Asuka Fujii

Infrared Spectra and Hydrogen-Bonded Network Structures of Large Protonated Water Clusters H+(H2O)n (n=20–200)†

Because H-bonding causes unique properties of water, structures of H-bonded water networks are of fundamental interest. For molecular-level understanding of H-bonded network structures in water and aqueous solutions, water clusters (H2O)n and hydrated clusters M(H2O)n (M: solute) in the gas phase have been extensively studied. These clusters are microscopic models for bulk water and aqueous solutions, respectively, and they provide detailed structural information on H-bonded networks. It is expected that investigation of cluster structures for each cluster size n would lead to insight into the bulk picture. Because Hbonding environments are sensitively reflected in OH stretching frequencies of water, IR spectroscopy of sizeselected clusters is a powerful tool to elucidate size-dependent H-bonded water network structures. 5–12, 14, 15] In the case of neutral clusters such as (H2O)n, rigorous size selection is difficult and has been practically limited to n values up to ten. 12] These sizes are too small to construct bulky water networks, which contain interior and fourcoordinate (4-coord) water molecules. Because 4-coord water would be in the majority in bulk water, as in hexagonal ice, spectral identification of 4-coord water is a key to studying large-scale H-bonded water networks. Studies of larger clusters are thus required. For ionic clusters, on the other hand, application of mass spectrometric techniques removes the difficulty of size selection and opens the way to study larger clusters (n> 10), which were first studied in protonated water clusters H(H2O)n (n9 30). [5–7,9] They are models for the hydrated proton and provide structural information about water networks around an excess proton. Studies on H(H2O)n (n9 30) have shown that clusters larger than n 10 form closed net (multiple ring) structures, and they develop into closed cage structures with a dramatic decrease in 2-coord water molecules at n = 21. 9] This network development of H(H2O)n has been discussed by analyzing free (non-H-bonded) OH stretching bands corresponding to the 1–3-coord water molecules. In larger clusters, 4-coord water is expected to be dominant; however, no direct observations have been reported. Because 4-coord water molecules do not have a free OH group, analyses of the H-bonded OH stretching region are required to reveal their contribution for more bulky networks. Previously, we succeeded in extending the application of size-selected IR spectroscopy to n = 100, but only free OH bands were covered. For H(H2O)n, IR spectra in the H-bonded OH region have been reported only for clusters with n 27 so far. In the case of other hydrated clusters, Williams et al. have reported IR spectra of Ca(H2O)n 69 and SO4 2 (H2O)n 80. [14,19] These are the largest cationic and anionic clusters investigated by size-selected IR spectroscopy in the whole OH stretching region. In these studies, the center of the H-bonded OH stretching band was found to approach that of bulk water with increasing cluster size; however, the band width of the H-bonded OH stretching band accounts only for part of the bulk spectrum, possibly because the doubly positively charged Ca and/or negatively charged SO4 2 ions strongly affect water networks. It is expected that, in much larger clusters with a singly charged ion, the ion effect would be further diluted and the cluster structure would be much closer to those of neat water networks. To identify the 4-coord water, and to discuss less perturbed H-bonded network structures of hundreds of water molecules, we report here IR spectra of precisely size selected, large H(H2O)n in the OH stretching region (2200– 4000 cm ) up to a size of n = 200, which is expected to be large enough to form a bulklike H-bonded network in which 4-coord water is dominant. For example, the lowest-energy structures of (H2O)n (n9 1000) on the empirical potentialenergy surface have been reported, and these studies suggested that crystal cores are formed in the size region of a few hundred water molecules or more. We show clear spectroscopic signatures for the abundance of the interior (4coord) water. Furthermore, the fact that IR spectral patterns approach those of supercooled water and ice with increasing cluster size suggests formation of more ordered H-bonded network structures. Figure 1a shows IR photodissociation spectra of H(H2O)n (n = 20–200). Depths of cluster-ion depletion, caused by the vibrational predissociation, are plotted as a function of the IR wavenumber. The spectra of H(H2O)20,21 are similar to those reported previously. Bands around 3700 and [*] K. Mizuse, Prof. N. Mikami, Prof. A. Fujii Department of Chemistry, Graduate School of Science Tohoku University, Sendai 980-8578 (Japan) Fax: (+ 81)22-795-6785 E-mail: asukafujii@m.tohoku.ac.jp Homepage: http://www.mikamilab.chem.tohoku.ac.jp

Tuning of the internal energy and isomer distribution in small protonated water clusters H(+)(H2O)(4-8): an application of the inert gas messenger technique.

