Benzene-noble gas complexes were one of the earliest topics of interest in spectroscopic investigation of van der Waals (vdW) complexes. Smalley et al.¹ 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 M. Hayashi et al.²at higher resolution (250 MHz). Here, we present the infrared observation of C₆D₆−Rg_n (n=1,2) with the rare gas being He, Ne, or Ar, in the regions of ν₁₂ fundamental band of C₆D₆ ( ∼ 2289 cm⁻¹) and the ν₂ + ν₁₃ combination band ( ∼ 2275 cm⁻¹) which are coupled by a Fermi resonance. The spectra were observed at a resolution of  60 MHz using a tunable optical parametric oscillator to probe a pulsed supersonic-jet expansion from a slit nozzle. In the case of C₆D₆−Rg dimers, the spectra were assigned to a symmetric top with C_6v symmetry with the rare gas atom being located on the C₆ symmetry axis. To observe C₆D₆−Rg₂ trimers, the nozzle was cooled using a closed-cycle methanol refrigerator and the spectra were simulated with a rotational temperature of 1.3K. The spectra of the C₆D₆−Rg₂ trimers were in agreement with a D_6h symmetry structure, where the rare gas atoms are positioned above and below the C₆D₆ plane. Data analysis and observation are presently ongoing.
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¹S. M. Beck, M. G. Liverman, D. L. Monts and R. E. Smalley, J. Chem. Phys. 70, 232 (1979). ²M. Hayashi, Y. Ohshima , Chem. Phys. 419, 131 (2013).

Excitation spectrum of the benzene–D₂ van der Waals (vdW) complex in the vicinity of the S₁ ← S₀ 6₀¹ vibronic transition of the monomer was recorded by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Extensive adiabatic cooling with the rotational temperature of < 0.5 K was conducted by the high-pressure pulsed expansion, and sub-Doppler resolution yielding the line width of 250 MHz was realized in a collimated molecular beam by employing Fourier-transform-limited ultraviolet pulses for the excitation.¹ In contrast to our previous study on the benzene–H₂ complex,² the weaker binding ortho nuclear-spin isomer, correlating to the j = 0 state of a freely rotating D₂, was observed in addition to the stronger binding para isomer (with j = 1), by using a gas sample of normal D₂. Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho isomers, respectively. By comparing the present results with those of the benzene–H₂ complex, we made unambiguous assignments on the vdW modes involved in each observed band, and obtained complete sets of vibrational frequencies of all the three vdW modes for the both H₂ and D₂ isotopomers in the S₁ 6¹ manifold. One of the vdW frequency correlates to the splitting between the m = 0 and ±1 sublevels in the j = 1 state of a freely rotating H₂/D₂ molecule, and the potential barrier for the hindered internal rotation has been evaluated to be ca. 60 cm⁻¹ from the values. Ratio of the vdW frequencies between the H₂ and D₂ species deviate significantly from the value for the harmonic vibration (i.e., √2 ≈ 1.4), indicating substantial anharmonic character of the vdW modes in the complex.
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¹M. Hayashi and Y. Ohshima, Chem. Phys. 419, 131–137 (2013). ²M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117, 9819–9830 (2013).

