To investigate the temperature effect on microscopic hydration structures in clusters, we have recorded ultraviolet photodissociation spectra of hydrated phenol cation, [PhOH(H₂O)₅]⁺, under the temperature-controlled conditionH. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017). The temperature dependence in the spectra clearly exhibits that there are two isomers in the present experimental condition and that the relative populations between them changes with an elevation of the temperature. Among many optimized structures obtained by the DFT calculations, two distinct hydration motifs, ring-with-tail and chain type motifs, are assigned for the isomers observed in our experiment. The change in the relative populations based on our observation is quantitatively interpreted by statistical mechanical estimation based on the DFT calculations. A ring with tail type hydration motif is dominant in cold condition, whereas a chain-like motif is dominant in hot condition. Moreover, possible cooling paths from the chain-like to ring-with-tail type motifs are discussed. In the present paper, temperature effects on the structures of the other hydrogen-bonded phenol cation clusters than [PhOH(H₂O)₅]⁺ are also introduced.

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Footnotes:

H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017)..

Gas-phase hydrated clusters are treated as a microscopic model of hydration networks. Recently, we have revealed the temperature-dependence of hydration structures of hydrated phenol cation, [PhOH(H₂O)₅]⁺ H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017). In the cold condition (30 K), only an isomer having a ring-with-tail type hydration motif (Rt isomer) exists, whereas chain-like (C) isomers are dominant in the hot condition (150 K). Since isomerizations among the isomers having distinct hydration motifs can be related to structural fluctuations in the bulk systems, we have been investigating the isomerization between these two isomers induced by the IR vibrational excitation for the further understanding of the microscopic hydration. At first, we observed an IR spectrum of the Rt isomer at 30 K, and found a OH stretch band that is specific for the Rt isomer at 3330 cm⁻¹. Next, the IR laser light at 3330 cm⁻¹ was irradiated to the Rt isomers in the cold trap. After 3 μs from the IR excitation, we observed UV photodissociation spectra. As a result, an increase of the intensity at the band of the C isomer (25400 cm⁻¹) was clearly observed. This change indicates the IR induced isomerization of the Rt isomer to the C isomer. Moreover, we observed a cooling of the C isomer produced by the IR excitation by the collisions with He buffer gas in the trap.

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Footnotes:

H. Ishikawa, I. Kurusu, R. Yagi, R. Kato, Y. Kasahara, J. Phys. Chem. Lett. 8, 2641 (2017)..

Benzene-noble gas complexes were one of the earliest topics of interest in spectroscopic investigation of van der Waals (vdW) complexes. Smalley et al.S. M. Beck, M. G. Liverman, D. L. Monts and R. E. Smalley, J. Chem. Phys. 70, 232 (1979). observed C₆H₆-(He)_1,2 vdW complexes in the late 1970s by means of electronic spectroscopy. A recent study on the same species was done by Hayashi and OshimaM. Hayashi and Y. Ohshima , Chem. Phys. 419, 131 (2013). at higher resolution (250 MHz). Here, we present an extensive infrared observation of C₆H₆-Rg_n (n=1,2) with the rare gas being He, Ne, or Ar, in the 3 micron region. The spectra were observed using a tunable optical parametric oscillator to probe a pulsed supersonic-jet expansion from a slit nozzle.
Benzene monomer is known to have a complex band system in this region.J. Pliva and A.S. Pine, J. Mol. Spectrosc. 126, 82 (1987). The strongest band, centered around 3047.91 cm⁻¹, belongs mainly to the C-H stretching fundamental ν₁₂ of symmetry E_1u. Other strong perpendicular bands occurring just above the main band as a result of intensity borrowing via anharmonic resonances between the fundamental ν₁₂ and the combinations are ν₂+ν₁₃+ν₁₈, occurring near 3079 cm⁻¹, and ν₁₃+ν₁₆ and ν₃+ν₁₀+ν₁₈, both occurring near 3100 cm⁻¹. The latter two bands are separated by merely 1.45 cm⁻¹. Although data analysis and observation are presently ongoing, we observe analogous bands for C₆H₆-Rg_n (n=1,2). Spectra were assigned to a symmetric top with C_6v symmetry with the rare gas atom being located on the C₆ symmetry axis. Spectra of the C₆H₆-Rg₂ trimers are in agreement with a D_6h symmetry structure, where the rare gas atoms are positioned above and below the plane of the Benzene monomer. Although jet conditions have resulted in excellent signal to noise for the dimer and trimer spectra, we have not been able to identify any lines which might be due to tetramers or larger clusters. We intend to pursue the search for large clusters using a cooled nozzle.

