecules. Theoretical calculations show that charged betaine tetramers have a square like structure with the central ion lying above the cluster plane and explain the difference in fragmentation patterns as a result of the charge distribution within the betaine molecule.