Hydrogen-bonded networks in ethanol proton wires:: IR spectra of (EtOH)qH+-Ln clusters (L = Ar/N2, q≤4, n≤5)

Isolated and microsolvated protonated ethanol clusters, (EtOH)(q)H+-L-n with L = Ar and N-2, are characterized by infrared photodissociation (IRPD) spectroscopy in the 3 mu m range and quantum chemical calculations. For comparison, also the spectrum of the protonated methanol dimer, (MeOH)(2)H+, is presented. The IRPD spectra carry the signature of H-bonded (EtOH)(q)H+ chain structures, in which the excess proton is either strongly 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-HI-HOEt for q = 2, 4), respectively. In contrast to neutral (EtOH), clusters, no cyclic (EtOH)(q)H+ 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 ethanol network. The acidity of the two terminal OH groups in the (EtOH)(q)H+ chains decreases with the length of the chain (q). Comparison between (ROH)(q)H+ with R = CH3 and C2H5 shows that the acidity of the terminal O-H groups increases with the length of the aliphatic rest (R). The most stable (EtOH)(q)H+-L-n clusters with n <= 2 feature intermolecular H-bonds between the inert ligands and the two available terminal OH groups of the (EtOH)(q)H+ chain. Asymmetric microsolvation of (EtOH)(q)H+ with q = 2 and 4 promotes a switch from Zundel-type to Eigen-type cores, demonstrating that the fundamental structural motif of the (EtOH)(q)H+ proton wire sensitively depends on the environment. The strength of the H-bonds between L and (EtOH)(q)H+ is shown to provide a rather sensitive probe of the acidity of the terminal OH groups.