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Footnotes:

S. M. Beck, M. G. Liverman, D. L. Monts and R. E. Smalley, J. Chem. Phys. 70, 232 (1979). M. Hayashi and Y. Ohshima , Chem. Phys. 419, 131 (2013). J. Pliva and A.S. Pine, J. Mol. Spectrosc. 126, 82 (1987).

The infrared spectra of the weakly-bound dimers OCS-HCCCCH, in the region of the ν₁ fundamental band of OCS (2050 cm⁻¹), and N₂O-HCCCCH, in the region of the ν₁ fundamental band of N₂O (2200 cm⁻¹), are observed in a pulsed supersonic slit jet expansion probed with tunable diode/QCL lasers. Both OCS-HCCCCH and N₂O-HCCCCH were found to have planar structure with side-by-side monomer units having nearly parallel axes. These bands have hybrid rotational structure which allow for estimates of the orientation of OCS and N₂O in the plane of their respective dimers. Analogous bands for OCS-DCCCCD and N₂O-DCCCCD were also observed and found to be consistent with the normal isotopologues. Various levels of ab initio calculations were performed to find stationary points on the potential energy surface, optimized structures and interaction energies. Three stable geometries were found for OCS-HCCCCH and two for N₂O-HCCCCH. The rotational parameters at CCSD(T*)-F12c level of theory give results in very good agreement with those obtained from the observed spectra. In both dimers, the experimental structure corresponds to the lowest energy isomer.

Dihydrogen bond is a hydrogen bond which acts between two H atoms having opposite partial charges. Among various kinds of dihydrogen bond systems, we have been investigating the Si-H…H-O type dihydrogen bondH. Ishikawa, A. Saito, M. Sugiyama, N. Mikami, J. Chem. Phys. 123, 224309 (2005).H. Ishikawa, T. Kawasaki, R. Inomata, J. Phys. Chem. A 119, 601 (2015). On the course of our study, we found that the competition between the dihydrogen bond and dispersion interactions determines the structures of phenol−alkylsilane 1:1 dihydrogen−bonded clusters. However, since there are many isomers due to intermolecular orientation as well as conformation of alkyl groups, we have not yet determined their structures completely. In the present study, we have carried out a laser spectroscopic study on the phenol−ethyldimetylsilane (PhOH−EDMS) dihydrogen bonded clusters. Since EDMS has a simple structure, the number of the isomers is expected to be small. We recorded laser−induced fluorescence (LIF), UV−UV hole−burning, and IR spectra of jet−cooled PhOH−EDMS clusters. As a result, we identified two isomers, A and B, based on the UV−UV hole−burning spectra. The 0−0 band of the isomer A is redshifted by -83.3 cm^-1 compared with that of the PhOH monomer and exhibits a simple and long progression of 16.6 cm^-1 interval of the intermolecular vibration. On the other hand, the redshift of the 0−0 band of the isomer B is much smaller (-20.3 cm^-1) and exhibits rather congested band patterns. The redshifts of the OH stretching band of these isomers are −27 and −20 cm^-1 H.Ishikawa, T.Kawasaki, R.Inomata, J. Phys. Chem. A 119, 601 (2015)..

