Experiments carried out at the CLIO Free Electron Facility have been used to probe a range of novel bonding motifs and dissociation dynamics in a variety of chemical systems. Among these are species which exhibit anion-pi interactions in complexes of halide ions with aromatic ring systems with electron withdrawing substituents; charge solvated and zwitterionic clusters of protonated methylamines with phenylalanines; hydrogen bonded dimers of nucleic acid analogues and Pd complexes potentially involving agnostic hydrogen bond interactions. Accompanying DFT computational work is used to assist in identifying the most probable structure(s) present in the IRMPD experiments.

r0pt Figure Far-Infrared (IR) spectroscopy has been labeled as a promising method for identifying structural motifs in large molecules. However, several hurdles have kept this promising spectral region from breaking through to widespread use for gas phase experiments. Normal modes in the far-IR mostly have weak intensities, and high brightness sources of far-IR radiation are rare. Moreover, standard density functional theory - applied to identify the specific molecular structure responsible for the measured IR spectra - does not reproduce features in the far-IR well. This mismatch can be attributed to the high degree of anharmonicity of many of the normal modes present in the far-IR. We have overcome these hurdles by combining an advanced laser source with novel experiments and high-level dynamical calculations. We present far-IR spectra of a family of phenolic molecules and solvated clusters, obtained using the free electron laser FELIX. By employing IR-UV ion-dip spectroscopy in the gas phase, we are able to obtain conformer specific far-IR spectra of isolated molecules or solvated clusters. The studied systems display both intra- and intermolecular hydrogen bonding, enabling us to study the merits of far-IR action spectroscopy for direct probing of these weak interactions. Moreover, the combination of far-IR experiments with quantum chemical calculations allows us to test the limits of the harmonic approximation in DFT calculations, and to test the possibilities of employing a more sophisticated technique, namely Born-Oppenheimer molecular dynamics.

Cryogenic multipole ion traps have become popular devices in the development of sensitive action-spectroscopic techniques. The low ion temperature leads to enhanced spectral resolution, and less congested spectra. In the early 2000s, a 22-pole ion trap was coupled to the Free-Electron Laser for Infrared eXperiments (FELIX), yielding infrared Laser Induced Reaction (LIR) spectra of the molecular ions C₂H₂ and CH₅Asvany et al.: Phys. Rev.Lett. 94, 073001 (2005), Asvany et al.: Science 309, 1219-1222 (2005) This pioneering work showed the great opportunities combining cold mass-selected molecular ions with widely tunable broadband IR radiation. In the past year a cryogenic (T > 3.9 K) 22-pole ion trap designed and built in Cologne (FELion) has been successfully coupled to FELIX, which in its current configuration provides continuously tunable infrared radiation from 3 μm to 150 μm, hence allowing to probe characteristic vibrational spectra in the so-called ”fingerprint region” with a sufficient spectral energy density also allowing for multiple photon processes (IR-MPD). Here we present the first infrared predissociation spectra of He-tagged protonated methanol and ethanol (MeOH₂/EtOH₂) stored at 4 K. These vibrational spectra were recorded with both a commercial OPO and FELIX, covering a total spectral range from 3700 cm⁻¹to 550 cm⁻¹at a spectral resolution of a few cm⁻¹. The H-O-H stretching and bending modes clearly distinguish the protonated alcohols from their neutral analoga. For EtOH₂, also IR-MPD spectra of the bare ion could be recorded. The symmetric and antisymmetric H-O-H stretching bands at around 3 μm show no significant shift within the given spectral resolution in comparison to those recorded with He predissociation, indicating a rather small perturbation caused by the attached He. The vibrational bands were assigned using quantum-chemical calculations on different levels of theory. The computed frequencies correspond favorably to the experimental spectra. Subsequent high resolution measurements could lead to a better structural characterization of these protonated alcohols.

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Footnotes:

Asvany et al.: Phys. Rev.Lett. 94, 073001 (2005), Asvany et al.: Science 309, 1219-1222 (2005).

While the Watson-Crick structure of DNA is among the most well-known molecular structures of our time, alternative base-pairing motifs are also known to occur, often depending on base sequence, pH, or presence of cations. Pairing of two cytosine (C) bases induced by the sharing of a single proton (C-H⁺-C) gives rise to the so-called i-motif, occurring particularly in the telomeric region of DNA, and particularly at low pH. At physiological pH, silver cations were recently suggested to form cytosine dimers in a C-Ag⁺-C structure analogous to the hemiprotonated cytosine dimer, which was later confirmed by IR spectroscopy.¹ Here we investigate whether Ag⁺ is unique in this behavior. Using infrared action spectroscopy employing the free-electron laser FELIX and a tandem mass spectrometer in combination with quantum-chemical computations, we investigate a series of C-M⁺-C complexes, where M is Cu, Li and Na. The complexes are formed by electrospray ionization (ESI) from a solution of cytosine and the metal chloride salt in acetonitrile/water. The complexes of interest are mass-isolated in the cell of a FT ion cyclotron resonance mass spectrometer, where they are irradiated with the tunable IR radiation from FELIX in the 600 - 1800 cm⁻¹range. Spectra in the H-stretching range are obtained with a LaserVision OPO. Both experimental spectra as well as theoretical calculations indicate that while Cu behaves as Ag, the alkali metal ions induce a clearly different dimer structure, in which the two cytosine units are parallelly displaced. In addition to coordination to the ring nitrogen atom, the alkali metal ions coordinate to the carbonyl oxygen atoms of both cytosine bases, indicating that the alkali metal ion coordination favorably competes with hydrogen bonding between the two cytosine sub-units of the i-motif like structure.

