Infrared spectra for as eries of asymmetric protonbound dimers with protonated trimethylamine (TMA-H +)a s the proton donor were recorded and analyzed.T he frequency of the N-H + stretching mode is expected to red shift as the proton affinity of proton acceptors increases.T he observed band, however,s hows ap eculiar splitting of approximately 300 cm À1 with the intensity shifting pattern resembling at wolevel system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm À1 is aresult of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes,t hat is commonly knowna sF ermi resonance (FR). We also provide ageneral theoretical model to link the strong FR coupling to the quasi-two-level system. Since the model does not depend on the molecular specification of TMA-H + ,t he strong coupling we observed is an intrinsic property associated with proton motions.

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Infrared spectra of protonated trimethylamine (TMA)-water clusters, (CH3)3N-H(+)-(H2O)n (n = 1-22) were measured in the OH stretching vibrational region by size-selective photodissociation spectroscopy. Density functional theory calculations of stable structures were performed, and temperature dependence of the isomer populations and infrared spectra was also simulated by the harmonic superposition approximation approach to analyze hydrogen bond network structures in the clusters. It was shown that the excess proton (H(+)) in this system localizes on the TMA moiety regardless of cluster size. In the small-sized clusters, many isomers coexist and their hydrogen bond networks are highly polarized to induce the large charge-dipole interaction to stabilize the excess proton. Magic number behavior is not observed at around the magic number size (n = 21) of protonated water clusters and its implication on the hydrogen bond network structures is discussed.