Interest in the hydroxymethyl radical, CH₂OH, stems primarily from its importance as a reaction intermediate. However, this radical is also of interest from a spectroscopic point of view with large amplitude COH torsional tunneling and out of plane CH₂ wagging motions. The first IR detection of CH₂OH was accomplished via matrix isolation spectroscopy over 40 years ago by Jacox.M.E. Jacox, D.E. Milligan, J. Mol. Spec. 47, 148-162 (1973).eisler and co-workersL. Feng, J. Wei, H. Reisler, J. Phys. Chem. A 108, 7903-7908 (2004).etected CH₂OH in the gas-phase using the sensitivity of double resonance ionization detected IR spectroscopy to probe the OH stretch, asymmetric CH stretch, and symmetric CH stretch vibrational modes with partial rotational resolution (0.4 cm⁻¹). Most recently, the Nesbitt group publishedM.A. Roberts, E.N. Sharp-Williams, D.J. Nesbitt, J. Phys. Chem. A 117, 7042-7049 (2013).he first fully rotationally resolved IR spectrum of CH₂OH via the K_a=0←0 band of the symmetric CH stretch. These researchers were able to unambiguously assign the identified transitions to a Watson A-reduced symmetric top Hamiltonian thereby producing improved values for the symmetric CH stretch rotational constants and vibrational band origin. However, in this same work the authors point out a number of remaining unresolved issues. Motivated by these gas-phase observations, we decided to return to the matrix isolation studies of CH₂OH, however utilizing solid parahydrogen as a matrix host to improve upon the sensitivity and resolution of the previous matrix isolation studies. Based on our measurements, while the end-over-end rotation of the CH₂OH radical is quenched, rotational motion around the a-axis is nearly free permitting both A-type and B-type transitions to be resolved. In the case of the OH stretch mode, both A-type and B-type transitions are observed with an energy difference that makes sense based on the gas-phase CH₂OH rotational constants. However, for the symmetric CH stretch mode, the same mode recently assigned by Nesbitt and co-workers, two absorption features are also observed but the energy difference and intensities of the two features do not match predictions based on the rotational constants. M.E. Jacox, D.E. Milligan, J. Mol. Spec. 47, 148-162 (1973).R L. Feng, J. Wei, H. Reisler, J. Phys. Chem. A 108, 7903-7908 (2004).d M.A. Roberts, E.N. Sharp-Williams, D.J. Nesbitt, J. Phys. Chem. A 117, 7042-7049 (2013).t

The high-resolution infrared rovibrational spectroscopy of OCS clustered with multiple hydrogen molecules has previously been studied in helium nanodropletsS. Grebenev, B. Sartakov, J.P. Toennies, A.F. Vilesov, Science 289, 1532-1535 (2000).nd in the gas-phaseJ. Tang, Y. Xu, A.R.W. McKellar, W. Jäger, Science 297, 2030-2033 (2002).n search of another substance other than helium that displays superfluidity. Para-hydrogen (pH₂) is one of the most likely candidates because it is a spinless (I=0) composite boson with a light mass similar to helium. However, compared to helium, the pH₂-pH₂ intermolecular potential is significantly stronger and thus pH₂ solidifies at higher temperatures than the predicted superfluid transition temperature thereby blocking access to the superfluid state. Both of these previous studies reveal intriguing results linked to the microscopic details of superfluidity. We were therefore interested to characterize the IR spectrum of OCS in solid pH₂. The conventional wisdom is that because pH₂ solidifies into a quantum solid, the effects of superfluidity detected in the finite sized clusters should not be manifest in solid pH₂. However, the OCS-H₂ intermolecular potential strongly favors arranging the first  5 pH₂ molecules in a ring around the equator of the OCS (R=3.2 Å). Isolation of OCS in bulk pH₂ therefore may result in a solvation site where 6 pH₂ molecules in the same basal plane form a ring around the OCS and are pulled inward decoupling them from the bulk. If the periodic barriers to motion around the ring are small, one might expect the 6 equatorial pH₂ molecules to become delocalized while still maintaining the permutation symmetry of bosons. These 6 particles-on-a-ring may only show this behavior at low temperatures when thermal excitations are minimized. Analysis of the IR spectroscopy of OCS in solid pH₂ indicates 1) the OCS molecule does not freely rotate and 2) there are at least two preferred OCS solvation sites. In principle, the measured OCS peak frequency for these two solvation sites should depend sensitively on the “structure” of the first pH₂ solvation shell and therefore provide indirect evidence of this delocalization. We are currently trying to model the effect of pH₂ delocalization on the OCS vibrational frequency to compare with experiment and test this hypothesis. S. Grebenev, B. Sartakov, J.P. Toennies, A.F. Vilesov, Science 289, 1532-1535 (2000).a J. Tang, Y. Xu, A.R.W. McKellar, W. Jäger, Science 297, 2030-2033 (2002).i

