1. An Analysis of the Rotation Spectrum of Acetonitrile (CH3CN) in Excited Vibrational States
Christopher F. Neese, James McMillian, Sarah Fortman, Frank C. De Lucia

2. Highly Sensitive Modern Telescopes
The success of radio astronomy has lead to bigger and better spectrometers such as
Atacama Large Millimeter Array (ALMA)
Herschel Space Observatory
The spectrometers are highly sensitive and spectra contain many unidentified lines.
Many of these lines are from the low-lying vibrational states of a few common carriers
We are starting to observe regions such as hot molecular cores that are warmer than 100 K.

3. Astrochemical Weeds Sulfur Dioxide Acetonitrile (Methyl cyanide)
Acrylonitrile (Vinyl cyanaide)
Propionitrile (Ethyl cyanide)
Methanol
Methyl formate
Dimethyl ether

4. Complete Temperature Resolved Spectrum
Record multiple spectra over a slow temperature ramp.
Complete
Doppler-limited lineshape
Carefully process spectra to calculate 𝐴(𝜈).
Temperature dependent background
Calculate a temperature 𝑇 and a scaled column density 𝑁 =𝑛𝐿/𝑄(𝑇) using carefully selected reference lines from the catalogs.

5. The 600-650 GHz Band Complete Spectrum at 300K Data Taken: Processing:
Temperature Range: K
150 Spectral Averages
~1K per Spectral Average
Processing:
Spectroscopically determine number density and temperature from Q.M. catalogs
Fit data to acquire parameters to generate CTRS at arbitrary temperatures
Result:
~1% intensity error in large lines
Returns lower state energies and line strengths for each line

6. Complete Temperature Resolved Spectrum
Acetonitrile
210.0 – GHz
406 Spectra
247 – 385 K
J = 11 – 13

7. Baseline Removal

8. Lower State Energies For ALL Lines
2𝜈 8
𝜈 4
𝜈 7
3𝜈 8
Displayed: Lines of S/N ≥ 3 at 300K
Catalog lines removed

9. Spectroscopic Details
Sym.
No.
Desc.
Freq / cm-1 a1 1
CH3 s-str
2954 2
CN str
2267 3
CH3 s-deform
1385 4
CC str
920 e 5
CH3 d-str
3009 6
CH3 d-deform
1448 7
CH3 rock
1041 8
CCN bend
362
Three-fold symmetric top
𝐶 3𝑣
mod 𝐾−ℓ ,3 = a 1 ⊕ a 2
mod 𝐾−ℓ ,3 ≠ e
𝜇 𝑎 = (13) D
Rotational selection rules: Δ𝐾=0 Δℓ=0
2:1 spin statistics

10. Survey Spectrum
Room Temperature
164.5 – GHz
J = 8 – 37

11. 𝜈 4

12. 3 𝜈 8

13. Resonances

14. Modeling |𝐾−ℓ| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ℓ 𝜈 8 -1 fit obs1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ℓ
𝜈 8 -1
fit
obs
2 𝜈 8 -2
𝜈 4
𝜈 7
3 𝜈 8 -3
𝝂 4 + 𝝂 8

15. Summary Complete Temperature Resolved Spectra
Provides catalog data for all lines (above an intensity threshold) without a complete QM assignment.
Two bands: 210 – 270 GHz, 600 – 650 GHz
Improved algorithm for modeling baseline with spline functions
Developed procedure for decontaminating spectra
Traditional QM assignment and modelling
Acetonitrile is a good candidate for benchmarking CTRS approach, because we can assign many hot bands.
Assignment is aided by low partition function and rigidity, but hindered by a large number of resonances and unfavorable selection rules.
Infrared line centers would help…
We have assignments and a reasonable model for lines up to 3 𝜈 8 .
We have signal to noise to see the next set of bands ( 𝜈 4 + 𝜈 8 , 𝜈 7 + 𝜈 8 , 4 𝜈 8 , 𝜈 6 )
Can we use a temperature ramp to improve accuracy of lower state energy and aid QM assignment and modeling? Fortman, et al. Chem. Phys. Lett. 493, 212–215 (2010)

16. Acknowledgements
We would like to thank the National Science Foundation and the National Aeronautics and Space Administration for their support of this work.