Infrared spectroscopy of large-sized neutral and protonated ammonia clustersElectronic supplementary information (ESI) available: Evaluation of the maximum number of evaporated ammonia molecules upon the ionization of large-sized (NH3)n. See DOI: 10.1039/c4cp00178h

Size-selective infrared spectroscopy was applied to neutral and protonated ammonia clusters, (NH 3 ) n ( n = ∼5-∼80) and H + (NH 3 ) n ( n = 8-100), to observe their NH stretching vibrations. The moderate size selection was achieved for the neutral clusters by the infrared-ultraviolet double resonance scheme combined with mass spectrometry. The size dependence of the observed spectra of (NH 3 ) n is similar to that of the average size-controlled clusters doped in He droplets. The ν 1 (NH sym stretch)/ ν 3 (NH asym stretch) band intensity ratio shows a rapid decrease in the size range n ≤ ∼20. This demonstrates that ammonia begins to form crystalline like hydrogen bond networks at the much smaller size region than water. The precise size selection was achieved for H + (NH 3 ) n by infrared photodissociation spectroscopy combined with a tandem type quadrupole mass spectrometer. The spectra of the protonated clusters become almost identical with those of the corresponding neutral clusters at n ≥ ∼40, demonstrating that the radial chain structures, which are characteristic of the small-sized protonated clusters, develop into the crystalline like structures seen in the neutral clusters up to n = ∼40. Size selective IR spectroscopy shows the nature of hydrogen bond networks in neutral and protonated ammonia clusters.