1. Rotational Spectroscopy of the Lowest Energy Conformer of 2-Cyanobutane and Search for it in Sagittarius B2(N2)
H. S. P. Müller, N. Wehres, O. Zingsheim, F. Lewen, S. Schlemmer,
J.-U. Grabow, R. T. Garrod, A. Belloche, K. M. Menten
72nd ISMS, Urbana-Champaign, IL, 19 – 23 June 2017, MF03

2. Motivation ratio i-PrCN : n-PrCN: 0.40 ± 0.06 BuCN may be observable
ALMA Cycles 0/1 survey at 3 mm toward Sgr B2(N),
i-PrCN detected toward Sgr B2(N2);
ratio i-PrCN : n-PrCN: ± 0.06
A. Belloche et al., Science 345 (2014) 1584–1587
BuCN may be observable
4 isomers; rot. spectra of 2 known: n- & t-BuCN
astrochemical modeling (Garrod et al., A&A 601 (2017) Art. No. A48):
some reproduced i- to n- ratio & BuCN not much less than i-PrCN;
branched 2-cyanobutane most abundant isomer,
n-BuCN and 3-methylbutyronitrile slightly less, t-BuCN almost negligible
Spectra of 2-cyanobutane and 3-methylbutyronitrile taken

3. Conformers of 2-CyanobutaneCN
CH3 H
2-cyano-anti-butane
(2-methyl-gauche'-butyronitrile)
ΔE = 0 cm−1
H3C
2-cyano-gauche-butane
(2-methyl-anti-butyronitrile)
~249 cm−1
2-cyano-gauche'-butane
(2-methyl-gauche-butyronitrile)
~283 cm−1
Energies from molecular mechanics calculations (+ IR & Ra + FF):
G. A. Crowder & G. O. Carlisle, J. Comp. Chem. 12 (1991) 880

4. Model of 2-Cyano-anti-butane

5. Recorded Spectra in Hannover
chirped pulse FTMW measurements with jet expansion (Trot a few K)
1 GHz wide scans around 14.9, 15.9, 16.7, 17.0, and 22.7 GHz
cavity FTMW measurements with jet expansion (Trot a few K)
2 − 24 GHz near lines of anti conformer (~ 1 MHz wide scans)

6. Chirped Pulse FTMW Spectrum
calculated stick 3 K ≤ 26.5 GHz
b-, a-, a-, b-type; Kc" = J" = 3, Ka = 0, 1

7. Rotational and Quartic Distortion Parameters
Exptl.
B3LYP
MP2
MP3
A (MHz)
4070
.37
4104
.35
4073
.64
4097
.39
B (MHz)
3145
.92
3093
.05
3184
.78
3148
.54
C (MHz)
1919
.40
196
.51
1935
.93
1927
.03
DK (kHz) 1
.942
.831 2
.015
DJK (kHz)
­−4
.280
.097
.422
DJ (kHz)
.572
.495
.649
d1 (Hz)
−37
.26 15
−83
.52
d2 (Hz)
−561 .4
−569 .2
−604 .5 κ
.1404
.0797
.1684
.1256
κ = (2B – A – C)/(A – C)
μa: 2.3 D, μb: 3.3 D, μc: 0.6 D

8. The 110 − 101 Transition with HFS

9. 14N Nuclear Quadrupole Coupling Parameters
Exptl.
B3LYP
MP2
MP3//MP2
MP3
χaa
0.296
0.201
0.269
0.288
0.257
χbb
−2.144
−2.270
−1.957
−2.181
−2.272
χcc
1.849
2.069
1.688
1.893
2.015
χab
2.90
3.145
2.559
2.840
3.052
basis set: aug-cc-pVTZ; except MP3: cc-pVTZ

10. Recorded Spectra in Köln and Overview of Results
2 x 7 m long double path glass cells; RT (heated); Schottky diode detector
► 36.0 − 47.7 & 52.0 − 65.0 GHz: synthesizer
J ≤ 87, Ka ≤ 47; R, Q; (HFS)
► 90 − 126 GHz: synthesizer + tripler (VDI)
J ≤ 100, Ka ≤ 56; R, Q
5 m long double path glass cells; RT (heated); Schottky diode detector
18 = 3, Verstärker, 3, 2
36 = 3, Verstärker, 3, 2, 2
► 175 − 200 GHz: synthesizer + multiplier (x18; VDI)
J ≤ 11, Ka ≤ 73; R, Q
► 360 − 401 GHz: synthesizer + multiplier (x36; VDI)
J ≤ 104, Ka ≤ 68; R

11. Detail of the Millimeter Wave Spectrum of 2-Cyanobutane

12. Search for 2-Cyanobutane in Sgr B2(N2)
ALMA Cycle 0 and 1 survey of Sgr B2(N) at 3 mm (84.0 − GHz)
Exploring Molecular Complexity with ALMA (EMoCA)
previous talk and Belloche et al., A&A 587 (2016) Art. No. A91
Assuming Trot = 150 K and a source size of 1", as for i- and n-PrCN
(Belloche et al., Science 345 (2014) 1584–1587):
i-PrCN : 2-CAB > & n-PrCN : 2-CAB > 16

13. Conclusion and Outlook
Ground vibrational state of lowest conformer of 2-cyanobutane characterized
Lines identified for v39 = 1 and for 2nd lowest conformer
Ground vib. states of two conformers of 3-methylbutyronitrile almost done
Search for 3rd conformer of 2-cyanobutane (+ vib. states) continues
2-Cyanobutane and other BuCN isomers may be found in new ALMA data;
higher sensitivity and higher spatial resolution observations partially taken
in Cycle 4

14. Acknowledgments Marius Hermanns (help during measurements in Hannover)
Support by Deutsche Forschungsgemeinschaft (DFG)
via collaborative research grant SFB 956
"Conditions and Impact of Star Formation", project area B3
OZ is grateful for a stipend from the Bonn-Cologne Graduate School
of Physics and Astronomy (BCGS)

15. The Star-forming Region Sagittarius B2
most massive star-forming region
in our Galaxy (~ 107 M)
~100 pc from Galactic Center
2 massive clumps, (M) and (N),
hosting clusters of UC HII regions
Central Molecular Zone at 870 µm
ATLASGAL/LABOCA & Planck; © MPIfR/A. Weiß
Sgr B2(N)
Sgr B2(N) at 850 µm (SMA; Qin et al., 2011)
2 hot cores: N1 (or LMH) & N2
different vlsr (10 km/s); 5" apart (0.2 pc)
many COMs first detected there
very high column densities (> 1025 cm–2)
key for detection of COMs

16. Exploring Molecular Complexity with ALMA
3 mm spectral line survey of Sgr B2(N) in Cycles 0 and 1 (84.0 – GHz)
angular resolution: 1.6" (median)
~ 12 % der Linien > 3 K sind U; zu vermuten: hauptsächlich höhere Zustände von EtCN und VyCN.
sensitivity to compact emission: factor ~20 compared to our IRAM 30 m survey
status: analysis ongoing; published: detection of i-PrCN (Science, 2014);
deuterated COMs, ROH & RSH, C2H5CN with 2 13C (all A&A, 2016)