Naohiko Mikami

Vibrational spectroscopy of small-sized hydrogen-bonded clusters and their ions

Vibrational spectroscopies of small-sized hydrogen-bonded clusters of organic acids and related molecules, as well as their ions, are reviewed based on our recent results. OH stretching vibrations of the jet-cooled clusters generated by supersonic expansions are observed by the various size-selected and population-labelling spectroscopic methods; ionization detected infrared (IR) and or stimulated Raman spectroscopies for the neutral clusters in the electronical ground state (S ) and fluorescence detected IR spectroscopy for the clusters in the electronically excited state (S ). The hydrogen-bond structures of phenol-(H O) clusters are n extensively investigated on the basis of the spectral analysis combined with ab initio calculations of their stable forms and vibrations. Remarkable enhancement of the hydrogen-bond strength upon electronic excitation is demonstrated for the IR spectra of the S clusters of phenol. For tropolone-(H O) and- (CH OH) n n clusters, (phenol), and fluorobenzene-(CH OH) clusters,...

OH stretching vibrations of phenol—(H2O)n (n=1–3) complexes observed by IR-UV double-resonance spectroscopy

Abstract OH stretching vibrations of jet-cooled phenol and phenol—(H 2 O) n ( n =1–3) complexes were investigated by IR—UV double-resonance spectroscopy. The stretching vibrations of the H 2 O moieties as well as that of the phenol moiety were observed for the complexes. A remarkable red-shift with increase of n was found for the phenolic OH vibration. The symmetric (ν 1 ) vibrations of the H 2 O moieties exhibit large red-shifts and splittings, while the antisymmetric (ν 3 ) vibrations show small red-shifts with small splittings. The spectral feature of the H 2 O moieties in the complexes was found to be quite similar to that of the pure (H 2 O) n . The result suggests the complexes have structures such that the hydrogen bond is formed between phenol and the (H 2 O) n clusters.

Size-selected vibrational spectra of phenol-(H2O)n (n=1–4) clusters observed by IR–UV double resonance and stimulated Raman-UV double resonance spectroscopies

OH and CH stretching vibrations of bare phenol, phenol‐(H2O)n clusters (n=1–4), and partially deuterated clusters in the S0 state were observed by using IR–UV double resonance and stimulated Raman‐UV double resonance spectroscopies. Characteristic spectral features of the OH stretching vibrations of the phenol as well as of the H2O sites were observed, which are directly related to their structures. The cluster structures were investigated by comparing the observed spectra with the calculated ones obtained by the ab initio molecular orbital calculation with (self‐consistent field) SCF 6‐31G and SCF 6‐31G* basis sets given by Watanabe and Iwata. It was found that for the clusters with n≥2, the isomer of ring form hydrogen‐bonded structure is most stable and the simulated IR spectra based on the calculated structure showed good agreements with the observed ones. For a particular cluster, which was assigned as an isomer of the n=4 cluster, an anomalous IR spectrum was observed. Two forms of the isomer are pr...

Vibrational spectroscopy of 2-pyridone and its clusters in supersonic jets: Structures of the clusters as revealed by characteristic shifts of the NH and C=O bands

Vibrational spectroscopy of the key functional vibrations of 2-pyridone and its hydrogen-bonded clusters with water, methanol, dioxane, dimethylether, as well as its dimer, has been carried out by using infrared-ultraviolet (IR–UV) and stimulated Raman–UV double resonance methods combined with fluorescence detection. The characteristic spectral changes upon the cluster formation have been observed for the NH and C=O stretching vibrations of the bare molecule and also for the OH stretching vibrations of the solvent molecules. The cluster structures were investigated by comparing the observed spectra with the simulated ones of the energy-optimized structures obtained by ab-initio molecular orbital calculations. It was found that the “ring-type” hydrogen-bonded structure is appropriate for the cluster with water or methanol, while the “linear-type” hydrogen-bonded structure is appropriate for the cluster with dioxane or dimethylether. The symmetric form of 2-pyridone dimer was confirmed by the observed spect...

The fluorescence excitation spectrum of aniline in a supersonic free jet: Double minimum potential for the inversion vibration in the excited state

Abstract The fluorescence excitation spectrum of the first singlet transition of aniline in a supersonic free jet has been measured. Vibronic transitions involving the inversion vibration of the NH 2 group in the excited state have been observed. The double minimum potential function in the excited state has been determined from the observed vibrational levels.

Resonance Raman effect of I−3 ion by ultraviolet laser excitation

Abstract The resonance Raman effect of the I −1 3 ion was observed by the use of an ultraviolet nitrogen pulsed laser. It was found that the totally symmetric stretching vibration of 112 cm −1 undergoes selective enhancement and the overtones up to the seventh order of this vibration could be observed. The result agrees well with the prediction by Nafie et al.

Electronic spectra and ionization potentials of rotational isomers of several disubstituted benzenes

Abstract The electronic spectra of several disubstituted benzenes ( o -fluorophenol, m -fluoroanisole, hydroquinone, and p -dimethoxybenzene) were investigated in supersonic free jets. For all the molecules studied, the existence of rotational isomers was established, and the ionization potentials of the individual rotational isomers were determined by two-color multiphoton ionization spectroscopy.

Electronic spectra of uracil in a supersonic jet

Abstract The fluorescence excitation and dispersed fluorescence spectra of uracil in a supersonic jet have been observed. The n,π∗ states of 2,4-diketo tautomer and enol-keto tautomer are found. The spectral analyses show an out-of-plane deformation of the molecules in the n,π∗ state. The coexistence of diketo and enol-keto tautomer in the vapor is discussed.

Two-photon excitation spectra of naphthalene and naphthalene-d8

Abstract The two-photon excitation spectra of naphthalene and naphthalene- d 8 in solution and as crystals were measured. The results obtained revealed the vibronic coupling of the lowest excited singlet state 1 B 2u with the higher “g” states through the b 2u vibrations. The 1 A g → 1 A g and 1 A g → 1 B 3g absorptions, and step-wise two-photon absorption were also found.

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