The spectrum of the weakly-bound CO₂-CS₂ complex was originally studied by the USC group,C.C. Dutton, D.A. Dows, R. Eikey. S. Evans, R.A. Beaudet, J. Phys. Chem. A 102, 6904 (1998).sing a pulsed supersonic expansion and a tunable diode laser in the CO₂ ν₃ region. Their derived structure was nonplanar X-shaped (C_2v symmetry), a relatively unusual geometry among linear molecule dimers. Very recently, there has been a detailed theoretical study of this complex based on a high-level ab initio potential surface.J. Brown, X.-G. Wang, T. Carrington, Jr., G.S. Grubbs II, and R. Dawes, J. Chem. Phys. 140, 114303 (2014).he theoretical ground state is X-shaped, in good agreement with experiment, and a very low-lying (3 cm⁻¹ at equilibrium, or 8 cm⁻¹ zero-point) slipped-parallel isomer is also found. We report here two new combination bands of X-shaped CO₂-CS₂ which involve the same ν₃ fundamental (2346.546 cm⁻¹) plus a low-frequency intermolecular vibration. The first band has b-type rotational selection rules (the fundamental is c-type). This, and its location (2361.838 cm⁻¹), clearly identify it as being due to the intermolecular torsional mode. The second band (2388.426 cm⁻¹) is a-type and can be assigned to the CO₂ rocking mode. Both observed intermolecular frequencies (15.29 and 41.88 cm⁻¹) are in extremely good agreement with theory (15.26 and 41.92 cm⁻¹).^b The torsional band is well-behaved, but the 2388 cm⁻¹ band is bizarre, with its K_a = 2 ← 2 and 4 ← 4 components displaced upward by 2.03 and 2.62 cm⁻¹ relative to the K_a = 0 ← 0 origin (odd K_a values are nuclear spin forbidden). A qualitatively similar shift (+2.4 cm⁻¹) was noted for the (forbidden) K_a = 1 level of this mode by Brown et al.,^b but the calculation was limited to J = 0 and 1. These huge shifts are presumably due to hindered internal rotation effects.

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Footnotes:

C.C. Dutton, D.A. Dows, R. Eikey. S. Evans, R.A. Beaudet, J. Phys. Chem. A 102, 6904 (1998).u J. Brown, X.-G. Wang, T. Carrington, Jr., G.S. Grubbs II, and R. Dawes, J. Chem. Phys. 140, 114303 (2014).T

The dispersed fluorescence (DF) spectra of the C₃Ne, C₃Ar, C₃Kr, and C₃Xe complexes near the 0 2⁻ 0- 000, 0 4⁻ 0- 000, 0 2⁺ 0- 000 and 100-000 bands of the Ã-~X system of C₃G. Zhang, B.-G. Lin, S.-M. Wen, and Y.-C. Hsu, J. Chem. Phys. 120, 3189(2004); J.-M. Chao, K. S. Tham, G. Zhang, A. J. Merer, Y.-C. Hsu, and W.-P. Hu, J. Chem. Phys. 134, 074313(2011)ave been revisited. Some of the DF spectra of the Ne and Ar complexes have been recently obtained with a slightly improved resolution of 6-10 cm⁻¹. All the DF spectra have been reassigned as emission from van der Waals (vdW) complexes and C₃ fragments. The optically excited C₃-Rg (Rg = rare-gas atom) complexes fluorescence and/or decay down to slightly lower (about 2-30 cm⁻¹) vibrational levels without changing the internal energy of C₃ and then predissociate via the continua of the nearby vibronic states of C₃. The available dissociation channels depend on the binding energy of the ground electronic state complex. Exceptions have been found at the vdW bands near the 0 4⁻ 0- 000 band of C₃. The binding energies of the ground electronic states of these four complexes will be discussed.