Internal rotation of the methyl group in substituted toluenes is one of prototypes of large amplitude motions in polyatomic molecules. The internal rotation of o-fluorotoluene is strongly hindered in the S₀ state, but that of m-fluorotoluene is almost free. For the S₁ state, however, the substantial changes of the internal rotation potentials have been reported; while the potential barrier in the o-isomer drastically decreases and the methyl group becomes almost a free rotor, the barrier in the m-isomer largely increasesK,Okuyama.;N,Mikami.;M,Ito. J.Phys.Chem. 1985, 89, 5617. These surprising barrier changes have been attributed to the methyl conformation-dependent stabilization in the S₁ state by the π*-σ* hyperconjugationH,Nakai.;M,Kawai. Chem.Phys.Lett 1999, 307, 272. In the present study, to test this interpretation, we observed infrared spectra of o- and m-fluorotoluenes in the S₀ and S₁ states. Both the isomers showed decrease of the methyl CH stretch frequencies upon the electronic excitation. We concluded that this frequency decrease is the evidence of the π*-σ* hyperconjugation.

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Footnotes:

K,Okuyama.;N,Mikami.;M,Ito. J.Phys.Chem. 1985, 89, 5617.. H,Nakai.;M,Kawai. Chem.Phys.Lett 1999, 307, 272..

We present high resolution rovibrational spectra of buffer gas cooled vinyl bromide (C₂H₃Br) and nitromethane (CH₃NO₂) in the 3 μm CH stretch region, acquired via cavity-enhanced direct frequency comb absorption spectroscopy. The  ∼ 10 K translational and rotational temperatures of the molecular gas, as well as the narrow linewidth of the frequency comb, yield well resolved rotational structure, isotope shifts, and nuclear hyperfine splittings. Given the wide bandwidth of the light source and the long path length of the enhancement cavity, we measure entire vibrational bands in a single shot with high signal-to-noise ratios. We discuss spectra of the entire fundamental CH stretch manifolds of both C₂H₃Br and CH₃NO₂, which provide contrasting examples of rovibrational structure of rigid and non-rigid systems. C₂H₃Br is a relatively normal asymmetric top, exhibiting local perturbations to its rotational structure. Conversely, CH₃NO₂ contains an essentially unhindered methyl rotor. Of particular interest are its quasi-degenerate asymmetric CH stretch modes. Here, one must consider multiple couplings between torsional, rotational, and vibrational angular momentum, leading to qualitatively new level patterns and structure.

The infrared Fourier transform spectrum of the asymmetric C-H stretching bands of CH₃SH has been recorded in the 2950-3100 cm⁻¹ region at Doppler limited resolution using synchrotron radiation at the FIR beamline of the Canadian Light Source in Saskatoon. Assignment of numerous torsion-rotation sub-bands for the asymmetric stretches has revealed a surprising pseudo-symmetric behavior, in which each band is seen in only one of the two possible ∆K selection rules. The upper states of the two asymmetric stretching vibrational bands thus appear to behave more like l = ± 1 components of a degenerate E state of a symmetric top rather than distinct vibrational states. The two components are separated by about 1.5 cm⁻¹ at K = 0, and then diverge linearly at higher K with torsional oscillation amplitude similar to that of the ground state of about 1.3 cm⁻¹. The divergence is consistent with an a-type Coriolis splitting picture with an effective Coriolis constant ζ ≈ 0.075.

A new global study of the acetone (CH₃)₂CO spectrum is reported. The new microwave measurements covering the frequency range from 34 GHz to 940 GHz have been carried out using spectrometers in IRA NASU (Ukraine) and PhLAM Lille (France). The far infrared spectrum of acetone has been recorded on the AILES beamline of the synchrotron SOLEIL using a Fourier transform infrared spectrometer coupled to a long path cell. The transitions belonging to the three lowest torsional states as well as to the observed fundamental band associated with the methyl-top torsion mode (ν₁₇ = 1) have been analyzed using recently developed model for the molecules with two equivalent methyl rotors and C_2v symmetry at equilibrium (PAM_C2v_2tops program) V. Ilyushin, J.T. Hougen J. Mol. Spectrosc. 289 (2013) 41-49. The dataset consisting of more than 26100 microwave and 1100 FIR line frequencies and including transitions with J up to 89 was fit using a model consisting of 119 parameters and weighted root-mean-square deviation of 0.89 has been achieved. In the talk the details of this new study will be discussed.

