Ground State Proton Transfer in Phenol–(NH3) n (n ≤ 11) Clusters Studied by Mid-IR Spectroscopy in 3–10 μm Range

The infrared (IR) spectra of size-selected phenol–ammonia clusters, PhOH–(NH3) n (n ≤ 11) in the 3–10 μm wavelength region were measured to investigate the critical number of solvent molecules necessary to promote the ground state proton transfer (GSPT) reaction. While the N–H stretching vibrations did not provide clear information, characteristic changes that are assigned to the GSPT reaction were observed in the skeletal vibrational region. The production of phenolate anion (PhO–), which is a product of the GSPT reaction, was established from the appearance of characteristic bands assignable to C–C stretching and C–H bending vibrations of PhO– and from the corresponding disappearance of C–O–H bending vibration of PhOH at n = 9. The mid-IR spectroscopy directly proves the structural change induced by the deprotonation from the O–H bond and thus establishes the GSPT reaction as complete at n = 9. No such absorptions were observed for n ≤ 5 in line with a previous report. For n = 6–8, both the proton transferred and the nontransferred signatures were observed in the spectra, showing coexistence of both species for the first time.