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Footnotes:

G. Zhang, B.-G. Lin, S.-M. Wen, and Y.-C. Hsu, J. Chem. Phys. 120, 3189(2004); J.-M. Chao, K. S. Tham, G. Zhang, A. J. Merer, Y.-C. Hsu, and W.-P. Hu, J. Chem. Phys. 134, 074313(2011)h

We present infrared photodissociation spectra of Mn(CO₂)_n⁻ (n=2−10) cluster ions. The spectra are interpreted in the framework of density functional theory and compared to other first-row transition metals in anionic clusters with CO₂, allowing to draw conclusions to the structure and spin state of the charge carrier.

We report infrared photodissociation spectra of nitrous oxide cluster anions, (N₂O)_n⁻ (n=7−11) and (N₂O)_mO⁻ (m=1−13). Structural changes of the charge carrier in the clusters are driven by increasing levels of solvation. The spectra are interpreted by comparison with quantum chemical calculations.

Dihydrogen bond is known to be one of the unconventional hydrogen bonds. When a hydrogen atom is bonded to an electropositive atom, such as B, Al, and so on, the hydrogen atom has a partial negative charge. Then, a hydrogen-bond type interaction are formed between the oppositely charged two hydrogen atoms. This interaction is called a dihydrogen bond. In the previous study, we reported the infrared spectroscopy of neutral phenol (PhOH)-triethylsilane (TES) clusterH. Ishikawa, T. Kawasaki, RJ02, the 68th International Symposium on Molecular Spectroscopy (2013) It was suggested that the Si-H…H-O dihydrogen bond should be as strong as the π-type hydrogen bond. In the present study, to investigate the intrinsic strength of the Si-H…H-O dihydrogen bond, infrared photodissociation spectroscopy on the PhOH⁺-TES and PhOH⁺-diethylmethysilane (DEMS) cationic clusteris was carried outH. Ishikawa, T. Kawasaki, R. Inomata, J. Phys. Chem. A 119, 601 (2015). Both of the clusters exhibit a very broad and intense band centered at about 2860 cm⁻¹. This band is assigned as the OH stretching band of the PhOH⁺ moiety. Based on the amount of the red-shift of the OH stretching band and the results of the theoretical calculations, the intrinsic strength of the Si-H…H-O dihydrogen bond is evaluated to be stronger than that of the π-type hydrogen bond. The proton affinities of TES and DEMS estimated by the theoretical calculation are larger than those of benzene and ethylene. These results are consistent with our experimental observations.

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Footnotes:

H. Ishikawa, T. Kawasaki, RJ02, the 68th International Symposium on Molecular Spectroscopy (2013). H. Ishikawa, T. Kawasaki, R. Inomata, J. Phys. Chem. A 119, 601 (2015)..

Histidine(His), one of the essential amino acids, is involved in active sites in many enzyme proteins, and known to play fundamental roles in human body. Thus, to gain detailed information about intermolecular interactions of His as well as its structure is very important. In the present study, we have recorded IR spectra of hydrogen-bonded clusters of protonated His (HisH⁺) in the gas phase to discuss the relation between the molecular structure and intermolecular interaction of HisH⁺. Clusters of HisH⁺-(MeOH)_n (n = 1, 2) were generated by an electrospray ionization of the MeOH solution of L-His hydrochloride monohydrate. IR photodissociation spectra of HisH⁺-(MeOH)_1,2 were recorded. By comparing with the results of the DFT calculations, we determined the structures of these clusters. In the case of n = 1 cluster, MeOH is bonded to the imidazole ring as a proton acceptor. The most of vibrational bands observed were well explained by this isomer. However, a free NH stretch band of the imidazole ring was also observed in the spectrum. This indicates an existence of an isomer in which MeOH is bounded to the carboxyl group of HisH⁺. Furthermore, it is found that a protonated position of His is influenced by a hydrogen bonding position of MeOH. In the case of n = 2 cluster, one MeOH molecule is bonded to the amino group, while the other MeOH molecule is separately bonded to the carboxyl group in the most stable isomer. However, there is a possibility that other conformers also exist in our experimental condition. The details of the experimental and theoretical results will be presented in the paper.

