1. Rotational spectroscopy as a tool to investigate interactions between vibrational polyads in symmetric top molecules: low-lying states v8  2 of methyl cyanide
Holger S. P. Müller, M. Ordu, F. Lewen, L. R. Brown,
B. J. Drouin, J. C. Pearson, K. Sung, I. Kleiner, R. L. Sams
70th ISMS, Urbana-Champaign, IL, 22 – 26 June 2015, FB04

2. Motivation important molecule in ISM, especially hot cores, and CSE
− many isotopic species detected (including CH2DCN, 13CH313CN)
− transitions of vibrationally excited states detected (up to v4 = 1)
− v8 transitions detected for 13C species
− used as temperature probe
− considered to be a weed species
interaction between polyads known e.g. for propyne, CH3CCH,
prominent: Δv10 = ±1, ΔK = 0, Δl = ±3

3. Previous and Present Work
v = 0: Cazzoli & Puzzarini, JMSp 240 (2006) 153 (with hfs; + ref.)
+ Müller et al., A&A 506 (2009) (Σ 196 lines retained)
JPL & U Köln: 120 new or rem. lines
5 × ΔK = 3 (R. Antilla, JMSp 157 (1993) 198)
v8 = 1, 2: R. Bocquet et al., JMSp 127 (1988) ref. ( lines retained)
JPL & U Köln , ~ 1218 new or rem. lines
ν8: M. Koivusaari et al., JMSp 152 (1992) 377; 1697 lines; unc.:  cm−1
PNNL spectrum not as good
2ν8: PNNL, p = 40 Pa, l = 19 m, res.: cm−1
1172 incl. 237 with l = ±2; unc.:  cm−1
ν4, ν7, 3ν8: parameters from A. M. Tolonen et al., JMSp 160 (1993) 198

4. Present Rotational Spectroscopy of CH3CN
measurements at JPL:
much of 400 − 1200, ~1600 GHz
typical unc.: 50 kHz (10 – 200 kHz)
measurements at U Köln:
~37, 55, 74 GHz, parts of 1330 – 1501 GHz
typical unc.: 10 /30 kHz

5. The 28 Band of CH3CN

6. Detail of the 28 Band

7. Low-Lying Vibrational States of CH3CN
1200
800
400
E / cm−1
v8 = 4
v8 = 3
v8 = 2
v8 = 1
v = 0
v4 = 1
v4 = v8 = 1
v7 = 1
v7 = v8 = 1
v3 = 1
v6 = 1

8. K Level Structure of v = 0 and v8 = 1
400
800
1200
E / cm−1 5 10 15
v8 = 1
l = +1
l = −1 Aζ
q22 5 10 15 14 12
v = 0

9. Interaction between v = 0, K = 14 and v8 = 1+1, K = 12

10. Purely K-dependent Parameters1 (MHZ) of v = 0 and v8 = 1
Herb Pickett’s SPFIT/SPCAT was used
H. S. P. Müller et al., JMSp 312 (2015) 22

11. Interactions between v8 = 1 and v8 = 2 Detail of the K Level Structure
1000
1250
1500
1750
E / cm−1 10
l = 0 15
l = +1
l = −1 15
l = −2 10 10 15
l = +2 14 15 13 11 13

12. Reduced Energy Diagram
K = 14, v8 = 1−1 & v8 = 2+2
K = 13, v8 = 1−1 & K = 11, v8 = 20
K = 15, v8 = 1+1 & K = 13, v8 = 2+2

13. Fortrat Diagram K = 13, v8 = 1−1 K = 11, v8 = 20 K = 15, v8 = 1+1

14. Interaction Parameters (MHz) between v8  2

15. IR Simulations (8) and Intensities
0.5 1 n 8 +1 –1
320
340
360
380
400
wavenumber (cm )
Transmittance 2 – +2 –2 ±1
Intensities (10–19 cm/molecule)
8 region: (1.77 (4), 1.81, 2.79 (28), )
28 region: (2.50 (4), 3.43, , 2.63 (18))