The Microwave Spectrum of Monodeuterated Acetamide CH
2DC(=O)NH
2
I. A. Konov,a L. H. Coudert,b C. Gutle,b
T. R. Huet,c L. Margulès,c R. A. Motiyenko,c
H. Mollendal,d and J.-C. Gillemine
aDepartment of Physics, Tomsk State University, Tomsk, RussiabLISA, CNRS/Universités Paris Est et Paris Diderot, Créteil, FrancecPhLAM, CNRS/Université de Lille I, Villeneuve d’Ascq, FrancedCTCC, Dept. of Chemistry, University of Oslo, Oslo, NorwayeISCR, UMR 6226, Rennes, France
CH2DC(=O)NH
2 is of astrophysical relevance &
theoretically interesting
• Normal species detected in Sagittarius B2(N)1
• Almost free internal rotation of its CH2D
methyl group
• Oblate asymmetric top
1. Hollis, Lovas, Remijan, Jewell, Ilyushin, and Kleiner, Astr. J. 643 (2006) L25
Overview
• Hindering potential changes upon deuteration
• Torsional energy levels
• Available microwave data
• Tentative assignment & fit
The hindering potential of the normal species
1. Ilyushin, Alekseev, Dyubko, Kleiner, and Hougen, J. Mol. Spec. 227 (2004) 115
Deuteration effects: potential energy function
Upon deuteration the potential energy function remains an even function of α but no longer has 2π/3 periodicity, only 2π.
Vi' coefficients are unknown.
Deuteration effects: potential energy function
For deuterated methanol,1 V3i' = V
3i, and
1. Lauvergnat, Coudert, Klee, and Smirnov, J. Mol. Spec. 256 (2009) 204
2. Ilyushin, Alekseev, Dyubko, Kleiner, and Hougen, J. Mol. Spec. 227 (2004) 115
{ V1' = 9.95 cm-1 CH
2D
V1' = -10.36 cm-1 CD
2H
In the present case,2 V3' = 25.043, V
6' = -10.048, and V
1' = ±10 cm-1
Deuteration effects: potential energy function
V1 = +10 cm-1
V1 = -10 cm-1
Deuteration effects: torsional energy levels
Normalspecies V
1 = 0
V1 = -10 cm-1V
1 = +10 cm-1
Quade and Suenram, J. Chem. Phys. 73 (1980) 1127
Torsion-rotation energy levels
Torsion-rotation energies are calculated using the model developed for CH
2DOH.1 It depends on 8 kinetic energy
parameters describing the 4x4 generalized intertia tensor, on 6 potential energy parameters V
1, V
2, V
3, …, and on distortion
parameters.
1. Paper RF10, Columbus 2013; and Coudert, Zemouli, Motiyenko, Margulès, and Klee,
J. Chem. Phys. 140 (2014) 064307
Torsion-rotation energy levels
Due to the fact that acetamide:• Oblate asymmetric top• Axis of internal rotation is // to the a-axis
It is more difficult to understand torsion-rotation energy levels than in methanol. A J-dependence arises in addition to the K-dependence.
Torsion-rotation energy levels
V1 = +10 cm-1
e0 torsional state: J
0,J J
1,J-1 J
2,J-2 J
3,J-3 J
4,J-4
V1 = -10 cm-1
V1 can be determined analyzing the microwave data
The microwave spectra
7 80
StarkModulation
75 91
MillimeterWave
150 165
MillimeterWave
Frequencies are in GHz
5.8 19
MolecularBeam
3 room temperature spectra
Cold temperature spectrum
The Stark modulation spectrum
Line assignment will be an issue
First assignments
14 transitions were assigned in the cold spectrum.
Their assignment in terms of rotational quantum numbers was performed with the help of the hyperfine structure.
Due to the low temperature in the beam they were assigned to the e
0 torsional state.
Assignment problem
When J increases, clusters of lines characterized by the same K
c-value arise leading to many
broadened transitions in the spectrum.
Assigning the room temperature spectra
The 3 room temperature spectra were assigned using a bootstrap method with the Watson-type Hamiltonian.
305 transitions could be assigned up to J = 23 and Ka = 10 for
the e0 torsional state.
The RMS of 0.9 MHz.
The fit degrades when Ka increases.
Torsion-Rotation Hamiltonian fit
171 transitions with J ≤ 12 and Ka ≤ 7 were fitted with the
Torsion-Rotation Hamiltonian.1 The RMS of the fit is 0.7 MHz and 21 parameters were determined.
We did not try to go to higher J- or Ka-values because we are not
sure about the line assignment. Labeling the torsion-rotation levels arising from the Torsion-Rotation Hamiltonian was also a problem.2
1. Paper RF10, Columbus 2013; and Coudert, Zemouli, Motiyenko, Margulès, and Klee,
J. Chem. Phys. 140 (2014) 064307
2. Ilyushin, Alekseev, Dyubko, Kleiner, and Hougen, J. Mol. Spec. 227 (2004) 115
Torsion-Rotation Hamiltonian fit
aConstrained value.
56.9102.5
48.7-2.0
Torsion-Rotation Hamiltonian fit
Deuteration effects are dominant.
Conclusion
Assignment of the spectrum is an issue.
We need to identify transitions involving o1 and e
1.