1. Rotational Spectroscopy and Search for Methoxymethanol in the ISM
Roman A. Motiyenko, Laurent Margulès
Laboratoire PhLAM, UMR 8523 CNRS - Université Lille 1, Villeneuve d’Ascq, France
Jean-Claude Guillemin
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS - ENSCR, Rennes, France
Didier Despois
Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, Floirac, France.
Arnaud Belloche
Max-Planck-Institut für Radioastronomie, Bonn, Germany
06/27/2016 – 60 years of Molecules, Motion and Matrix Elements (NIST)

2. The Lille THz spectrometer
10 – 18 GHz
Millitech AMC (x4) : 50 – 75 GHz
VDI AMC-10 (x6) : 75 – 110 GHz
VDI AMC-35x (x12) : 118 – 170 GHz x2 x3 x5
x2x3
x3x3
x3x3
f, GHz

3. The fast-scan box
Inspired by Medvedev et al. Optics Letters 35 (2010) 1533 – 1535
Agilent E8257D
Frequency switching time min: 25 ms/point
Frequency switching time typical: 35 ms/point
Time to cover 150 – 1500 GHz: ~ 180h
Agilent E8257D + Fast-scan box
Frequency switching time: 50 µs/point
Frequency switching time typical: 4 ms/point
Time to cover 150 – 1500 GHz: ~ 23h
~ 3 decrease in S/N ratio
Synthesized fast-scan – at each point the frequency is determined with the accuracy of the reference clock (frequency standard); line measurement accuracy – the same as for « slow » scan
The principle – up-conversion of direct digital synthesizer into RF
RF synthesizer provides frequency switching with large step once per scan
DDS synthesizer provides fast frequency switching with small frequency step
Typically, a scan consists to acquire points

4. The fast-scan box
No signal-to-noise difference between old and new setup when the acqusition time is the same

5. Garrod, Widicus Weaver & Herbst, ApJ, 682 : 283-302, 2008
Methoxymthanol - Astrophysical interest
Both CH3O and CH2OH may be formed from methanol photolysis in ices
Methoxymethanol (CH3OCH2OH) is predicted to form from reactions of CH3O and CH2OH in ices
CH3O detected in the ISM (Cernicharo et al., ApJL 759 : L43, 2012)
CH2OH is not yet detected
Garrod, Widicus Weaver & Herbst, ApJ, 682 : , 2008

6. Stable conformations MP2/aug-cc-pVQZ Gg+ Gg- Tg E (kJ/mol) 8.5 10.9
8.5
10.9
A (MHz)
B (MHz)
C (MHz)
µa (D)
0.22
0.77
1.48
µb (D)
0.08
1.22
0.97
µc (D)
0.13
2.0
1.26

7. Stable conformations Gauche-gauche Gauche-gauche’ Trans-gauche 0.51
E (kJ/mol)
8.5
10.9 K 1
0.033
0.012
µa (D)
0.22
0.77
1.48
Normalized intensities without taking the partition function into account: Ia
0.51
0.20
0.29
I ~ µ2×exp(-ħω/kT)

8. Experiment Sample synthesized by J.-C. Guillemin
Methoxymethanol unstable under room temperature
Spectra measured in flow mode
Pyrex absorption cell
CH3OH and H2CO as major impurities
THz source
Absorbtion cell
Detector
to diffusion +
rotary pump
Sample at ~ 250 K
At room temperature
In this study: ∆t = 1 ms/point due to weak intensities
Each spectrum averaged 8 times – effective acquisition time 8 ms/point
Frequency range: 150 – 470 MHz

9. CH3OH and H2CO as major impurities
Analysis
235 – 265 GHz, ~ points, recorded in 92 minutes
CH3OH and H2CO as major impurities

10. Analysis Gg: B+C = 10.506 GHz Tg: B+C = 8.485 GHz

11. Internal rotation Gg Gg’ V3 = 517 cm-1 V3 = 607 cm-1
B3LYP/ (3df, 2pd)
250,25 – 240,24 A
251,25 – 241,24 E Gg
Gg’
V3 = 517 cm-1
V3 = 607 cm-1
249,15 – 239,14 A
249,16 – 239,15 A
249,15 – 239,14 E
249,16 – 239,15 E
No symmetry: XIAM code used for fitting and predicting the spectra
XIAM allows fitting of structural parameters (angles ε and δ)

