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
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
Previous and Present Work
• v = 0: Cazzoli & Puzzarini, JMSp 240 (2006) 153 (with hfs; + ref.)
+ Müller et al., A&A 506 (2009) 1487 (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) 178 + ref. (95 + 51 lines retained)
JPL & U Köln , 1028 + ~ 1218 new or rem. lines
• ν8: M. Koivusaari et al., JMSp 152 (1992) 377; 1697 lines; unc.:  0.00020 cm−1
PNNL spectrum not as good
• 2ν8: PNNL, p = 40 Pa, l = 19 m, res.: 0.0016 cm−1
1172 incl. 237 with l = ±2; unc.:  0.00020 cm−1
• ν4, ν7, 3ν8: parameters from A. M. Tolonen et al., JMSp 160 (1993) 198
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
The 28 Band of CH3CN
Detail of the 28 Band
Low-Lying Vibrational States of CH3CN
E / cm−1
v8 = 4
v7 = v8 = 1
v4 = v8 = 1
1200
v8 = 3
v7 = 1
v4 = 1
800
v8 = 2
400
v8 = 1
0
v=0
v3 = 1
v6 = 1
K Level Structure of v = 0 and v8 = 1
E / cm−1
1200
15
15
14
12
10
10
800
5
10
5
400
l = −1
l = +1
Aζ
5
0
v=0
q22
v8 = 1
Interaction between v = 0, K = 14 and v8 = 1+1, K = 12
Purely K-dependent Parameters1 (MHZ) of v = 0 and v8 = 1
present
(Δ)1 A − B
Anttila2
148900.103 (66) 148900.166 (78)
present
Koivusaari3
−115.930 (26)
−90.079 (36)
−18.11 (81)
(Δ) DK × 103
2830.6 (18)
2831.8 (45)
−11.46 (48)
(Δ) HK × 106
164.6 (66)
156. (72)
14.9 (22)
Aζ
−
−
ηK
−
−
ηKK × 106
−
−
−834.0 (41)
−45.3 (174)
ηKKK × 106
−
−
0.0
15.0 (9)
1) ΔX = X8 − X0;
2) JMSp 152 (1992) 377;
138656.195 (73)
10.3329 (72)
3) JMSp 157 (1993) 198.

Herb Pickett’s SPFIT/SPCAT was used
H. S. P. Müller et al., JMSp 312 (2015) 22
−215.0 (51)
138647.150 (72)
10.448 (93)
Interactions between v8 = 1 and v8 = 2
Detail of the K Level Structure
v8 = 2
v8 = 1
E / cm−1
1750
15
15
14
1500
14
10
15
13
13
11
1250
10
10
1000
l = +1
l = −1
l = +2
l=0
l = −2
Reduced Energy Diagram
K = 14, v8 = 1−1 & v8 = 2+2
K = 15, v8 = 1+1 & K = 13, v8 = 2+2
K = 13, v8 = 1−1 & K = 11, v8 = 20
Fortrat Diagram
K = 13, v8 = 1−1
K = 11, v8 = 20
K = 15, v8 = 1+1
K = 13, v8 = 2+2
Interaction Parameters (MHz) between v8  2
F(81,82)
FJ × 103
FJJ × 106
F2(0,81) × 103
F2(8−1,82,0) × 103
F2(8+1,8+2) × 103
CH3CN
53157.7 (33)2
−369.89 ( 44)
1.681 (87)
CH3CCH1
51744.8 (27)
−342.00 (69)
−3.96 (15)
−70.897 (27)
−
−65.491 (24)3
−130.982 (48)3
−
−
1) P. Pracna et al., Mol. Phys. 102 (2004) 1555.
2) FK/FJ ≈ A/B assumed.
3) Ratio constrained.
F + FJJ(J + 1) + FKJa2 + F2(J+2 + J−2) + . . .
IR Simulations (8) and Intensities
Transmittance
1
0.5
 –1
 +1
8
0
1
8
0.5
0
±1
0
320
–2
–1
+2 +1
2 8 –  8
2 8 –  8
340
360
380
400
–1
wavenumber (cm )
2 8 –  8
Intensities (10–19 cm/molecule)
8 region: 2.285 (1.77 (4), 1.81, 2.79 (28), )
28 region: 2.580 (2.50 (4), 3.43, , 2.63 (18))