Gas phase rovibrational spectroscopy of DMSO, Part I: « When a synchrotron source reveals an unusual rotational behaviour » Arnaud Cuisset, Dmitrii Sadovskii Laboratoire de Physico-Chimie de l’Atmosphère, Maison de la Recherche en Environnement Industriel, Dunkerque, France. Olivier Pirali ISMO, unversité Paris-Sud AILES beamline of the SOLEIL synchrotron facility

Outline Part I: High resolution FT-FIR synchrotron spectroscopy of DMSO: Context of the DMSO study FT-FIR high resolution spectroscopy with synchrotron sources Spectroscopic analysis: the 2 bending modes of DMSO Observation of an unusual rotational behaviour for the ν23 mode Part II: High resolution subTHz analysis of DMSO: General and specific motivations A versatile subTHz spectrometer for gas phase spectroscopy Analysis of the DMSO sub THz spectra Rotational structure of the 5 lowest frequency modes of DMSO Conclusions

Context of the DMSO study: DiMethylSulfOxyde (DMSO): SO(CH3)2 Environmental interest: - a currently used industrial solvent - a significant actor of the sulfure chemistry in marine atmosphere Civil protection interest: DMSO is considered as a toxical agent simili of the yperit (mustard gas) The monitoring of DMSO concentrations in gas phase is of considerable interest!

Context of the DMSO THz study: A recent analysis of the ground state in the submillimeter domain have been in 2010 performed by our colleagues from Lille Typke and Dakkouri were the last to investigate the vibrational spectra of DMSO from 30 years old spectroscopic data ! In the FIR, the low-frequency rovibrational transitions have never been previously resolved!

Preliminary study: FTIR spectroscopy and quantum chemistry calculations Using the FTIR spectrometer of the AILES beamline with its internal sources, the low resolution (0.5 cm-1) vibrational spectrum of DMSO have been recorded on the full IR domain. Only 2 vibrational bands 11 and 23 are observed below 600 cm-1 The anharmonic contribution significantly reduced the relative errors of the DFT calculations except for the low-frequency modes. The energy levels associated to the unassigned vibrational modes 12 , 13 & 24 are expected as perturbing states for the 11 and 23 bands

FT-FIR synchrotron high resolution spectroscopy set-up Synchrotron beam entrance Bruker 125 Interferometer Multipass cell : Max = 200m detectors Maximum spectral resolution = 0.001 cm-1 Spectral range= 7 - 18000 cm-1

FT-FIR synchrotron high resolution spectroscopy of DMSO Due to the low volatilty of DMSO (0.42 Torr at 293K) a high sensitivity was required for these experiments: The White type multipass-cell was adjusted to reach a 150 m. optical path length The exceptional properties of the AILES synchrotron beamline were required to resolve the FIR spectrum of DMSO! Brubach et al. AIP Conf. Proc 1214, 81, (2010)

FIR synchrotron high resolution spectroscopy of DMSO Resolution: 0.0015 cm-1 (45 MHz) Calibration accuracy: 0.00015 cm-1 (4,5 MHz) 700 scans (S/N > 100 at 380 cm-1) Acquisition time: 46h Several months of continuous scans will be necessary without the SR! The symmetric 11 and the asymmetric 23 bending vibrations of the OSC2 frame have been resolved in a limited time thanks to synchrotron source

Spectral analysis: DMSO a slightly asymetric top DMSO [(CH3)2SO] is close to an oblate symmetric top ( ~ 0.91). In the ground state, rotational transitions are mainly c-type ( > 85%) and the axis C is a natural choice of the quantization axis for the rotational basis functions |J , K=Kc> used to construct the effective Hamiltonian 11 23 The 11 and 23 modes correspond to the in plane and out of plane bending vibrations of the OSC2 frame related respectively to the ‘parallel’ and ‘perpendicular’ bands

Spectral analysis: Determination of the molecular parameters for the |11=1> state The |11=1> fundamental has been treated as an isolated vibrational state An effective Hamiltonian has been used including purely rotational terms and their corrections in the |11=1> state: The centrifugal distortion has been developed to degree 8 using the Watson’s S-III reduced expression. For the assignment, two components of the induced dipole moment have been considered: strong c-type transitions and weaker a-type transitions. Combining recent pure rotational transitions measured by Margulès et al. and our FIR measurements, we adjusted all parameters in H and we reproduced the experimental data close to their experimental accuracy.

Spectral analysis: Determination of the molecular parameters for the |11=1> state ☺ We have been able to reach all levels within assigned J-multiplet, and in particular levels of type ☹ We observe the limitations of our fit considering |11=1> as an isolated state: the effects of a clear perturbing dark state are observed.

High resolution analysis of the rovibrational spectrum of DMSO : the asymmetric band ν23 Compared to ν11 the P and R branches of the ν23 band are very congested. Only in the Q branch, ΔKc=±1 transitions show regularities allowing to start a Loomis-Wood analysis. We managed to model pQ and rQ branches well enough for picking combination frequencies and making unambiguous assignments.

High resolution analysis of the rovibrational spectrum of DMSO : the asymmetric band ν23 A computer aided assignment procedure based on a systematic search of all combination frequencies was performed About 7500 transitions with 4 ≤ J ≤ 60, including more than 400 A-type transitions are attributed and reproduced essentially the experimental accuracy In the effective Watson Hamiltonian, 86 molecular parameters are correctly fitted for the ground and the two bending states!

The energy levels structure of ν23: an unusual rotational behaviour Observed and predicted levels of ν23 Zoom on the left figure and normalization on the B axes energy We observe for J > 35, the rotational energy around the A axis is not maximal, and there are quantum levels above it. Most of these X-type levels participate in the transitions have been observed experimentally (≃ 90 transitions) While stable principal axes A and C correspond to sequences of nearly degenerate A and C-type doublets, a common feature of any asymmetric top J-multiplet, the pair of X axes manifests itself in the formation of quadruples, or 4-clusters.

Rotational dynamics of DMSO: observation of a gyroscopic destabilization in a polyatomic molecule Pauli and Bohr watching a tippe top Institute of Physics, University of Lund, Sweden on May 31 1951. In the case of the tippe top, rotation about its lower point is stable at low values of angular momentum j and becomes unstable at large j. In the ground state and in the ν11 state and generally for low J, DMSO is similar to a rigid oblate asymmetric Euler top. Contrary to the tippe top, in the ν23 state, we observe a gyroscopic destabilization at high J value and a pair of tilted axes become stationary for J > 27: 2 new stable stationary X axes emerge in the plane AB at a certain tilt angle to A

Thanks for your attention! Aknowledgements The AILES team specially Olivier Pirali 60 years after… Pr Dimitrii Sadovskii Thanks for your attention!