On the Detection of Methyl Cyanide, CH3CN, in Space

Methyl cyanide, CH3CN, also known as acetonitrile or cyanomethane, was among the molecules detected early by radioastronomical means. The J = 6 – 5 transitions with Ka = 0 to 5 were observed with the 36 foot NRAO antenna at 2.7 mm toward Sagittarius B; some lines were probably also detected toward Sgr A where the lines were much broader.
P. M. Solomon, K. B. Jefferts, A. A. Penzias, and R. W. Wilson,
Detection of Millimeter Emission Lines from Interstellar Methyl Cyanide
Astrophys. J. 168, L107–L110 (1971).

The molecule was also detected toward dark clouds:
H. E. Matthews and T. J. Sears,
Detection of the J = 1 – 0 transition of CH3CN
Astrophys. J. 267, L53–L57 (1983).

More recently, it was detected toward a low mass proto-star:
S. Cazaux, A. G. G. M. Tielens, C. Ceccarelli, A. Castets, V. Wakelam, E. Caux, B. Parise, D. Teyssier,
The Hot Core around the Low-mass Protostar IRAS 16293-2422: Scoundrels Rule!
Astrophys. J. 593, L51–L55 (2003).

V. Thiel, A. Belloche, K. M. Menten, R. T. Garrod, and H. S. P. Müller,
reported on
Complex Organic Molecules in Diffuse Clouds along the Line of Sight to Sgr B2
Astron. Astrophys. 605 (2017), Art. No. L6.
The J = 5 – 4 and 6 – 5, K = 0 transitions of CH3CN were detected with ALMA near 92.0 and 110.4 GHz in absorption in two velocity components corresponding to Galactic center diffuse clouds and in one velocity component corresponding to diffuse clouds in the Scutum arm. The absorption was detected toward the ultracompact H II region K4 in the Sagittarius B2(N) complex.


CH313CN was probably detected because its lines are very close to those of the main isotopolog:
S. E. Cummins, S. Green, P. Thaddeus, and R. A. Linke,
The Kinetic Temperature and Density of the Sagittarius B2 Molecular Cloud from Observations of Methyl Cyanide
Astrophys. J. 266, 331–338 (1983);
both 13C species were detected by
E. C. Sutton, G. A. Blake, C. R. Masson, and T. G. Phillips,
Molecular Line Survey of Orion A from 215 to 247 GHz
Astrophys. J. Suppl. Ser. 58, 341–378 (1985).

CH2DCN was detected in several transitions at 2 and 1.3 mm with the IRAM 30 m telecope toward the IRc2 hot core of the Orion Molecular Cloud. The CH2DCN/CH3CN ratio is roughly 0.01, suggesting considerable molecular deuterium enrichment even in hot cores. Note that the H/D ratio is lower by a factor of three because of the three equivalent H atoms in the parent species.
M. Gerin, F. Combes, G. Wlodarczak, T. Jacq, M. Guélin, P. Encrenaz, and C. Laurent,
Interstellar Detection of Deuterated Methyl Cyanide
Astron. Astrophys. 259, L35–L38 (1992).

CH3C15N was detected first toward Sgr B2(N) in the course of a line survey around 1.3 mm with SEST by
A. Nummelin, P. Bergman, Å. Hjalmarson, P. Friberg, W. M. Irvine, T. J. Millar, M. Ohishi, and S. Saito,
A Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHz. I. The Observational Data
Astrophys. J. Suppl. Ser. 117, 427–529 (1998).

A. Belloche, H. S. P. Müller, R. T. Garrod, and K. M. Menten
were
Exploring Molecular Complexity with ALMA (EMoCA): Deuterated Complex Organic Molecules in Sagittarius B2(N2)
Astron. Astrophys. 587, Art. No. A91 (2016).
They detected 13CH313CN unumbiguously about a factor of 480 lower than the main species. They also observed CH2DCN with a ratio of about 0.004 with respect to CH3CN.


CH3CN in its lowest v8 = 1 excited vibrational state was detected with the FCRAO near 111 GHz by
P. F. Goldsmith, R. Krotkov, R. L. Snell, R. D. Brown, and P. Godfrey,
Vibrationally excited CH3CN and HC3N in Orion
Astrophys. J. 274, 184–194 (1983).

S. M. Fortman, J. P. McMillan, C. F.Neese, S. K. Randall, A. J. Remijan, T. L. Wilson, and F. C. De Lucia
gave a brief account on the detection of v8 = 2 in their work
An analysis of a preliminary ALMA Orion KL spectrum via the use of complete experimental spectra from the laboratory
J. Mol. Spectrosc. 280, 11–20 (2012).

A. Belloche, H. S. P. Müller, K. M. Menten, P. Schilke, and C. Comito
investigated
Complex Organic Molecules in the Interstellar Medium: IRAM 30 m Line Survey of Sagittarius B2(N) and (M)
Astron. Astrophys. 559, Art. No. A47 (2013).
Besides observing these two lower lying vibrational states, they detected transitions pertaining to v4 = 1, the next higher state, as well as transitions of the 13C isotopologs in their v8 = 1 vibrational states.


Contributor(s): H. S. P. Müller 01, 2010; 10, 2017