12. Internal rotation XIAM fit Theory Gg Theory Gg’ A (MHz) 17237.9491(12)
B (MHz)
(29)
C (MHz)
(33)
V3 (cm-1)
545.93(39)
517
607
δ (rad)
(40)
0.932
0.942
ε (rad)
0.3819(13)
0.397
0.359
+13 parameters
rms (MHz)
0.04
N lines
1189
J max, Ka max
48, 14
δ – the angle between the internal rotation axis and the principal axis z
ε – the angle between the principal axis x and the projection of the internal rotation axis onto xy-plane

13. Internal rotation Strong µa lines Theory Weak µb lines Weak µb lines
µa (D)
0.22
µb (D)
0.08
µc (D)
0.13
µa (D)
0.77
µb (D)
1.22
µc (D)
2.0 Gg
Gg’
Experiment

14. OH tunneling in Tg conformation
Splittings ~40 MHz
Theory
Experiment

15. OH tunneling in Tg conformation
J+12,J – J2,J-1
J’’ 19 20 21 22 23 24 25 26 27 28 29

16. OH tunneling in Tg conformation
J+12,J-1 – J2,J-2
J’’ 19 20 21 22 23 24 25 26 27 28 29

17. Hindered internal rotation of the hydroxyl group
Tunneling between the two equivalent Tg configurations split each rotational level into two components.
The analysis of rotational spectrum is complicated by Coriolis-type coupling between the tunneling substates
00,0
DE = 90.6 GHz
(3.02 cm-1) 0+ 0-
selection rules:
µa, µb: +  + & -  -
µc: +  -

18. The Hamiltonian and the global fit
Pickett’s Reduced Axis Method (RAM, Pickett H.M. J. Chem. Phys. 1972, 56, ) formalism was used
The Hamiltonian in the matrix form:
Where:
Hrot – Watson A-reduction Hamiltonian in the Ir coordinate representation
H∆ – the rotational dependence of the energy difference, it allows fitting a single set of rotational constants for both 0+ and 0- states
HI – Coriolis interaction terms and their centrifugal distortion corrections

19. The Hamiltonian and the global fit
The ∆E and F terms in the Hamiltonian are usually highly correlated
In the absence of v = 0  1 transitions one of the terms should be fixed
Usually it is better to fix ∆E and to let F to vary
We performed a series of fits with ∆E fixed in the range 60 – 200 GHz to find a global rms minimum
global rms
minimum
rms as a function of (11) parameter
(410001) as a function of (11) parameter
preliminary fit:
Parameter
Experiment
Theory
MP2/aug-cc-pVTZ
A (MHz)
(38)
B (MHz)
(267)
C (MHz)
(267)
+8 parameters
ΔE (MHz)
( 32)
Fbc
(264)
Fac
(165)
N lines
175
rms (MHz)
0.044
quest for µc transitions to decorrelate ∆E term

20. Assignment of Gg’ conformation
Experim. Gg
Theory Gg
Theory Gg’
Scaled Gg’
A (MHz)
B (MHz)
C (MHz)
J+10,J+1 – J0,J

21. Assignment of Gg’ conformation
B3LYP/ (3df, 2pd)

22. Results
The most stable Gg conformation of methoxymethanol assigned and analyzed
Accurate predictions of transition frequencies for the v=0 state of Gg conformation in the frequency range up to 500 GHz and for J up to 50
Gg’ and Tg conformations have pure spectroscopic interest
Tg conformation assigned, its spectrum exhibits splittings due to OH tunneling
Gg’ conformation tentatively assigned
A. Belloche: methoxymethanol is not detected in ALMA Sgr B2 (N2) survey due to high line blending. Its abundance upper limit is at least 4 times lower in comparison with methyl formate (to be corrected by vibrational partition function)
Hays & Widicus Weaver, JPCA 117: , 2013
Acknowledgements
Action sur Projets de l'INSU : "Physique et Chimie du Milieu Interstellaire"
ANR-13-BS IMOLABS