1. ADVANCES IN GLOBAL MODELLING OF METHYL MERCAPTAN CH332SH TORSION-ROTATION SPECTRUM
V. Ilyushin1, I. Armieieva1, O. Zakharenko2, H. S. P. Müller2, F. Lewen2, S. Schlemmer2, L.-H. Xu3, R. M. Lees3
1 Institute of Radio Astronomy of NASU, Kharkiv, Ukraine
2 I. Physikalisches Institut, Universität zu Köln, Köln, Germany
3 Centre for Laser, Atomic and Molecular Physics, Department of Physics, University of New Brunswick, New Brunswick, Canada

2. Motivation
=0.65
Assignments: Jmax=40, fitted microwave data Jmax=30

3. Motivation
Current goal: increase the J coverage for CH3SH spectrum

4. Fits in molecules with large amplitude motions and K>2 Hamiltonian terms4 8 11 14 17 20 J
130
640
385
895
1150
Ered
cm-1
vt=0
vt=1
vt=2
vt=3
vt=4
vt=5
K=3
K=3
K=4
Acetic acid, vt=0,1, J up to 79
V. V. Ilyushin, C. P. Endres, F. Lewen, S. Schlemmer, B. J. Drouin „Submillimeter wave spectrum of acetic acid“Journal of Molecular Spectroscopy Vol. 290 pp (2013).
Acetaldehyde vt=0,1,2 J up to 66
I.A. Smirnov, E.A. Alekseev, V.V. Ilyushin, L. Margulés, R.A. Motiyenko, B.J. Drouin « Spectroscopy of the ground, first and second excited torsional states of acetaldehyde from 0.05 to 1.6 THz » Journal of Molecular Spectroscopy Vol. 295 pp (2014).
Toluene, m =0,1,2,+/ J up to 94
V. V. Ilyushin, E.A. Alekseev, Z.Kisiel, Lech Pszczółkowski, accepted by Journal of Molecular Spectroscopy

5. RAM36 program
V. Ilyushin, Z. Kisiel, L. Pszczółkowski, H. Mäder, J. T. Hougen // J. Mol. Spectrosc. Vol. 259, pp (2010).
The RAM36 (rho-axis-method for 3 and 6-fold barriers) code realizes the Rho-axis-method approach for the molecules with the C3v top attached to a molecular frame of Cs or C2v symmetry and having 3- or 6-fold barrier to internal rotation respectively. The program carries out a global fit of rotational transitions in several torsional states simultaneously using a two step diagonalization procedure.

6. Cologne (Sub-)Millimeter Spectrometer
Experimental details:
Frequency range: , GHz
Pressure: 40 μbar
Temperature: 300 K
Measurement accuracy: from 20 to 65 kHz depending on the S/N ratio
Spectral resolution: Doppler limited
Data acquisition rate: 20 ms/point
Output power: mW
Frequency modulation: 47.8 kHz
Frequency step: 100 kHz

7. Portion of the recorded spectrum of CH3SH around 327.5 GHz.
experiment
theory
327.5
326.25
328.75
GHz

8. Progress in fitting FTFIR data
Li-Hong Xu et al., The J. Chem. Phys. 137, (2012)
18366
FTFIR transitions were assigned uncertainties of and cm−1 (6 and 10.5 MHz) for lower states of νt = 0 and higher, respectively.
NOW
# data Weight cm rms = cm-1

9. Progress in fitting MW data
Li-Hong Xu et al., The J. Chem. Phys. 137, (2012)
0.687
0.89
0.065
4481
0.88
0.063
2312
1.06
0.060
1403
1.36
0.091
766
0.92
0.046
1459
1.00
0.100
1223
0.81
0.161
157
Jmax =

10. Second order parameters of the RAM Hamiltonian for CH3SH molecule
Operator
Parameter
Valued (cm-1)
current work
Xu et al.
pα2 F
(92)
(66)
½(1-cos3α) V3
(60)
(10)
½(1-cos6α) V6
(30)
− (15)
½(1-cos9α) V9
4.471(13)
(31)
½(1-cos12α)
V12
(32) -
pαPa ρ
(19)
(13)
Pa2
ARAM
(35)
(84)
Pb2
BRAM
(28)
(87)
Pc2
CRAM
(28)
(83)
{Pa,Pb}
Dab
− (70)
− (59)
Li-Hong Xu et al., The J. Chem. Phys. 137, (2012)

11. Fits in molecules with large amplitude motions and K>2 Hamiltonian terms4 8 11 14 17 20 J
130
640
385
895
1150
Ered
cm-1
vt=0
vt=1
vt=2
vt=3
vt=4
vt=5
CS stretch
708 cm-1
Reduced energies Ered for torsion-rotation levels of methyl mercaptan of E species.

12. Conclusions
The new experimental data on the methyl mercaptan (CH3SH) spectrum were obtained in the GHz range.
The assignments in the vt=0,1,2 torsional states of methyl mercaptan were extended up to Jmax= 50.
The analysis using RAM36 program allowed significant improvement of the quality of the fit in the vt=0,1,2 torsional states, at the expense of anomalous behavior of the torsional potential function expansion. This suggests significant perturbation from a low-lying small amplitude vibration state.

13. Thank you for your attention
This work was done under support of the Volkswagen foundation. The assistance of Science and Technology Center in Ukraine is acknowledged (STCU partner project #P686).