Infrared spectroscopy of gas-phase hydrated clusters provides us much information on structures and dynamics of water networks. However, interpretation of spectra is often difficult because of high internal energy (vibrational temperature) of clusters and coexistence of many isomers. Here we report an approach to vary these factors by using the inert gas (so-called messenger)-mediated cooling technique. Protonated water clusters with a messenger (M), H(+)(H(2)O)(4-8)·M (M = Ne, Ar, (H(2))(2)), are formed in a molecular beam and probed with infrared photodissociation spectroscopy in the OH stretch region. Observed spectra are compared with each other and with bare H(+)(H(2)O)(n). They show clear messenger dependence in their bandwidths and relative band intensities, reflecting different internal energy and isomer distribution, respectively. It is shown that the internal energy follows the order H(+)(H(2)O)(n) >> H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·Ne, while the isomer-selectivity, which changes the isomer distribution in the bare system, follows the order H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ne ~ (H(+)(H(2)O)(n)). Although the origin of the isomer-selectivity is unclear, comparison among spectra measured with different messengers is very powerful in spectral analyses and makes it possible to easily assign spectral features of each isomer.

CH/π interactions in methane clusters with polycyclic aromatic hydrocarbons

Geometries and interaction energies for methane clusters with naphthalene and pyrene were studied. Estimated CCSD(T) interaction energies for the clusters at the basis set limit were -1.92 and -2.50 kcal mol(-1), respectively. Dispersion is mainly responsible for the attraction. Electrostatic interaction is very small. Although the benzene-methane cluster prefers a monodentate structure, in which a C-H bond of the methane points toward the benzene, the methane clusters with the polycyclic aromatic hydrocarbons do not prefer monodentate structures. In the benzene-methane cluster, the weak electrostatic interaction stabilizes the monodentate structure. On the other hand the dispersion interaction controls the orientation of methane in the naphthalene and pyrene clusters. The dispersion interactions in these clusters are significantly larger than those in the benzene-methane cluster. The methane prefers the orientation which is suitable for stabilization by dispersion. Hydrogen atoms of the methane locate above the centers of hexagonal rings of the polycyclic aromatic hydrocarbons in the stable structures. The structures have a small steric repulsion and this positions them only a short distance from the aromatic plane. The large dispersion contribution to the attraction shows that interactions between carbon atoms are mainly responsible for the attraction, and that hydrogen atoms are not important for the attraction. This shows that the interactions between the methane and polycyclic aromatic hydrocarbons are not pi-hydrogen bonds.

Comprehensive characterization of the photodissociation pathways of protonated tryptophan

The photofragmentation of protonated tryptophan has been investigated in a unique experimental setup, in which ion and neutral issued from the photofragmentation are detected in coincidence, in time and in position. From these data are extracted the kinetic energy, the number of neutral fragments associated with an ion, their masses, and the order of the fragmentation steps. Moreover, the fragmentation time scale ranging from tens of nanoseconds to milliseconds is obtained. From all these data, a comprehensive fragmentation mechanism is proposed.

Long range influence of an excess proton on the architecture of the hydrogen bond network in large-sized water clusters

Infrared spectra of completely size-selected protonated water clusters H+(H2O)n are reported for clusters ranging from n=15 to 100. The behavior of the dangling OH stretch bands shows that the hydrogen bond structure in H+(H2O)n is uniquely different to that of (H2O)n up to the size of n=100, at least. This finding indicates that the presence of an excess proton creates a characteristic morphology in the hydrogen bond network architecture of more than 100 surrounding water molecules.

Characterization of a solvent-separated ion-radical pair in cationized water networks: infrared photodissociation and Ar-attachment experiments for water cluster radical cations (H2O)n+(n = 3-8).

