J/A+A/628/A53 Rotational spectroscopy of imidazole (Giuliano+, 2019)
Rotational spectroscopy of imidazole: improved rest frequencies for
astrophysical searches.
Giuliano B.M., Bizzocchi L., Pietropolli Charmet A., Arenas B.E.,
Steber A.L., Schnell M., Caselli P., Harris B.J., Pate B.H.,
Guillemin J.-C., Belloche A.
<Astron. Astrophys. 628, A53 (2019)>
=2019A&A...628A..53G 2019A&A...628A..53G (SIMBAD/NED BibCode)
ADC_Keywords: Atomic physics
Keywords: molecular data - methods: laboratory: molecular -
techniques: spectroscopic - radio lines: ISM
Abstract:
Organic ring compounds play a key role in terrestrial biochemistry,
and they were also most likely pivotal ingredients in Earth's
prebiotic chemistry. The five-membered ring imidazole, c-C3N2H4, is a
substructure of fundamental biological molecules such as the purine
nucleobases and the amino acid histidine.
An unsuccessful search for imidazole in a sample of cold-core clouds
and massive star-forming regions was performed almost 40 years ago. At
that time, the spectroscopic knowledge of this species was scarce: the
existing laboratory study was limited to the centimetre-wave region,
and the precision of the rest frequencies in the millimetre regime was
not adequate.
The goal of the present work is to perform a comprehensive
investigation of the rotational Spectrum of imidazole in its ground
vibrational state from the microwave region to the 1mm wavelength
regime.
The rotational spectrum of imidazole was recorded in selected
frequency regions from 2 to 295GHz. These intervals were covered
using various broadband spectrometers developed at DESY (Hamburg) and
at the University of Virginia. High-level ab initio calculations were
performed to obtain reliable estimates of the quartic and sextic
centrifugal distortion constants. We used the EMoCA imaging spectral
line survey to search for imidazole towards the hot molecular core Sgr
B2(N2).
About 700 rotational transitions spanning a J interval from 0 to 59
and Kc interval from 0 to 30 were analysed using the Watson S-reduced
Hamiltonian. These new data allowed the determination of a much
extended set of spectroscopic parameters for imidazole in its
vibrational ground state. The improved spectral data allow us to set
an upper limit to the column density of imidazole in SgrB2(N2). Its
non-detection implies that it is at least 3400 times less abundant
than ethyl cyanide in this source.
With the new set of spectroscopic constants, it has been possible to
compute reliable rest frequencies at millimetre wavelengths. We
suggest a search for imidazole towards TMC-1, where the aromatic
molecule benzonitrile was recently detected.
Description:
The rotational spectrum of imidazole was recorded across the
2-295GHz frequency range employing several broadband chirped-pulse
Fourier transform rotational spectrometers located at the Deutsches
Elektronen-Synchroton (DESY; Hamburg) and at the University of
Virginia. The Hamburg COMPACT spectrometer was used to record the
spectrum in the 2-8 and 12-15.5GHz regions.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 58 133 Assignments, measured line positions and least-squares
residuals for the analysed transitions of imidazole
with hyperfine structure
table2.dat 46 617 Assignments, measured line positions and least-squares
residuals for the analysed transitions of imidazole
without hyperfine structure
imida.dat 73 19584 CDMS-like file for imidazole without hyperfine
structure
imidahfs.dat 77 5122 CDMS-like file for imidazole with hyperfine structure
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- J1 Upper state J
4- 5 I2 --- Ka1 Upper state Ka
7- 8 I2 --- Kc1 Upper state Kc
10- 11 I2 -- F11 Upper state F1
13- 14 I2 -- F1 Upper state F
16- 17 I2 --- J0 Lower state J
19- 20 I2 --- Ka0 Lower state Ka
22- 23 I2 --- Kc0 Lower state Kc
25- 26 I2 -- F10 Lower state F1
28- 29 I2 -- F0 Lower state F
31- 42 F12.5 MHz Observed ?=- Measured line position
44- 51 F8.5 MHz O-C ?=- Least-squares residual (fit I)
53- 58 F6.4 --- Weight ? Relative weight in a line blend
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- J1 Upper state J
4- 5 I2 --- Ka1 Upper state Ka
7- 8 I2 --- Kc1 Upper state Kc
10- 11 I2 --- J0 Lower state J
13- 14 I2 --- Ka0 Lower state Ka
16- 17 I2 --- Kc0 Lower state Kc
19- 30 F12.5 MHz Observed ?=- Measured line position
32- 39 F8.5 MHz O-C ?=- Least-squares residual (fit I)
41- 46 F6.4 --- Weight ? Relative weight in a line blend
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Byte-by-byte Description of file: imida.dat imidahfs.dat
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Bytes Format Units Label Explanations
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3- 13 F11.4 MHz Freq Frequency of the line
15- 21 F7.4 MHz e_Freq uncertainty on the line
23- 29 F7.4 [nm+2.MHz] logInt base 10 logarithm of the integrated
intensity at 300K
31 I1 --- dof [3] Degree of freedom in the rotational
partition function (1)
33- 41 F9.4 cm-1 E0 Lower state energy
42- 44 I3 --- gup Upper state degeneracy
47- 52 I6 --- MolTag [686011] Molecule tag (2)
53- 55 I3 --- Code [403-405] Coding of the quantum numbers
56- 57 I2 --- J0 Lower state J
58- 59 I2 --- Ka0 Lower state Ka
60- 61 I2 --- Kc0 Lower state Kc
62- 63 I2 --- F10 ? Lower state F1
64- 65 I2 --- F0 ? Lower state F
68- 69 I2 --- J1 Upper state J
70- 71 I2 --- Ka1 Upper state Ka
72- 73 I2 --- Kc1 Upper state Kc
74- 75 I2 --- F11 ? Upper state F1
76- 77 I2 --- F1 ? Upper state F
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Note (1): Degree of freedom as follows:
0 = atoms
2 = linear molecules
3 = non-linear molecules
Note (2): The six digit molecule tag consists of the molecular weight in atomic
mass units for the first three digits (here: 2x1 + 12 + 18 = 32), a 0,
and the last two digits are used to differenciate between entries with the
same molecular weight.
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Acknowledgements:
Luca Bizzocchi, bizzocchi(at)mpe.mpg.de
(End) Patricia Vannier [CDS] 12-Jul-2019