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

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.

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

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.

The 600-650 GHz Band Complete Spectrum at 300K Data Taken: Processing: Temperature Range: 230-390 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

Complete Temperature Resolved Spectrum Acetonitrile 210.0 – 270.0 GHz 406 Spectra 247 – 385 K J = 11 – 13

Baseline Removal

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

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 =0 a 1 ⊕ a 2 mod 𝐾−ℓ ,3 ≠0 e 𝜇 𝑎 = 3.92197(13) D Rotational selection rules: Δ𝐾=0 Δℓ=0 2:1 spin statistics

Survey Spectrum Room Temperature 164.5 – 717.8 GHz J = 8 – 37

𝜈 4

3 𝜈 8

Resonances

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

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)

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