Formamide formation routes in the interstellar medium from electronic structure and kinetics calculations
- Dr. Dimitrios SKOUTERIS
- Dr. Dimitrios SKOUTERIS (Scuola Normale Superiore)
- Prof. Balucani NADIA (DCBB-University of Perugia)
- Prof. Cecilia CECCARELLI (IPAG, Universitè J.Fourier de Grenoble)
- Ms. Fanny VAZART (Scuola Normale Superiore, 56125 Pisa, Italy)
- Dr. Camille LATOUCHE (Scuola Normale Superiore, 56125 Pisa, Italy)
- Prof. Vincenzo BARONE (Scuola Normale Superiore, 56125 Pisa, Italy)
Formamide has been detected a long time ago in the direction of Sgr B2 . A systematic search of this molecule has been recently achieved with the IRAM 30-m telescope . In that study formamide was detected in five (IRAS 4A, IRAS 16293, SVS13A, Cep E, and OMC-2) objects, while it was not detected in other colder sources devoid of hot corinos . This was explained by invoking that NH2CHO (i) forms in the gas phase at temperatures above ∼ 100 K, and/or (ii) forms predominantly on the icy mantles of dust grains at low temperatures, and subsequently sublimates into the gas-phase when the temperature in the inner regions rises .
The reaction NH2 + H2CO has been already considered and disregarded as a possible formamide formation route by Garrod et al.  who estimated an entrance barrier of ca. 1000 K. Our calculations at the CBS-QB3 level, instead, demonstrate that this reaction is characterized by a slightly submerged barrier in the entrance channel. Moreover, there is an exit barrier in the channel leading to NH2COH + H slightly above the reactants asymptote. However, this barrier is so low that it is easily surmounted by the tunneling effect. This mechanism explains very well the abundance of formamide observed in two very different interstellar objects: the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular shock L1157-B2. Therefore, there is no need to invoke grain-surface chemistry to explain the presence of formamide provided that its precursors, NH2 and H2CO, are available in the gas-phase.
An alternative entrance pathway is the OH + CH2NH reaction which is also characterized by a very slightly submerged barrier. The two pathways are compared and contrasted from the RRKM viewpoint.
 A. Lopez-Sepulcre et al. 2015, MNRAS 449, 2438
 R. H. Rubin et al. 1971, Astrophys. J. Lett., 169, L39
 R. T. Garrod et al. 2008, Astrophys. J., 682, 283
 V. Barone et al. 2015, MNRAS Letters, in press.