l0pt Figure A common intermediate in the high temperature combustion of benzene is cyclopentadienone, C₅H₄=O. Cyclopentadienone is considered to be an “anti-aromatic” molecule. It is certainly a metastable species; samples persist at LN₂ temperatures but dimerize upon warming to -80^°C. It is of great interest to physically characterize this “anti-aromatic” species. The microwave spectrum, the infrared spectrum, the ionization energy, and the electron affinity of cyclopentadienone have been measured. Flash pyrolysis of o–phenylene sulfite (C₆H₄O₂SO) provides molecular beams of C₅H₄=O entrained in a rare gas carrier. The beams are interrogated with time-of-flight photoionization mass spectrometry, confirming the clean, intense production of C₅H₄=O. a) Chirped-pulse Fourier transform microwave spectroscopy and CCSD(T) electronic structure calculations have combined to determineKidwell et al. J. Phys. Chem. Letts. 2201 (2014)he r_e molecular structure of C₅H₄=O. b) Guided by CCSD(T) electronic structure calculations, the matrix infrared absorbance spectrum of C₅H₄=O isolated in a 4 K neon matrix has been usedOrmond et al. J. Phys. Chem. A 118, 708 (2014)o assign 20 of the 24 fundamental vibrational frequencies. c) Imaging photoelectron photoion coincidence (iPEPICO) spectraOrmond et al. Mol. Phys. in press (2015)f cyclopentadienone establishes the ionization energy, IE(C₅H₄=O), to be 9.41 ± 0.01 eV. d) Prof. A. Sanov’s groupKhuseynov et al. J. Phys. Chem. A 118, 6965 (2014)as reported the electron affinity, EA(C₅H₄=O), to be 1.06 ± 0.01 eV.

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Kidwell et al. J. Phys. Chem. Letts. 2201 (2014)t Ormond et al. J. Phys. Chem. A 118, 708 (2014)t Ormond et al. Mol. Phys. in press (2015)o Khuseynov et al. J. Phys. Chem. A 118, 6965 (2014)h

Thiophosgene (Cl₂CS) is a favorite model system for studies of photophysics, vibrational dynamics, and intersystem interactions. But at high resolution its infrared spectrum is very congested due to hot bands and multiple isotopic species. Previously, we reported the first high resolution IR study of this molecule, analyzing the ν₂ (504 cm⁻¹) and ν₄ (471 cm⁻¹) fundamental bands.A.R.W. McKellar, B.E.Billinghurst, J. Mol. Spectrosc. 260, 66 (2010).ere we continue, with analysis of the ν₁ (1139 cm⁻¹) and ν₅ (820 cm⁻¹) fundamentals for the two most abundant isotopologues, ³⁵Cl₂CS and ³⁵Cl³⁷ClCS, based on spectra with a resolution of about 0.001 cm⁻¹obtained at the Canadian Light Source far-infrared beamline using synchrotron radiation and a Bruker IFS125 Fourier transform spectrometer. The ν₂ + ν₄ (942 cm⁻¹) and ν₂ + 2ν₆ (1104 cm⁻¹) bands are also studied here. But so far the ν₂ + ν₆ combination band (795 cm⁻¹) resists analysis, as do the weak ν₃ (292.9 cm⁻¹) and ν₆ ( ≈ 300? cm⁻¹) fundamentals.

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A.R.W. McKellar, B.E.Billinghurst, J. Mol. Spectrosc. 260, 66 (2010).H

The fundamental vibrational bending mode ν₂ of disulfur monoxide, S₂O, and the associated hot band 2ν₂ − ν₂ have been observed at high spectral resolution for the first time at the SOLEIL synchrotron facility using Fourier-transform far-infrared spectroscopy. This transient species has been produced using a radio-frequency discharge by flowing SO₂ over elemental sulfur. The spectroscopic analysis has been performed using an Automated Spectral Assignment Procedure (ASAP) which has enabled the accurate determination of more than 3500 energy levels of the v₂=1 and v₂=2 vibrational states. In addition to the high-resolution synchrotron study, pure rotational spectra of S₂O in the v₂=1 and 2 vibrational states were observed in the frequency range 250 - 500 GHz in a long-path absorption cell.