To reveal detailed characters of the dihydrogen bond at molecular level, we have been carrying out IR spectroscopic study on the Si-H…H-O type dihydrogen-bonded clustersH. Ishikawa, A. Saito, M. Sugiyama, N. Mikami, J. Chem. Phys. 123, 224309 (2005).^,H. Ishikawa, T. Kawasaki, R. Inomata, J. Phys. Chem. A 119, 601 (2015). It was found that the structures of the phenol−alkylsilane 1:1 clusters are determined by the competition between the dihydrogen bond and the dispersion interaction in the case of the S_0 state of the neutral clusters. On the other hand, the dihydrogen bond exhibit a dominant contribution in the cationic states. Based on these results, it is expected the balance between the dihydrogen bond and the dispersion interaction is expected to change in the S_1 state compared with the S_0 state. Thus, we have carried out an IR spectroscopic study on the phenol−alkylsilane clusters, in the present study. In the present paper, we will report mainly on the results of the phenol−triethylsilane (PhOH−TES) clusters. It is already reported that three isomers appear in the fluorescence excitation spectrum of PhOH−TES. Using the vibronic bands of these isomers as excitation transitions, IR spectra in the S_1 state were observed by the UV−IR double resonance technique. All of the isomers exhibit much larger redshifts of the OH stretching band compared with those in the S_0 state. It indicates the strengthening of the dihydrogen bond in the S_1 state. In addition, all the isomer exhibit Franck−Condon−like patterns. The patterns change by the intermediate vibrational levels selected by the UV transitions. Similar Franck−Condon−like pattern in the IR transition is reported in the literatureA. V. Zabuga, M. Z. Kamrath, T. R. Rizzo, J. Phys. Chem. A 119, 10494 (2015). This result indicates a strong coupling between the OH stretch and the intermolecular vibrational mode. This coupling is considered to be a characteristic feature in the S_1 H.Ishikawa, T.Kawasaki, R.Inomata, J. Phys. Chem. A 119, 601 (2015)..A.V.Zabuga, M.Z.Kamrath, T.R.Rizzo, J. Phys. Chem. A 119, 10494 (2015)..

A five-dimensional compound-model morphed potential has been generated for the prototype hydrogen-bonded dimer HCN-HF. The potential includes the intermolecular degree of freedom and the HF stretching vibration. Five morphing parameters only are optimized correcting for inadequacies in the underlying ab initio potentials. The morphing transformation utilized a rotationally resolved spectroscopic database composed of microwave and infrared spectroscopic information. Band origin fundamental vibrational frequencies in HCN-HF are fitted to an average absolute error of 0.006 cm⁻¹. The calculated value of the ground state dissociation energy, D₀ = 1969 cm⁻¹is in excellent agreement with the experimental value of 1970(10) cm⁻¹[Oudejans and Miller, Chem. Phys. 239 (1998) 345]. Limitations of the morphing methodology and its potential applications will be discussed.

High-resolution infrared spectra of β-alanine and its water clusters have been studied using solid para-H₂ FT-IR matrix-isolation spectroscopy. It is known that zwitterion forms of amino acids are more stable than neutral forms in water solutions and biological environments, but it is still under debate whether zwitterions are stable in small alanine-water clusters. We have investigated the stabilization effect of water molecules on the zwitterion form of β-alanine by codepositing H₂O and β-alanine in solid para-H₂. Through a comparison with theoretical calculations, as well as with crystalline β-alanine FT-IR spectra, the characteristic NH₃ N-H bending vibrational frequency for the zwitterionic form was identified. Analysis of the spectral peak temporal behavior shows that other proposed zwitterion peaks behave similarly to the characteristic NH₃ spectral peak. It has been shown that water can stabilize the zwitterionic form of gas phase amino acids, causing the zwitterion to form preferentially over the neutral form under certain conditions. The β-alanine zwitterion formation rate may be attributed to aggregation of small water clusters in the solid para-H₂ matrix. These findings provide insight into the behavior of amino acid zwitterion formation.