On the Detection of Methyl Formate, HC(O)OCH3, in the ISM

Methyl formate, HC(O)OCH3, is a typical hot core molecule. It is very abundant in massive star-forming regions. It was detected first in emission in Sgr B2 by
R. D. Brown, J. G. Crofts, P. D. Godfrey, F. F. Gardner, B. J. Robinson, and J. B. Whiteoak,
Discovery of Interstellar Methyl Formate
Astrophys. J. 197, L29–L31 (1975).
The A component of the 110 – 111 near 1610 GHz was clearly seen with the Parkes 64 m telescope, and there was evidence for the E component close by.

More recently, it has also been detected toward low-mass star-forming regions:
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).

The molecule was also detected in several Galactic center molecular clouds which have low dust temperatures (10–20 K), even lower rotational temperatures (around 10 K), which are moderately dense (a few tenthousand molecules per cubic centimeter), but which are kinetically moderately warm (about 100 K or more). Several transitions were detected with the IRAM 30 m telescope in the 3 mm range by
M. A. Requena-Torres, J. Martín-Pintado, A. Rodríguez-Franco, S. Martín, N. J. Rodríguez-Fernández, and P. de Vicente,
Organic Molecules in the Galactic Center – Hot Core Chemistry without Hot Cores
Astron. Astrophys. 455, 971–985 (2006).

In addition, the identification of torsionally excited HC(O)OCH3 was reported in
K. Kobayashi, K. Ogata, S. Tsunekawa, and S. Takano,
Torsionally Excited Methyl Formate in Orion KL
Astrophys. J. 657, L17–L19 (2007).
20 U-lines from four different line surveys in the frequency range 97 to 160 GHz were assigned.

More recently,
S. Takano, Y. Sakai, S. Kakimoto, M. Sasaki, and K. Kobayashi reported on the
Detection of Methyl Formate in the Second Torsionally Excited State (vt = 2) in Orion KL
Publ. Astron. Soc. Japan 64, Art. No. 89 (2012).
Four new lines were observed newly with the Nobeyama 45 m radio telescope near 90 GHz together with several transitions of the ground and the first excited torsional state. 15 additional lines were detected in two published line surveys. The derived vibrational temperature of about 125 K was higher than the rotational temperature of around 50 K, possibly indicating infrared pumping. Since the torsional modes are rather weak, it may be more likely that infrared pumping took place via some small amplitude vibration and subsequent intermolecular energy redistribution. However, as the authors indicated, collisional excitation cannot be ruled out a being responsible for the vibrational temperature being apparently higher than the rotational one.

The detection of both 13C isotopomers of methyl formate was reported by
M. Carvajal, L. Margulès, B. Tercero, K. Demyk, I. Kleiner, J. C. Guillemin, V. Lattanzi, A. Walters, J. Demaison, G. Wlodarczak, T. R. Huet, H. Møllendal, V. V. Ilyushin, and J. Cernicharo,
Rotational Spectrum of 13C2-Methyl Formate (HCOO13CH3) and Detection of the Two 13C-Methyl Formate in Orion
Astron. Astrophys. 500, 1109–1118 (2009).

L. Margulès, T. R. Huet, J. Demaison, M. Carvajal, I. Kleiner, H. Møllendal, B. Tercero, N. Marcelino, and J. Cernicharo
reported on the
Rotational Spectrum and Tentative Detection of DCOOCH3-Methyl Formate in Orion
Astrophys. J. 714, 1120–1132 (2010).
Their Orion KL molecular line survey carried out with the IRAM 30 m telescope was used to search for the acid-deuterated isotopolog. The tentative D to H ratio of 0.02 to 0.06 in the compact ridge part of the source is not unusual.

K. Demyk, S. Bottinelli, E. Caux, C. Vastel, C. Ceccarelli, C. Kahane, and A. Castets
reported shortly thereafter on
TIMASSS: the IRAS16293-2422 Millimeter and Submillimeter Spectral Survey: Tentative Detection of Deuterated Methyl Formate (DCOOCH3)
Astron. Astrophys. 517, Art. No. A17 (2010).
The degree of deuteration is similar in both sub-sources A and B, about 0.06.

B. Tercero, L. Margulès, M. Carvajal, R. A. Motiyenko, T. R. Huet, E. A. Alekseev, I. Kleiner, J. C. Guillemin, H. Møllendal, and J. Cernicharo
reported on the
Microwave and Submillimeter Spectroscopy and First ISM Detection of 18O-Methyl Formate
Astron. Astrophys. 538, Art. No. A119 (2012).
Their Orion KL molecular line survey carried out with the IRAM 30 m telescope was used again to search for the 18O isotopomers. The 13C to 18O of about 13 is rather reasonable. The 12C to 13C and 16O to 18O appear to be too small by a factor of about two, likely because of neglect of opacity.

L. H. Coudert, B. J. Drouin, B. Tercero, J. Cernicharo, J.-C. Guillemin, R. A. Motiyenko, and L. Margulès
reported on
The First Astrophysical Detection, Terahertz Spectrum, and Database for the Monodeuterated Species of Methyl Formate HCOOCH2D
Astrophys. J. 779, Art. No. 119 (2013).
Their Orion KL molecular line survey carried out with the IRAM 30 m telescope was used once more to search for the methyl-deuterated isotopolog. The D to H ratio of 0.02 to 0.06 in the compact ridge part of the source is not unusual, but is three times lower per H atom than the tentatively derived value for the acid-deuterated isotopolog.

The so-called trans conformer, with the carbonyl oxygen pointing away from the methyl group, is very much higher (about 25 kJ/mol or about 3000 K) in energy than the so-called cis conformer. Nevertheless, it has been characterized in the laboratory recently and was detected tentatively in the cooler, probably more diffuse parts of Sagittarius B2(N) where the chemistry deviates significantly from LTE. The work has been described by
J. L. Neill, M. T. Muckle, D. P. Zaleski, A. L. Steber, B. H. Pate, V. Lattanzi, S. Spezzano, M. C. McCarthy, and A. J. Remijan,
Laboratory and Tentative Interstellar Detection of Trans-Methyl Formate Using the Publicly Available Green Bank Telescope Primos Survey
Astrophys. J. 755, Art. No. 153 (2012).
The line survey covers the 1 to 50 GHz region. The three K = 0 transitions with J" = 0 to 2 were detected for both A and E internal rotation components. In addition, one K = 1 was detected for the A component. Two additional K = 1 transitions each were barely recognizable in the noise, the intensities being compatible with the model. The detection is probably not far from a definitive one.

Contributor(s): H. S. P. Müller; 02, 2007; 04, 2011; 07, 2012; 08, 2012; 11, 2016