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Footnotes:

V. Ilyushin, J.T. Hougen J. Mol. Spectrosc. 289 (2013) 41-49..

The LWW program package is based on traditional methods used in assigning rotationally resolved IR molecular spectra. The Loomis-Wood diagrams, which are used to visualize spectral branches and facilitate their identification, are combined with the power of interactive lower state combination difference (LSCD) checking, which provides immediate verification of correct assignments of quantum numbers to spectral lines. The traditional Giessen/Cologne type Loomis-Wood algorithm is also implemented. Predictions of vibration-rotation wavenumbers are calculated from a table of vibration-rotation energies, which can be imported from any external fitting program. Program includes many additional tools like simulation of a spectrum from a catalog file (list of transitions with intensities), build-up of a vibration-rotation band from individual branches and simultaneous displaying of two IR spectra - active one used for assignments and a reference one, both with full link to their peak-list files. Importing energies as well as exporting assigned data for fitting in an external program is made easy and flexible by a user-programmed import/export interface, which facilitates iterative refining of energy levels and gives a possibility of using directly exact vibration-rotation energies. Program is available in tree versions: for symmetric top, asymmetric top and molecules with large amplitude motions. The program is designed for the Windows operating systems and is available with full documentation on www.lww.amu.edu.pl .

Within the past two decades, action vibrational spectroscopy has become an almost routine experimental method to probe the structures of molecules and clusters in the gas phase (neutral and ions). Such experiments are mainly performed in the 1000-4000 cm⁻¹ fingerprint regions. Though successful in many respects, these spectral domains can be however restrictive in the information provided, and sometimes reach limitations for unravelling structures without ambiguity. In a collaborative work with the group of Dr A.M. Rijs (FELIX laboratory, Radbout University, The Netherlands) we have launched a new strategy where the far-IR/Tera-Hertz domain (100-800 cm⁻¹ domain) is experimentally probed for neutral gas phase molecules. Our group in Paris apply finite temperature DFT-based molecular dynamics (DFT-MD) simulations in order to unravel the complex signatures arising in the far-IR domain, and provide an unambiguous assignment both of the structural conformation of the gas phase molecules (taking into account the experimental conditions) and an understanding of the spectral signatures/fingerprints. We will discuss our experimental and theoretical investigations on two neutral peptides in the 100-800 cm⁻¹ far-IR spectral domain, i.e. Z-Ala₆ and PheGly dipeptide, that represent two systems which definitive conformational assignment was not possible without the far IR signatures. We will also present our very recent results on the Phe-X peptide series, where X stands for Gly, Ala, Pro, Val, Ser, Cys, combining experiments and DFT-MD simulations, providing a detailed understanding of the vibrational fingerprints in the far-IR domain. In all exemples, we will show how DFT-MD simulations is the proper theoretical tool to account for vibrational anharmonicities and mode couplings, of prime importance in the far-IR domain. References : J. Mahé, S. Jaeqx, A.M. Rijs, M.P. Gaigeot, Phys. Chem. Chem. Phys., 17 :25905 (2015)
        S. Jaeqx, J. Oomens, A. Cimas, M.P. Gaigeot, A.M. Rijs, Angew. Chemie. Int., 53 :3663 (2014)

The rotational-resolved vibrational spectra of AsH₃ ⁺ have been measured for the first time with vibrational energies up to 6000 cm⁻¹above the ground state using zero-kinetic energy photoelectron spectroscopic method. The inversion vibrational energy levels (ν₂) and the corresponding rotational constants for the ν₂ =0-16 have been determined. The tunneling splittings of the inversion vibration energy levels have been observed for the ground and the first excited vibrational states. The geometric parameters of AsH₃ ⁺ as a function of inversion vibrational quantum states have been determined, indicating that the geometric structure of the cation changes from near planar structure to a pyramidal structure with more vibrational excitations. In addition to the experimental measurement, a two-dimensional theoretical calculation including the two symmetric vibrational modes was performed to determine the energy levels of the symmetric inversion and As-H stretching vibrations. The calculated vibrational energy levels are in good agreement with the experimental results. The first adiabatic ionization energy (IE) for AsH₃ was also accurately determined. The result of this work will be compared with our published result on the PH₃⁺.

