Infrared spectroscopy of protonated trimethylamine-(benzene)(n) (n = 1-4) as model clusters of the quaternary ammonium-aromatic ring interaction.

The essence of the molecular recognition of the neurotransmitter acetylcholine has been attributed to the attractive interaction between a quaternary ammonium and aromatic rings. We employed protonated trimethylamine-(benzene)n clusters (n = 1-4) in the gas phase as a model to study the recognition mechanism of acetylcholine at the microscopic level. We applied size-selective infrared spectroscopy to the clusters and observed the NH and CH stretching vibrational regions. We also performed density functional theory calculations of stable structures, charge distributions, and infrared spectra of the clusters. It was shown that the methyl groups of protonated trimethylamine are solvated by benzene one at a time in the n > 1 clusters, and the validity of these clusters as a model system of the acetylcholine recognition was demonstrated. The nature of the interactions between a quaternary ammonium and aromatic rings is discussed on the basis of the observed infrared spectra and the theoretical calculations.