Fragmentation, structure, and energetics of small sodium formate clusters : evidence for strong influence of entropic effects

The behavior of small cationic sodium formate clusters [NaOOCH]nNa+ was studied using a combination of experimental and theoretical approaches. Energy-dependent tandem mass spectra were measured using a triple quadrupole and an ion trap type instrument, while structure and energetics were obtained from density functional theory calculations. Analysis of the relative abundances and fragmentation patterns indicate the existence of certain unstable cluster sizes, like the tetramer [NaOOCH]4Na+. This observation is in contrast to alkali halides, where this species has enhanced stability. Surprisingly, this ‘anti-magic’ sodium formate cluster is shown to be compact, energetically low-lying by quantum chemical calculations. The apparent contradiction can be explained by taking into account entropy effects as calculated dissociation Gibbs free energy values show remarkably improved agreement with experimental trends. Fragmentation of these clusters is a good example of reactions that are predominantly directed by entropy and not by energy constraints