r0pt Figure The microwave spectra were measured for cyclopropanecarboxylic acid (CPCA), an excited state conformer of CPCA and a doubly hydrogen bonded dimer formed with formic acid (FA) using a pulsed-beam Fourier transform microwave spectrometer. The rotational constants were determined from the spectra and were used to obtain a best fit gas phase structure of both CPCA and CPCA-FA using a nonlinear least squares fitting program. We obtained the C-C bond lengths in the cyclopropane ring for CPCA and the hydrogen bond distances for the CPCA-FA dimer. For CPCA-FA, there was no evidence of a concerted double proton tunneling motion as singlet b-type transitions were observed. The absence of the tunneling motion is most likely due to the asymmetry of the dimer. The excited stated conformer of the CPCA-FA dimer was also searched for, but was also not observed.
*Supported by the NSF CHE-1057796

r0pt Figure The gas phase homodimer of 3-fluorobenzoic acid was detected and the spectra showed evidence of proton tunneling. Experimental rotational constants are A(0⁺)= 1151.8(5), B(0⁺)=100.3(5), C(0⁺)= 87.64(3) MHz and A(0⁻)=1152.2(5), B(0⁻)= 100.7(5), C(0⁻)=88.85(3) MHz for the two ground vibrational states split by the proton tunneling motion. The tunneling splitting (∆E) is approximately 560 MHz. This homodimer appears to be the largest carboxylic acid dimer observed with F-T microwave spectroscopy. Additionally, the microwave spectra of the mono-fluoro-benozic acids, (2-fluoro, 3-floro and 4-fluoro) benzoic acid have been measured in the frequency range of 4-14 GHz using a pulsed beam Fourier Transform microwave spectrometer. Measured rotational transition lines were assigned and fit using a rigid rotor Hamiltonian. Assignments were made for 3 conformers of 2-fluorobenzoic acid, 2 conformers of 3-fluorobenzoic acid and 1 conformer of 4-fluorobenzoic acid.

r0pt Figure The microwave spectrum was measured for the tropolone-formic acid doubly hydrogen bonded dimer using a pulsed-beam Fourier transform microwave spectrometer in order to search for the concerted double proton tunneling motion. The tunneling motion was expected for the dimer, as the transition state of this motion exhibits C_2V symmetry, which has been thought to be a requirement to observe the concerted double proton tunneling. The tunneling motion was not observed for this dimer, as the transitions measured did not show observable splittings into doublets. The barrier height calculated of the dimer using B3LYP/aug-cc-pVTZ was about 15000 cm⁻¹, significantly larger than the value determined for the propiolic acid-formic acid dimer (3800 cm⁻¹),^a which showed the tunneling motion. The estimated separation of the minima in the potential energy surface is estimated to be very similar to that of propiolic acid-formic acid (about 0.8 Å),^a so the large barrier height may be why the tunneling process was not observed.
^aDaly, A. M.; Bunker, P. R.; Kukolich, S. G. Communications: Evidence for Proton Tunneling from the Microwave Spectrum of the Formic Acid-Propiolic Acid Dimer. J. Chem. Phys. 132, 2010, 201101/1.
*Supported by the NSF CHE-1057796

The vibration-rotation-tunneling spectrum of formic acid dimer, (HCOOH)₂, in the spectral region 1369-1375 cm⁻¹has been measured by a multi-step rapid-can method in a slit jet expansion using a distributed-feedback quantum cascade laser. The observed spectrum is assigned to the O-C-H bending fundamental band. The tunneling splitting in the vibrational excited state is determined to be about 0.005 cm⁻¹, which is much smaller than that in the ground state, 0.0165 cm⁻¹(Goroya et al.,J. Chem. Phys. 140, 164311 (2014)). Strong local perturbations involving transitions with J > 9, K = 0 and 1 are identified in the observed spectrum. The deperturbation analysis will be presented.

r0pt Figure The microwave spectra were measured for the maleimide monomer and the maleimide-formic acid doubly hydrogen bonded dimer using a pulsed-beam Fourier transform microwave spectrometer. Many previously studied doubly hydrogen bonded dimers are formed between oxygen containing species, so it is important to also characterize and study other dimers containing nitrogen, as hydrogen bonding interactions with nitrogen are found in biological systems such as in DNA. The transition state of the dimer does not exhibit C_2V symmetry, so the tunneling motion was not expected to be observed based on the symmetry, but it would be very important to also observe the tunneling process for an asymmetric dimer. Single-line b-type transitions were observed, so the tunneling motion was not observed in our microwave spectra. The hydrogen bond lengths were determined using a nonlinear least squares fitting program.
*Supported by the NSF CHE-1057796