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.