An interesting experimental observation in the photodissociation dynamics of ICN^–(Ar)_n is that, even in Ar–ICN^–, a small fraction of the products recombine to form ICN^– following electronic excitation.A. S. Case, E. M. Miller, J. P. Martin, Y. J. Lu, L. Sheps, A. B. McCoy, and W. C. Lineberger, Angew. Chem., Int. Ed. 51, 2651 (2012).he two electronic states that are experimentally accessible dissociate into X^* + CN^– and X^– + CN (X=I or Br). The energy differences between these two asymptotes are roughly 0.14 eV and -0.04 eV for ICN^– and BrCN^–, respectively.^a,B. Opoku-Agyeman, A. S. Case, J. H. Lehman, W. Carl Lineberger and A. B. McCoy, J. Chem Phys. 141, 084305 (2014).he addition of an argon atom is expected to shift the relative energies of these potential energy surfaces, and provide a mechanism for dissipating some of the excess energy from the electronically excited ICN^– and BrCN^–, altering the product branching. In this study, the effects of argon solvation are investigated using classical dynamics approaches. In order to simulate the dynamics, potential energy surfaces for the argon clusters are developed using the results obtained from electronic structure calculations of the fragments in the clusters. Specifically, the potential energies are approximated as the interaction in the bare anion and pair-wise interactions between the argon and the dissociation products. The dynamics are then carried out using classical mechanics. Non-adiabatic effects are treated by incorporating surface hopping into the dynamics.J. C. Tully, J. Chem Phys. 93, 1061 (1990).o assess the accuracy of the approach, the branching ratios for the bare anions are calculated using classical dynamics and the results are then compared to the previously reported quantum dynamics results.^a,b Once the results from both the quantum and classical dynamics are shown to be consistent, classical dynamics simulations are then carried out on the clusters.

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Footnotes:

A. S. Case, E. M. Miller, J. P. Martin, Y. J. Lu, L. Sheps, A. B. McCoy, and W. C. Lineberger, Angew. Chem., Int. Ed. 51, 2651 (2012).T B. Opoku-Agyeman, A. S. Case, J. H. Lehman, W. Carl Lineberger and A. B. McCoy, J. Chem Phys. 141, 084305 (2014).T J. C. Tully, J. Chem Phys. 93, 1061 (1990).T

The non-covalent interactions responsible for π-stacking play crucial roles in many fields of modern chemistry, influencing topics ranging from assembly and recognition in biomolecular systems to the design and function of nano-materials. Owing to the propensity for stronger non-aryl forces (e.g., hydrogen bonding) to dominate complex formation, the number of detailed laser-spectroscopic studies on π-bound species is surprisingly limited, with most reported examples focusing on adducts involving combinations of benzene (Bz) and/or substituted-benzene derivatives. A concerted experimental and theoretical effort has been undertaken to explore novel π-stacking motifs based on the non-benzoidal framework of tropolone (TrOH), where the potentially frustrated (tunneling-mediated) transfer of a proton between donor and acceptor sites can afford an in situ probe of non-covalent binding. Laser-induced fluorescence spectra acquired for the binary TrOH·Bz complex synthesized under cryogenic free-jet expansion conditions show extensive vibronic features that are red-shifted from the intense ùB₂−~X¹A₁ absorption resonance of the bare TrOH substrate and display intensity patterns indicative of changing intermolecular potential-surface topography upon π^*←π electron promotion. These results, as well as spectral signatures from intramolecular TrOH reaction dynamics, will be discussed, with complimentary quantum-chemical calculations serving to provide new insights into the nature of weak, dispersion-dominated interactions.

