Remarkable amine-TFA self assembly

Supramolecular assemblies that are formed between amines and trifluoroacetic acid were studied using electrospray ionization mass spectrometry. Distinctive association behavior of primary, secondary, and tertiary amines with trifluoroacetic acid upon identical experimental conditions is observed and indicates that steric effects dominate in the formation of these protonated clusters. Extraordinary complexation behavior is observed in the case of R-(+)-alpha-methylbenzylamine and 4-tert-butyl-cyclohexylamine that form high-order clusters. The strong relation between stereochemistry and assembly results in the specific association characteristics of trans 4-tert-butyl-1-phenylcyclohexylamine when compared with the cis isomer. The cis isomer gives rise to a highly abundant M(4)TFA(3) + H](+) ion (M = amine molecule, TFA = trifluoroacetic acid), as observed for other primary amines. However, the trans isomer generates higher M(n)TFA(m) + H](+) cluster ions, the largest and most abundant being an M(7)TFA(6) + H](+) ion. Collision induced dissociation spectra that were recorded for several M(n)TFA(m) + H](+) cluster ions typically show the consecutive losses of M.TFA moieties. Density functional theory calculations indicate that the highly abundant M(4)TFA(3) + H](+) clusters are macrocycles and support the formation of these structures with TFA and not with acetic acid.