Chemistry in nanodroplets: Studies of protonation sites of substituted anilines in water clusters using FT-ICR

Water clusters of protonated substituted anilines generated by an electrospray ion source have been investigated using a Fourier Transform ion cyclotron resonance mass spectrometer. It is observed that evaporation kinetics and cluster distributions are highly dependent on sites of protonation in the substituted anilines. Based on the examination of the water cluster distributions of protonated aniline derivatives, the site of protonation is postulated to be the amine group for aniline, p-anisidine, p-thiomethylaniline, p-ethylaniline, and m-ethylaniline. The water cluster distributions of these compounds display magic number clusters ([M + nH₂O]⁺) for n = 20, 27, 50, and 52. However, there is no indication of clusters with special stability for m-anisidine and m-thiomethylaniline, suggesting that these compounds protonate on the ring. DFT calculations have been performed to obtain proton affinities for the different sites of protonation in the substituted anilines and are in good agreement with experimental observation.