Previous work on the benzene-(water)_n clusters with n=7 have focused attention on the main conformer, whose S₀-S₁ 6¹₀ R2PI transition appears +138 cm⁻¹above the benzene monomer. Using resonant ion-dip infrared spectroscopy with a higher resolution IR source, we have recently returned to this cluster to record improved OH stretch infrared spectra and more thoroughly consider the possible Bz-(H₂O)₇ structures that might give rise to it. Analysis of that spectrum led to its assignment as an inserted cube structure with pseudo-S₄ symmetry. This talk will consider the spectrum and structure of a minor conformer of Bz-(H₂O)₇ with R2PI transition  65 cm⁻¹to the blue of the monomer. This spectrum, recorded for the first time, shows a distinctive OH stretch infrared spectrum that is best matched with an expanded prism structure in which the seventh water molecule inserts into one edge of the hexamer prism. In this case, benzene acts as acceptor for an OH^...π H-bond, sitting on the surface of a preformed water heptamer structure. The infrared spectra of the two Bz-(H₂O)₇ structures are compared, and the results of a local mode Hamiltonian model are applied to make an assignment for the observed structure. The monomer Hamiltonians resulting from this model shed light on the unique two- and three-coordinate water molecules found in this structure.

The investigation of benzene-water clusters (Bz-(H₂O)_n) provides insight into the relative importance π-hydrogen bond interactions in cluster formation. Taking advantage of the higher resolution of current IR sources, isomer-specific resonant ion-dip infrared (RIDIR) spectra were recorded in the OH stretch region (3000-3750 cm⁻¹). A local mode Hamiltonian for describing the OH stretch vibrations of water clusters is applied to Bz-(H₂O)₆ and Bz-(H₂O)₇ and compared with the RIDIR spectra. These clusters are the smallest water clusters in which three-dimensional H-bonded networks containing three-coordinate water molecules begin to be formed, and are therefore particularly susceptible to re-ordering or re-shaping in response to the presence of a benzene molecule. The spectrum of Bz-(H₂O)₆ is assigned to an inverted book structure while the major conformer of Bz-(H₂O)₇ is assigned to an S₄-derived inserted cubic structure in which the benzene occupies one corner of the cube. The local mode model is used to extract monomer Hamiltonians for individual water molecules, including stretch-bend Fermi resonance and intra-monomer couplings. The monomer Hamiltonians divide into sub-groups based on their local H-bonding architecture (DA, DDA, DAA) and the nature of their interaction with benzene.

The local mode Hamiltonian that assigns RIDIR spectra for Bz-(H₂O)₆ and Bz-(H₂O)₇ is explored in detail for Bz-(H₂O)_n with n=3−7. In addition to contributions from OH stretches, the Hamiltonian includes the anharmonic coupling of each water monomer's bend overtone and its OH stretch fundamentals, which is necessary for accurately modeling 3150-3300 cm⁻¹ region of the spectra. The parameters of the Hamiltonian can be calculated using either MP2 or density functional theory. The relative strengths and weaknesses of these two electronic structure approaches are examined to gain further physical understanding. Initial assignments of Bz-(H₂O)₆ and Bz-(H₂O)₇ were based on a linear scaling of M06-2X harmonic frequencies. In most cases, counterpoise-corrected MP2 calculations obtain similar frequencies (across all cluster sizes) if stretch anharmonicity is taken into account. Individual "monomer Hamiltonians" are constructed via the application of fourth order Van Vleck perturbation theory to MP2 potential energy surfaces. These calculations elucidate the sensitivity of intra-monomer couplings to chemical environment. The presence of benzene has particularly important consequences for the spectra of the Bz-(H₂O)₃₋₅ clusters, in which the symmetry of the water cycles is broken by π-H-bonding to benzene. The nature of these perturbations is discussed.