We present infrared spectra of nominal water cluster radical cations (H(2)O)(n)(+) (n = 3-8), or to be precise, ion-radical complexes H(+)(H(2)O)(n-1)(OH), with and without an Ar tag. These clusters are closely related to the ionizing radiation-induced processes in water and are a good model to characterize solvation structures of the ion-radical pair. The spectra of Ar-tagged species show narrower bandwidths relative to those of the bare clusters due to the reduced internal energy via an Ar-attachment. The observed spectra are analyzed by comparing with those of the similar system, H(+)(H(2)O)(n), and calculated ones. We find that the observed spectra are attributable to ion-radical-separated motifs in n ≥ 5, as reported in the previous study (Mizuse et al. Chem. Sci.2011, 2, 868-876). Beyond the structural trends found in the previous study, we characterize isomeric structures and determine the number of water molecules between the charged site and the OH radical in each cluster size. In all of the characterized cluster structures (n = 5-8), the most favorable position of OH radical is the next neighbor of the charged site (H(3)O(+) or H(5)O(2)(+)). The positions and cluster structures are governed by the balance among the hydrogen-bonding abilities of the charged site, H(2)O, and OH radical. These findings on the ionized water networks lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions.

The Intermolecular SH⋅⋅⋅Y (Y=S,O) Hydrogen Bond in the H2S Dimer and the H2S–MeOH Complex

The nature of the S−H⋅⋅⋅S hydrogen-bonding interaction in the H2 S dimer and its structure has been the focus of several theoretical studies. This is partly due to its structural similarity and close relationship with the well-studied water dimer and partly because it represents the simplest prototypical example of hydrogen bonding involving a sulfur atom. Although there is some IR data on the H2 S dimer and higher homomers from cold matrix experiments, there are no IR spectroscopic reports on S−H⋅⋅⋅S hydrogen bonding in the gas phase to-date. We present experimental evidence using VUV ionization-detected IR-predissociation spectroscopy (VUV-ID-IRPDS) for this weak hydrogen-bonding interaction in the H2 S dimer. The proton-donating S−H bond is found to be red-shifted by 31 cm(-1) . We were also able to observe and assign the symmetric (ν1 ) stretch of the acceptor and an unresolved feature owing to the free S−H of the donor and the antisymmetric (ν3 ) SH stretch of the acceptor. In addition we show that the heteromolecular H2 S-MeOH complex, for which both S−H⋅⋅⋅O and O−H⋅⋅⋅S interactions are possible, is S-H⋅⋅⋅O bound.

Folding of the Hydrogen Bond Network of H+(CH3OH)7 with Rare Gas Tagging

A number of isomer structures can be formed in hydrogen-bonded clusters, reflecting the essential variety of structural motifs of hydrogen bond networks. Control of isomer distribution of a cluster is important not only in practical use for isomer-specific spectroscopy but also in understanding of isomerization processes of hydrogen bond networks. Protonated methanol clusters have relatively simple networks and they are model systems suitable to investigate isomer distribution changes. In this paper, isomer distribution of H(+)(CH(3)OH)(7) is studied by size-selective infrared spectroscopy in the OH and CH stretching vibrational region and density functional theory calculations. While the clusters produced by a supersonic jet expansion combined with electron ionization were predominantly isomers having open hydrogen bond networks such as a linear chain, the Ar or Ne attachment (so-called rare gas tagging) entirely switches the isomer structures to compactly folded ones, which are composed only of closed multiple rings. The origin of the isomer switching is discussed in terms of thermal effects and specific isomer preference.

Infrared predissociation spectroscopy of cluster cations of protic molecules, (NH3)n+, n=2-4 and (CH3OH)n+, n=2,3.

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH(4) (+)...NH(2) or CH(3)OH(2) (+)...OCH(3), is formed. Theoretical calculations at the MP2 and B3LYP levels support the formation of the proton-transferred type structures for the cluster cations, and indicate that they are formed by proton-transfer following the photoionization of the neutral clusters.

Long-range migration of a water molecule to catalyze a tautomerization in photoionization of the hydrated formamide cluster.

The dynamics on the vacuum-ultraviolet one-photon ionization of a formamide-water cluster is investigated by a combination of theoretical reaction-path search and infrared spectroscopic methods. A keto-enol tautomerization of the formamide moiety occurs after photoionization by a catalytic action of the water molecule accompanied with its long-distance migration; the water molecule in the cluster migrates almost one turn around the formamide moiety. During the migration, the water molecule abstracts the proton of CH in the formamide moiety and carries it to the O atom side in the carbonyl group through a catch and release-type catalytic action.