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

Abstract

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±BetN, 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.

Monte Carlo-Simulated Annealing and Machine Learning-Based Funneled Approach for Finding the Global Minimum Structure of Molecular Clusters

“…The most stable geometries are shown in Figure . The symmetrical structure shown in Figure a, which was also described in the previous study by our group, is very similar to the most stable structure of the neutral tetramer, where the four molecules are arranged as a square plane with the dipoles arranged in a circular fashion. The ion is located above the center of the molecules and causes a distortion such that the amine groups of each molecule are slightly elevated and point toward the ion.…”

“…The molecule of choice in most of these studies is trimethyl glycene betaine, which is a pure zwitterion with a dipole moment of 11.9 D. 1 In a previous study by our group, we have shown that one can readily form clusters containing an ion of choice with a large number of neutral betaine molecules attached and that these clusters have a "magic number" with size N = 4 no matter the identity of the charge. 66 Thus, by measuring the photoabsorption band as a function of cluster size, it may be possible to measure the shift in absorption as a function of the applied electric field. For this method, the identity of the lowest energy isomer is of crucial importance, as the electric field applied to the chromophore will be different for different isomers.…”

“…In our previous study, we measured the MS/MS (tandem mass spectrometry) spectrum of the chlorinated betaine tetramer, shown in Figure 11a. 66 In this earlier study, we found competition between monomer emission and cluster fission, which was not seen in the MS/MS of protonated betaine clusters (which fragment solely by monomer evaporation). This was deduced from the fact that the threshold collision voltages for the appearance of all cluster fragments are the same.…”