Owing to recent technical developments of various spectroscopies, microscopic hydration structures of various clusters in the gas phase have been determined so far. The next step for further understanding of the microscopic hydration is to reveal the temperature effect, such as a fluctuation of the hydration structure. Thus, we are carrying out photodissociation spectroscopy on the hydrated phenol cation clusters, [PhOH(H₂O)_n]⁺. Since electronic spectra of [PhOH(H₂O)_n]⁺ have been reported alreadyS. Sato, N. Mikami J. Phys. Chem. 100, 4765 (1996). this system is suitable for our purpose. In the present study, we use our temperature-variable 22-pole ion trap apparatusH. Ishikawa, T. Nakano, T. Eguchi, T. Shibukawa, K. Fuke Chem. Phys. Lett. 514, 234 (2011). The ions in the trap become thermal equilibrium condition by multiple collisions with temperature-controlled He buffer gas. By this way, the temperature of the ions can be controlled. In the electronic spectrum of the n = 5 cluster measured at 30 K, a sharp band is observed. It shows that the temperature of ions are well-controlled. Contrary to the n = 5 cluster, the n = 6 cluster exhibits a wider band shape. The temperature dependence of the band shape indicates the existence of several, at least two, isomers in the present experimental condition.

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Footnotes:

S. Sato, N. Mikami J. Phys. Chem. 100, 4765 (1996)., H. Ishikawa, T. Nakano, T. Eguchi, T. Shibukawa, K. Fuke Chem. Phys. Lett. 514, 234 (2011)..

To understand the temperature effect on the microscopic hydration, we have been carrying out the laser spectroscopy of temperature-controlled hydrated phenol cation clusters using our temperature-variable ion trap apparatusR. Yagi, Y. Kasahara, H. Ishikawa, the 70th International Symposium on Molecular Spectroscopy (2015).^,H. Ishikawa, T. Nakano, T. Eguchi, T. Shibukawa, K. Fuke Chem. Phys. Lett. 514, 234 (2011). In the present study, we have chosen an anilinium ion (AnH^+) as a solute. Since the phenol cation has ()^-1 configuration, the phenyl ring does not play as a proton−acceptor. On the contrary, the −orbitals in the AnH^+ are fulfilled and both the NH_3^+ and phenyl groups can behave as hydrogen−bonding sites. Thus, hydration structures around the AnH^+ are expected to be different from those of the phenol cation. Since there is no spectroscopic report on the hydrated AnH^+ clusters, we have carried out the UV and IR photodissociation spectroscopy of AnH^+(H_2O) clusters. In the present study, the AnH^+(H_2O) is produced by an electrospray ionization method. As the first step, spectroscopic measurements are carried out without temperature control. In the UV photodissociation spectrum, the 0−0 band appears at 36351 cm^-1 which is red−shifted by 1863 cm^-1 from that of the AnH^+ monomerG. Féraud, et al. Phys. Chem. Chem. Phys. 16, 5250 (2014). The band pattern is similar to that of the AnH^+ monomer. This indicates that the structure of the AnH^+ is not so affected by the single hydration. In the IR photodissociation spectrum, OH stretching band of the H_2O moiety and free NH stretching band of AnH^+ moiety are observed. Comparison with the results of the DFT calculation at M05−2X/6−31++G(d,p) level, we determined the structure of the AnH^+(H_2 H.Ishikawa, T.Nakano, T.Eguchi, T.Shibukawa, K.Fuke Chem. Phys. Lett. 514, 234 (2011)..G.Fraud, et al. Phys. Chem. Chem. Phys. 16, 5250 (2014)..

As an isolated species, the radical anion of pyridine (Py⁻) exists as an unstable transient negative ion, while in aqueous environments it is known to undergo rapid protonation to form the neutral pyridinium radical [PyH⁽⁰⁾] along with hydroxide. Furthermore, the negative adiabatic electron affinity (AEA) of Py⁻ can become diminished by the solvation energy associated with cluster formation. In this work, we focus on the hydrates [Py·(H₂O)_n]⁻ with n = 3-5 and elucidate the structures of these water clusters using a combination of vibrational predissociation and photoelectron spectroscopies. We show that H-trasfer to form PyH⁽⁰⁾ occurs in these clusters by the infrared signature of the nascent hydroxide ion and by the sharp bending vibrations of aromatic ring CH bending.