| Abstract
| - Isolated and microsolvated protonated ethanol clusters, (EtOH)qH+−Ln with L = Ar and N2, are characterizedby infrared photodissociation (IRPD) spectroscopy in the 3 μm range and quantum chemical calculations.For comparison, also the spectrum of the protonated methanol dimer, (MeOH)2H+, is presented. The IRPDspectra carry the signature of H-bonded (EtOH)qH+ chain structures, in which the excess proton is eitherstrongly localized on one or (nearly) equally shared between two EtOH molecules, corresponding to Eigen-type ion cores (EtOH2+ for q = 1, 3) or Zundel-type ion cores (EtOH−H+−HOEt for q = 2, 4), respectively.In contrast to neutral (EtOH)q clusters, no cyclic (EtOH)qH+ isomers are detected in the size range investigated(q ≤ 4), indicative of the substantial impact of the excess proton on the properties of the H-bonded ethanolnetwork. The acidity of the two terminal OH groups in the (EtOH)qH+ chains decreases with the length ofthe chain (q). Comparison between (ROH)qH+ with R = CH3 and C2H5 shows that the acidity of the terminalO−H groups increases with the length of the aliphatic rest (R). The most stable (EtOH)qH+−Ln clusters withn ≤ 2 feature intermolecular H-bonds between the inert ligands and the two available terminal OH groups ofthe (EtOH)qH+ chain. Asymmetric microsolvation of (EtOH)qH+ with q = 2 and 4 promotes a switch fromZundel-type to Eigen-type cores, demonstrating that the fundamental structural motif of the (EtOH)qH+ protonwire sensitively depends on the environment. The strength of the H-bonds between L and (EtOH)qH+ isshown to provide a rather sensitive probe of the acidity of the terminal OH groups.
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