In the course of our recent study of the ν₂ bending mode of S₂O (Martin-Drumel el al.; see Talk P1190), the S-S stretching mode ν₃ located at 679cm⁻¹ and first studied by Lindenmayer et al. in 1986 (J. Mol. Spectrosc. 119, 56) has been re-investigated at the French national synchrotron facility SOLEIL using Fourier-transform far-infrared spectroscopy. In addition to the vibrational fundamental, evidence for at least one more hot band, most likely ν₃+ν₂−ν₂, was found. Complementary submillimeter wave measurements of the pure rotational spectrum in the v₃=1 state were also performed.

Ammonia (NH₃) has been found ubiquitous, e.g., in the interstellar medium, low-mass stars, Jovian planets of our solar system, and possibly in the low temperature exoplanets. Their spectroscopic line parameters are essential in the accurate interpretation of the planetary and astrophysical spectra observed with Herschel, SOFIA, ALMA, and JWST. In our previous paperS. Yu, et al. J. Chem. Phys. (2010) 174317/1-174317/14. the NH₃ line positions in the far-IR region were studied for the ground state and ν₂ in an unprecedented accuracy, which revealed significant deficiencies in the NH₃ intensities, for instance, some weak ∆K = 3 lines were predicted to be  100 times stronger. Measurement of line intensity for these lines in a consistent manner is demanded because the ∆K = 3 forbidden lines are only way other than collisions and l-doubled states to excite NH₃ to K > 0 levels. Recalling that NH₃ transition lines in the high J and K up to 18 were detected toward the galactic center in the star forming region of Sgr B₂, their accurate intensity measurements are critical in explaining the observed high K excitation, which will provide insights into radiative-transfer vs. collision excitation mechanics of interstellar NH₃. For this, we obtained a series of spectra of ¹⁴NH₃ in the 50 – 550 cm⁻¹ using a Fourier-transform spectrometer, Bruker 125HR, and AILES beam line at Synchrotron SOLEIL, France. Line positions, intensities, and pressure-broadened half-widths have been measured using non-linear least squares spectrum fitting algorithm. In this presentation we report and discuss preliminary results of line position and intensity measurements for the inversion transitions in the ground state, ν₂, 2ν₂, ν₄ and for the vibration-rotation transitions of ν₂, 2ν₂, ν₄, 2ν₂−ν₂, ν₄−ν₂ and ν₄−2ν₂ in this region. Comparison of the new measurements with the current databases and ab initio calculations will be discussed.

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S. Yu, et al. J. Chem. Phys. (2010) 174317/1-174317/14.,

The H₂O-CH₃F complex could have two geometries, one with a hydrogen bond and one with the newly proposed carbon bond Mani, D; Arunan, E. Phys. Chem. Chem. Phys. 2013, 15, 14377. While in general carbon bonds are weaker than hydrogen bonds, this complex appears to have comparable energies for the two structures. Infrared (IR) and microwave (MW) spectroscopic measurements using, respectively, the Jet-AILES apparatus Cirtog, M; Asselin, P; Soulard, P; Tremblay, B; Madebene, B; Alikhani, M. E; Georges, R; Moudens, A; Goubet, M; Huet, T.R; Pirali, O; Roy, P. J. Phys. Chem. A. 2011, 115, 2523nd the FTMW spectrometer at the PhLAM laboratory Kassi, S; Petitprez, D; Wlodarczak, G. J. Mol. Struct. 2000, 517-518, 375 have been carried out to determine the structure of this complex. The IR spectrum shows the formation of the CH₃F- H₂O hydrogen bonded complex and small red-shifts in OH frequency most probably due to (CH₃F)_m-(H₂O)_n clusters. Noticeably, addition of CH₃F in the mixture promotes the formation of small water clusters. Preliminary MW spectroscopic measurements indicate the formation of the hydrogen bonded complex. So far, we have no experimental evidence for the carbon bonded structure. However, calculations of the Ar-CH₃F complex show three energetically equivalent structures: a T-shape, a “fluorine” bond and a carbon bond. The MW spectrum of the (Ar)_n-CH₃F complexes is currently under analysis.

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Mani, D; Arunan, E. Phys. Chem. Chem. Phys. 2013, 15, 14377.. Cirtog, M; Asselin, P; Soulard, P; Tremblay, B; Madebene, B; Alikhani, M. E; Georges, R; Moudens, A; Goubet, M; Huet, T.R; Pirali, O; Roy, P. J. Phys. Chem. A. 2011, 115, 2523a Kassi, S; Petitprez, D; Wlodarczak, G. J. Mol. Struct. 2000, 517-518, 375,