Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton-Bound Dimers.

Infrared spectra for a series of asymmetric proton-bound dimers with protonated trimethylamine (TMA-H+ ) as the proton donor were recorded and analyzed. The frequency of the N-H+ stretching mode is expected to red shift as the proton affinity of proton acceptors increases. The observed band, however, shows a peculiar splitting of approximately 300 cm-1 with the intensity shifting pattern resembling a two-level system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm-1 is a result of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes, that is commonly known as Fermi resonance (FR). We also provide a general 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+ , the strong coupling we observed is an intrinsic property associated with proton motions.

Vibrational spectroscopy of protonated amine-water clusters: tuning Fermi resonance and lighting up dark states.

Strong coupling between stretching fundamentals and bending overtones of vibrational modes, known as Fermi resonance (FR), has been observed for proton motions in the protonated trimethylamine-water cluster. To investigate the role of FR, we examined the vibrational spectra of other three protonated ammonia/amine-water clusters, including the NH4+ ion and its mono- and di-methylated analogues, respectively, with and without argon tagging. In these systems, a simple frequency-scaled harmonic oscillator model will predict only one strong band between 2600 and 3200 cm-1 uniquely due to the hydrogen-bonded NH stretching fundamental for a given conformer. In the experimental vibrational spectra, however, multiple sharp bands were observed. Such a discrepancy often leads to the notions of the coexistence of multiple conformers and/or the appearance of an overtone state as a result of FR. In this work, we applied a discrete variable representation (DVR) implementation of ab initio anharmonic algorithms and demonstrated how one N-H+ stretching fundamental can lead to multiple bands as a result of intrinsic anharmonic couplings. A prominent effect of tuning these FR bands and lighting up dark overtone states in this wide frequency range was investigated by changing the number of methyl groups in the protonated amine moiety. The effect of Ar-tagging was also analyzed and decent agreement between the experimental and simulated spectra certified the above-mentioned simple pictures. We also found that the coupling constant for trimethylamine is the largest among these protonated amine-water clusters, and the overall coupling strength decreases as the hydrogen-bonded NH stretching frequency redshifts in the order of dimethylamine, methylamine, and ammonia.

An infrared spectroscopic and theoretical study on (CH3)3N-H(+)-(H2O)(n), n = 1-22: highly polarized hydrogen bond networks of hydrated clusters.

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.

Infrared spectroscopy of large-sized neutral and protonated ammonia clusters.

Size-selective infrared spectroscopy was applied to neutral and protonated ammonia clusters, (NH3)n (n = ∼5-∼80) and H(+)(NH3)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 (NH3)n is similar to that of the average size-controlled clusters doped in He droplets. The ν1 (NH sym stretch)/ν3 (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(+)(NH3)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.

Infrared spectroscopy of protonated trimethylamine-(benzene)(n) (n = 1-4) as model clusters of the quaternary ammonium-aromatic ring interaction.

The essence of the molecular recognition of the neurotransmitter acetylcholine has been attributed to the attractive interaction between a quaternary ammonium and aromatic rings. We employed protonated trimethylamine-(benzene)n clusters (n = 1-4) in the gas phase as a model to study the recognition mechanism of acetylcholine at the microscopic level. We applied size-selective infrared spectroscopy to the clusters and observed the NH and CH stretching vibrational regions. We also performed density functional theory calculations of stable structures, charge distributions, and infrared spectra of the clusters. It was shown that the methyl groups of protonated trimethylamine are solvated by benzene one at a time in the n > 1 clusters, and the validity of these clusters as a model system of the acetylcholine recognition was demonstrated. The nature of the interactions between a quaternary ammonium and aromatic rings is discussed on the basis of the observed infrared spectra and the theoretical calculations.

Structures of hydrogen bond networks formed by a few tens of methanol molecules in the gas phase: size-selective infrared spectroscopy of neutral and protonated methanol clusters.

In this work, we report infrared spectra of large neutral and protonated methanol clusters, (MeOH)n and H(+)(MeOH)n, in the CH and OH stretching vibrational region in the size range of n = 10-50. The infrared-ultraviolet double resonance scheme combined with mass spectrometry was employed to achieve moderate size selection of the neutral clusters with the addition of a phenol molecule as a chromophore. Infrared dissociation spectroscopy was performed on the protonated methanol clusters by using a tandem quadrupole mass spectrometer to enable the precise size selection of the clusters. While the neutral clusters showed essentially the same spectra in all the observed size range, the protonated clusters showed remarkable narrowing of the H-bonded OH stretch band with increasing n. In n≥~30, the spectra of the neutral and protonated clusters become almost identical. These spectral features demonstrate that hydrogen bond networks of methanol prefer simple cyclic structures (or "bicyclic" structures in protonated methanol) and branching of the hydrogen bond networks (side-chain formation) is almost negligible. Implications of the spectra of the clusters are also discussed by comparison with spectra of bulk phases.

Structures and dissociation channels of protonated mixed clusters around a small magic number: infrared spectroscopy of ((CH3)3N)n-H(+)-H2O (n = 1-3).

The magic number behavior of ((CH(3))(3)N)(n)-H(+)-H(2)O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1-3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH(3))(3)N molecules. The ion core (protonated site) of the clusters is found to be (CH(3))(3)NH(+) for n = 1-3, but coexistence of an isomer of the H(3)O(+) ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.