Neon carrier gas at 0.3 MPa of backing pressure is flowed over a room-temperature ammonia hydroxide solution before being expanded into a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer operating between 2 and 8 GHz. A dense spectrum was observed and the investigation allowed unambiguous assignment of the (NH₃)₂(H₂O)_n with n=1,2 and NH₃(H₂O)_n with n=2,3,4,5,6,8 with a signal to noise of at least 3:1. The structures show a cyclic arrangement for clusters with up four monomer and then move to a 3D arrangement. These clusters are of interest because of the different possibilities for hydrogen bond network related to the isolated water clusters. Calculations indicate that there are several possible low-energy isomers, with different levels of theory identifying different isomers as the global minimum. The evidence for the assignment and a discussion of the derived properties for the species are presented.

We have used continuous-wave cavity ring-down spectroscopy to record part of the 2OH excitation range in an Ar/Kr supersonic expansion seeded with H₂O. Various bands were observed, and are being rotationally analyzed, of Ar−H₂O and Kr−H₂O. The analysis of experimental linewidths allowed us to determine the mean upper state predissociation lifetime to be 3 ns for Ar−H₂O and 4 ns for Kr−H₂O. In this talk, the latest results concerning Ar−H₂O will be presented. Several bands were identified and analyzed, highlighting some strong perturbations. The assignment of the many bands, as well as the perturbation processes, is complicated and is still in progress. The results of the analysis will be presented along with the perturbing effects of the dark states. Identification of these will be attempted using a multidimensional approach, based on the intramolecular potential energy surface of water monomerH. Partridge and D. W. Schwenke, J. Chem. Phys. 106 (1997) 4618.nd on the intermolecular potential energy surface of the complex, allowing us to evaluate the rovibrational energy levels of H₂O perturbed by the argon atom. Although several such intermolecular potentials are already available, like those reported by MakarewiczJ. Makarewicz, J.\ Chem. Phys. 129 (2008) 184310.nd by Hou et al.,D. Hou, Y.-T. Ma, X.-L. Zhang, and H. Li, J. Chem. Phys. 144 (2016) 014301.one of them can be used in the present investigation as they were designed for vibrational states of the water monomer below the (101) state, involved in the present spectra. A 6D intermolecular potential energy surface is currently being computed through ab initio calculations to deal with high lying states of the water monomer like the (101) state. With the help of this new surface and of the multidimensional approach, we are hoping to assign the bright and the dark states of the complex, near 7 500 cm⁻¹, involved in the present spectra.

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Footnotes:

H. Partridge and D. W. Schwenke, J. Chem. Phys. 106 (1997) 4618.a J. Makarewicz, J.\ Chem. Phys. 129 (2008) 184310.a D. Hou, Y.-T. Ma, X.-L. Zhang, and H. Li, J. Chem. Phys. 144 (2016) 014301.n

We measured IR spectra of the protonated methanol–water mixed clusters (H⁺(CH₃OH)_n(H₂O)₁, n =6 - 10) in the OH stretching vibrational region. Spectra of their Ar tagged clusters were also measured to explore hydrogen-bonded structure changes by the vibrational cooling. The temperature dependence of the isomer distribution was also examined by the harmonic superposition approximation (HSA) simulation. No essential change of the structures with the Ar tagging (lowering of temperature) was concluded in the size range of n = 8 - 10, indicating the remarkable stability of the inclusion structures in this size range. On the other hand, at n = 7, the large isomer distribution change with the Ar tagging is suggested. Moreover, at n = 6, the IR spectrum showed dramatic changes upon the Ar tagging. The protonated site switching from water to methanol well explained these observed changes.

Preferred isomer structures of hydrogen-bonded clusters should depend on their temperature because of the entropy term in the free energy. To observe such temperature dependence, we propose a new approach to control the internal energy (vibrational temperature) of protonated clusters in the gas phase. We performed IR spectroscopy of protonated methanol clusters, H⁺ (CH₃OH) _n, n= 5 and 7, with the tagging by various inert gas species (Ar, CO₂, CO, CS₂, C₂H₂, and C₆H₆). We found that vibrational temperature of the tagged clusters raises with increase of the interaction energy with the tag species, and the observed cluster structures follow the theoretical prediction of the temperature dependence of the isomer population.

We investigated IR spectra in the CH- and OH-stretching regions of size-selected methanol-water clusters, M_n(H₂O) with M representing CH₃OH and n = 1-4, in a pulsed supersonic jet by using the VUV (vacuum-ultraviolet)-ionization/IR-depletion technique. The VUV light at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser served as a source of dissociation for clusters before ionization. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increase, whereas spectra in the CH region are similar. For M(H₂O), absorption of a structure with H₂O as a proton donor was observed at 3570, 3682, and 3722 cm⁻¹, whereas that of methanol as a proton donor was observed at 3611 and 3753 cm⁻¹. For M₂(H₂O), the OH-stretching band of the dangling OH of H₂O was observed at 3721 cm⁻¹, whereas overlapped bands near 3425, 3472, and 3536 cm⁻¹correspond to the OH-stretching modes of three hydrogen-bonded OH in a cyclic structure. For M₃(H₂O), the dangling OH shifts to 3715 cm⁻¹, and the hydrogen-bonded OH-stretching bands become much broader, with a band near 3179 cm⁻¹having the smallest wavenumber. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted for the methanol-water clusters with the M06-2X/aug-cc-pVTZ method are consistent with our experimental results. For M₄(H₂O), observed spectrum agree less with theoretical predictions, indicating the presence of isomers other than the most stable cyclic one. Spectra of M_n(H₂O) and M_n+1 are compared and the cooperative hydrogen-bonding is discussed.