This talk will focus on the isomer-specific IR spectra of benzene-(water)n (BWn) clusters with n = 1-8, returning to a topic studied by our group¹ some 20 years ago, but now with higher resolution (OH stretch region), with inclusion of data from isotopically substituted clusters, and with extension into the HOH bending mode region. Spectra are recorded using resonant ion-dip infrared spectroscopy, an IR-UV double resonance method. Isomer-specific IR spectra in the regions of OH, OD stretches and HOH, HOD bend of benzene-H₂O, -D₂O, -HOD, -(H₂O)₂, -(D₂O)₂, -HOD-DOD were recorded in order to investigate in greater detail the intermolecular potential energy surface between water and benzene. These spectra show strong combination bands in addition to the OH/OD stretch fundamentals arising from large-amplitude “tumbling” and tunneling along internal rotation and torsion coordinates of water(s) on the surface of benzene. Interestingly, the number of extra bands and spectral patterns change dramatically depending on cluster size, the kind of deuterated isomer, and the spectral region probed. In larger clusters with n=3-8, the water HOH bending region is explored for the first time. The prominent bending mode transitions in BW1-8 are spread over a relatively small range (1610-1660 cm⁻¹), and shift with cluster size in a way that reflects the known structural changes that accompany the increase in size. By comparison of experiment with calculation, it is possible to assign the experimentally observed 1614 cm⁻¹transition of BW1 and 1615 cm⁻¹of BW2 bands to the π-bound water molecule. The 1620-1660 cm⁻¹bands of BW3-8 are due to water molecules that can be categorized as single-acceptor, single-donor (AD) hydrogen-bonded waters. In the case of single-acceptor, double-donor (ADD) water molecules, which are expected to be seen from BW6,^a they show higher-frequency bending vibrations and weaker IR intensity, which would correspond to very weakly observed bands in 1660-1750 cm⁻¹for BW6-8.
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¹R. N. Pribble and T. S. Zwier, Science, 1994, 265, 75-79.

A-type rotational spectra of the weakly bound complex formed from pyridine and acetylene are reported. Contrary to expectation based on the symmetric structure of HCCH···NH₃, the acetylene moiety in HCCH···NC₅H₅ does not lie along the symmetry axis of the pyridine. Rotational and ¹⁴N hyperfine constants instead indicate that, while the complex is indeed planar with an acetylenic hydrogen directed toward the nitrogen, the HCCH axis forms an angle of  ∼ 23^° with the C₂ axis of the pyridine. Spectra of HCCH···NC₅H₅, HCCD···NC₅H₅, DCCH···NC₅H₅, and DCCD···NC₅H₅ are all doubled, revealing the existence of a pair of low energy states. In light of the bent structure, this suggests a tunneling motion through a barrier at the C_2v configuration. Because the splitting persists in the singly deuterated species, we conclude that the motion does not involve interchange of the acetylenic hydrogens. Single ¹³C substitution in either the ortho or meta positions of the pyridine eliminates the doubling and gives rise to separate sets of spectra for which the rotational constants are well predicted by a bent geometry. In this case, the two sets correspond to distinct species in which the ¹³C is either on the same or the opposite side as the acetylene. This further suggests that the doubling observed with unsubstituted pyridine arises from wagging of the acetylene, as such a tunneling motion is expected to be quenched when the pyridine is rendered asymmetric. The bent structure of the system may arise due to a secondary hydrogen bonding interaction between the ortho hydrogens of the pyridine and the π system of the acetylene.

The rotational spectra of the 1:1 complexes of pyridine (PYR) with methane and its halides have been observed and assigned using pulsed jet Fourier transform microwave technique. Depending on the nature of the chemical species linked to PYR, π or σ type complexes have been observed, in relation to the PYR interaction sites, that is the σ system of the aromatic ring, or the n orbital of the nitrogen atom. In the case of PYR-CF₄, where the two subunits are held together by a CF₃•••N halogen bond, with the top undergoing a free rotation with respect to PYR [1]. Instead, a C-Cl•••N halogen bond characterizes PYR-CF₃Cl. In PYR with CHF₃ [2], CH₃F and CH₂F₂ two kind of weak hydrogen bonds, C-H•••N and C-H•••F, have been found to connect the two constituent units. The barriers to the internal rotations of the CHF₃ and of the CH₃F groups have been determined from the A-E splittings of the rotational transitions. The rotational spectrum of the adduct PYR-CH₄ shows that CH₄ links to an aromatic molecule through a C H•••π weak hydrogen bond. The shape and the internal dynamic behavior of this complex are very similar to that of the van der Waals complexes involving aromatic molecules with rare gases, which would suggest classifying CH₄, in relation to its ability to form molecular complexes with aromatic molecules, as a pseudo rare gas. References: [1] A. Maris, L. B. Favero, B. Velino, W. Caminati, J. Phys. Chem. A, 2013, 117, 11289. [2] L. B. Favero, B. M. Giuliano, A. Maris, S. Melandri, P. Ottaviani, B. Velino, W. Caminati, Chem. Eur. J. 2010, 16, 1761.

