1. 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

2. 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

3. 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!

4. 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!

5. 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

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

7. 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)

8. FIR synchrotron high resolution spectroscopy of DMSO
Resolution: cm-1 (45 MHz)
Calibration accuracy: 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

9. 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

10. 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.

11. 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.

12. 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.

13. 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!

14. 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.

15. 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
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

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