r0pt Figure Ceria (CeO₂) has been established as a good support in heterogeneous catalysts for the water gas shift reaction. This study looks into cerium’s reactivity with water, a water gas shift reagent, and aims to build an understanding of the three reactions which can occur: direct oxidation, -OH abstraction, and H₂O addition. Through the use of anion photoelectron spectroscopy and density functional theory calculations we have been able to determine that the reactivity is dependent on (1) the oxidation states of the metal centers, (2) the availability of 5d orbitals to form metal oxide bonds, and (3) the presence of electrons in the 6s* orbital. The results of this study can be used to inform design of catalytic materials for the water gas shift reaction.

Assigning the vibrational signatures between 800-1200 cm⁻¹is not a trivial task for (CH₃OH)₂H⁺.J.R. Roscioli, L.R. McCunn and M.A. Johnson. Science 2007, 316, 249^- T.D. Fridgen, L. Macaleese, T.B McMahon, J. Lemaire and P. Maitre. Phys. Chem. Chem. Phys. 2006, 8, 955−966uch complication in the assignment arises due to the intermode coupling between O−H^+−O stretch and flanking group motions. In this talk, we will examine the interactions between O−H^+−O stretch and four flanking group motions: 1) out−of−phase C−O stretch, 2) in−plane CH₃ rock, 3) out−of−plane CH₃, and 4) O−O stretch. Vibrational interactions were investigated by solving a reduced−dimensional Schrödinger equation by means of Discrete Variable Representation (DVR) with harmonic oscillators as basis. Potential and dipole moment surfaces were constructed by scanning along normal mode coordinates using MP2 and CCSD(T) level. It was found out that the O−H^+ T.D. Fridgen, L. Macaleese, T.B McMahon, J. Lemaire and P. Maitre. Phys. Chem. Chem. Phys. 2006, 8, 955-966SJ.A. Tan and J.-L. Kuo. J. Phys. Chem. A. 2015, 119, 11320-11328

The vibrational coupling between O-H⁺-O/O-D⁺-O stretch and flanking group motions were explored in the following isotopologues: (CH₃OH)₂H⁺, (CD₃OH)₂H⁺, (CH₃OD)₂D⁺, and (CD₃OD)₂D⁺. At present only measurements for (CH₃OH)₂H⁺ are available in the literature.J.R. Roscioli, L.R. McCunn and M.A. Johnson. Science 2007, 316, 249^- T.D. Fridgen, L. Macaleese, T.B McMahon, J. Lemaire and P. Maitre. Phys. Chem. Chem. Phys. 2006, 8, 955−966educed−dimensional calculations were performed by solving several vibrational Schrödinger equations using the method of Discrete Variable Representation (DVR) with harmonic oscillator as basis. Both potential and dipole moment surfaces were constructed at MP2/aug−cc−pVDZ by scanning along normal modes corresponding to: 1) O−H^+−O/O−D^+−O stretch, 2) out−of−phase C−O stretch, 3) in−plane CH₃/CD₃ rock, 4) out−of−plane CH₃/CD₃ rock, and 5) O−O stretch. It was found that vibrational states for isotopologues corresponding to O−H^+−O are more mixed than that of the O−D^+ T.D. Fridgen, L. Macaleese, T.B McMahon, J. Lemaire and P. Maitre. Phys. Chem. Chem. Phys. 2006, 8, 955-966R

Propargyl alcohol (hereafter abbreviated as PA) is a molecule of astrophysical interest and has been probed extensively using microwave spectroscopy.^1,2 It is a multifunctional molecule and offers multiple sites for hydrogen bonding interactions. Therefore, it has also attracted the attention of groups interested in weak intermolecular interactions. Recently, the Ar…PA complex³ and PA-dimer⁴ have been studied using microwave spectroscopy. More recently, there have been matrix-isolation infrared spectroscopic studies on PA-water⁵ and PA-acetylene⁶ complexes. In the present work, clusters of PA and water were formed in the helium nanodroplets and probed using a combination of infrared spectroscopy and mass spectrometry. Using ab-initio quantum mechanical calculations, PA-water clusters were optimised and five minimum structures were found on the potential energy hypersurface, which were used as a guidance to the experiments. We used D₂O for the experiments since our laser sources at Bochum do not cover the IR spectral region of H₂O. IR spectra of PA-D₂O complex were recorded in the region of symmetric and antisymmetric stretches of the bound D₂O. Multiple signals were found in these regions which were dependent on the concentration of PA as well as D₂O. Using pickup curves most of these signals could be assigned to 1:1 PA:D₂O clusters. The ab-initio calculations helped in a definitive assignment of the spectra to the different conformers of PA-D₂O complex. The details will be presented in the talk. References: 1. E. Hirota, J. Mol. Spec. 26, 335 (1968). 2. J.C. Pearson and B.J. Drouin, J. Mol. Spectrosc. 234, 149 (2005). 3. D. Mani and E. Arunan, ChemPhysChem 14, 754 (2013). 4. D. Mani and E. Arunan, J. Chem. Phys. 141, 164311 (2014). 5. J. Saini, K.S. Vishwanathan, J. Mol. Struct. 1118, 147 (2016). 6. K. Sundararajan et al., J. Mol. Struct. 1121, 26 (2016).