Previously, we have studied non-covalent interactions in mono-halogenated benzene clusters using mass selected resonant 2-photon ionization methods. We have extended our studies by investigating the interaction between these mono-halobenzenes with a prototypical N atom donor (NH₃). Thus, we have obtained electronic spectra of PhX^…(NH₃)n ( X=F, Cl, Br and n=1,2….) complexes in the region of the PhX monomer S₀-S₁ (ππ*) transition. Here we are mainly focusing on PhCl^…NH₃ dimer. We found that upon ionization of the dimer, three reactive pathways of the [PhCl^…NH₃]^+. have been evidenced. The primary pathway is the Cl atom elimination, previously evidenced. The second and third pathways, HCl elimination and H atom elimination are identified for the first time in the R2PI studies of the dimer. Electronic spectra obtained for the three pathways shows that they originate from a common precursor. The reactive pathways in this system were extensively characterized computationally. We used DFT and post-Hartree Fock electronic structure calculations, Frank-Condon analysis to support our experimental findings. The results were consistent with previous direct ab initio molecular dynamics calculations, we found two nearly iso-energetic Wheland intermediates which lie significantly lower in energy than the initially formed dimer cation radical [PhCl^…NH₃]^+..

Although α-cleavage is the preferred reaction channel upon photoionization of small carbonyl compounds, we present experimental and theoretical evidences that in a jet-cooled CH...O hydrogen bonded dimeric complex, enolization and intermolecular proton transfer turns out to be one of the prominent channels of cyclopentanone when photoionized at the threshold by a two-photon ionization scheme. On changing the ionization wavelength of the nanosecond laser pulses, we show that the fragmentation channels of dimer as well as monomer are immensely altered for photoionizations by the 2nd (532 nm), 3rd (355 nm)and 4th (266 nm) harmonics of a Nd: YAG laser. The observed changes are interpreted with the energetic predictions of electronic structure calculations.

Stabilizing α-helical structure of protein and dissolving a hard to dissolve polymer, polythene terphthalete, are some of the unique properties of the organic solvent Hexafluoroisopropanol (HFIP). After determining the complete microwave spectrum of HFIP monomer¹, we have recorded the spectrum of HFIP•••H₂O complex. Ab initio calculations were used to optimize three different possible structures. The global minimum, structure 1, had HFIP as proton donor. Another promising structure, Structure 2, has been obtained from a molecular dynamic study². A total of 46 observed lines have been fitted well for obtaining the rotational and distortion constants within experimental uncertainty. The observed rotational constants are A = 1134.53898(77) MHz, B = 989.67594(44) MHz and C = 705.26602(20) MHz. Interestingly, the rotational constants of structure 1, structure 2 and experiments were very close. Experimentally observed distortion constants were close to structure 1. b−type transitions were stronger than c−type which is also consistent with the calculated dipole moment components of structure 1. Calculations predict a non-zero a-dipole moment but experimentally a−type transitions were absent. Microwave spectra of two of the deuterium isotopologues of this complex i.e. HFIP•••D₂O (30 transitions) and HFIP•••HOD (33 transitions) have been also observed. Search for other isotopologues are in progress. To characterize the nature of hydrogen bonding, Atoms in Molecules and Natural Bond Orbital theoretical analysis have been done. Experimental structure and these theoretical analyses indicate that the hydrogen bonding in HFIP•••H₂O complex is stronger than that in water dimer.
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¹A. Shahi and E. Arunan, Talk number RK16, 68th International Symposium on Molecular Spectroscopy 2013, Ohio, USA. ² Yamaguchi, T.; Imura, S.; Kai, T.; Yoshida, K. Zeitschrift für Naturforsch. A 2013, 68a, 145.