The adduct formamide₂-H₂0 has been detected in a supersonic expansion and its rotational spectra in the 5-13 GHz frequency region characterized by narrow-band molecular beam Fourier transform microwave spectroscopy (MB-FTMW). The spectrum shows the hyperfine structure due to the presence of two ¹⁴N-nuclei. This hyperfine structure has been analyzed and the determined quadrupole coupling constants together with the rotational constants have been a key for the identification of the adduct structure on the light of ab initio computations. The rotational parameters are consistent with the formation of a three body cycle thanks to the double proton acceptor/proton donor character of both formamide and water. The low value of the planar moment of inertia P_cc indicates that the heavy atom skeleton of the cluster is essentially planar. A detailed analysis of the results reveals the subtle effects of hydrogen bond cooperative effects in this system.

The adduct formamide-(H₂0)₃ has been detected in a supersonic expansion and its rotational spectrum characterized in the 5-13 GHz frequency region by narrow-band molecular beam Fourier transform microwave spectroscopy (MB-FTMW). The spectrum shows the hyperfine structure due to the presence of one ¹⁴N-nuclei and small splittings due to a tunnelling motion of the complex. The spectra has been analyzed using a two-state Hamiltonian including Coriolis coupling terms to determine the vibrational spacing. The determined quadrupole coupling constants together with the rotational constants have been a key for the identification of the adduct structure on the light of ab initio computations. The rotational parameters are consistent with the formation of a four body cycle thanks to the double proton acceptor/proton donor character of both formamide and water. The rotational data are consistent with a non-planar heavy atom skeleton.

The investigation of the dynamics of biomolecules is crucial to understand biological processes. For this purpose, the initial research investigations on the conformational behavior of isolated biomolecules should go one further step by investigating the structure and conformation of complexes formed in supersonic jets by different biomolecules to model the interactions which take place in biological media. In this work, the imidazole-urea complex formed in a supersonic expansion has been investigated by using microwave spectroscopy. In parallel, the conformational space of the complex has been explored with ab initio calculations. The broadband microwave spectrum (8-18GHz frequency interval) has been recorded using a Chirped Pulse Fourier Transform Microwave spectrometer (CP-FTMW). The solid sample was formed by mixing pure samples of imidazole and urea within a solid copper matrix, and was vaporized using the second harmonic of a pulsed Nd:YAG laser. The analysis of the experimental data in the light of the theoretical predictions has allowed the unambiguous identification of the observed conformers in the microwave spectrum.

Anharmonic vibrational coupling among N-H stretching fundamental (ν₁ and ν₃) and N-H bending overtone (2ν₄) vibrations in (NH₃)_n (n = 1 to 5) are analyzed based a full dimensional Hamiltonian including third and quartic terms. In particular, we examine Fermi resonance between the symmetric N-H stretching (ν₁) and N-H bending overtone (2ν₄) vibrations. As the cluster size increases, enhancement of the hydrogen bond strength makes ν₁ red-shifted while 2ν₄ blue-shifted. These shifts result in the crossing of the frequencies of ν₁ and 2ν₄ levels, and their energy order reverses between n = 3 to n = 4. Because the nature of Fermi resonance, although the zero-order ν₁ and 2ν₄ levels are shifted, the resultant mixed levels do not show remarkable changes in frequency. Instead, the major component of each mixed level largely changes and this causes significant redistribution of the intensity. Our results offer a solution to resolve puzzles on the intensity distribution and assignments of the Fermi mixing bands in the previously reported infrared spectra of (NH₃)_n.

Weakly-bound clusters are important for studying intermolecular interactions such as van der Waals forces and hydrogen bonding. The geometry and the effect of intermolecular force can be retrieved from their rovibrational spectra. Using a millimeter/submillimeter direct absorption spectrometer coupled with a fast-sweep detection technique, we are able to probe the ground vibrational state of weakly-bound molecules with high precision. With this spectrometer, we have fully characterized the pure rotational spectrum of trans-HO₃ up to 450 GHz. We have also identified lines in the trans-HO₃ spectrum as arising from the Ar-H₂O complex, and followed up with a full study of this cluster in the 200-850 GHz range. We are additionally studying proton-bound complexes that might have an impact in astrochemical environment, such as H₂-HCO⁺. The experimental setup and the preliminary results for these complexes will be presented.