The potentially frustrated transfer of a proton between the hydroxylic (proton-donating) and ketonic (proton-accepting) oxygen atom centers in tropolone (TrOH) long has served as a model system for the study of coherent (symmetrical) proton-transfer events. A litany of hydrogen-bound complexes [TrOH · X_n] can be formed in situ by docking amphoteric ligands onto the TrOH substrate under supersonic free-jet expansion conditions. Binary (n = 1) and higher order (n = 2, 3, ...) complexes formed with formic acid, hydrogen fluoride, acetic acid and propiolic acid (X = FA, HF, AA, and PA) have been synthesized and interrogated using a variety of spectroscopic probes built upon the intense ùB₂−~X¹A₁ (π^*←π) near-ultraviolet absorption system of bare tropolone, thereby providing vibronically resolved information through combined use of laser-induced fluorescence (LIF), dispersed fluorescence (DF), fluorescence hole-burning (FHB), and stimulated emission pumping (SEP) methods. Experimental results reveal the propensity for binary complexes to adopt a higher-energy external binding motif (ligand attached to the seven membered aromatic ring) over the energetically preferred internal form (ligand bound to the O−H…O reaction center), where the latter cleft-bound species can undergo unique symmetric (coherent) double proton-transfer reactions. These findings will be discussed in light of supporting quantum-chemical calculations.

The ground-state rotational spectra of the formaldehyde-dimethyl ether (H₂CO−DME) and the formaldehyde-dimethyl sulfide (H₂CO−DMS) complexes have been studied by Fourier transform microwave spectroscopy, and a-type and c-type transitions have been observed and assigned for the H₂CO as well as D₂CO species of both the complexes. In the case of the H₂CO−DME, doublets were observed with the splitting of a few 100 kHz, whereas no such splitting was found for the H₂CO−DMS. The observed rotational constants were analyzed to conclude a C_s geometry, where DME and DMS were bound to H₂CO by the two C-H(DME/DMS)-O(H₂CO) and the two O(DME)/S(DMS)-H-C(H₂CO) hydrogen bonds. The distances R_cm between the centers of mass of the constituents were determined to be 3.102 and 3.200 Å for the two complexes, respectively. Both the H₂CO−DME and H₂CO−DMS complexes were shown by the natural bond orbital (NBO) analysis, to have strong charge transfer interactions between the constituents, as expected from the strong electron-accepting character of the H₂CO.

Equilibrium constants of the formation of gas-phase hydrogen bonded molecular complexes have been determined using a method combining experimental and theoretically simulated vibrational spectra. The equilibrium constants are determined from the monomer pressures and the pressure of the complex. The pressures of the monomers are measured experimentally, but the pressure of the complex is too low to detect. However, a combination of an experimentally measured integrated absorbance from Fourier-Transform Infrared spectra and a theoretically calculated oscillator strength allows us to estimate the pressure of the complex and determine the equilibrium constant of the complex formation. We have developed a vibrational model to improve the accuracy of the calculated oscillator strengths, which is used to determine the pressures of the molecular complexes, thus improving the accuracy of the equilibrium constants. Our model and results obtained using this model will be presented and discussed

A series of alcohol-amine complexes have been investigated to gain physical insight into the effect on the hydrogen bond strength as different substituents are attached. The series of complexes investigated are shown in the figure, Figure where R₁ = CH₃, CH₃CH₂ or CF₃CH₂ and R₂ = H or CH₃. To estimate the hydrogen bond strength, redshifts of the OH-stretching transition frequency upon complexation were measured using gas phase Fourier Transform InfraRed (FTIR) spectroscopy. Equilibrium constants for the formation of the complexes were also determined, exploiting a combination of a calculated oscillator strength and the measured integrated absorbance of the fundamental OH-stretching and second overtone NH-stretching transitions.