Carbodiimide, HNCNH, is a high energy isomer (1400 cm1
or 2000 K) of the fairly typical hot core molecule cyanamide,
H2NCN. The molecule is thus very hard to detect under local
thermodynamic equilibrium (LTE) conditions. Nevertheless,
B. A. McGuire, R. A. Loomis, C. M. Charness, J. F. Corby, G. A. Blake, J. M. Hollis, F. J. Lovas, P. R. Jewell, and A. J. Remijan,
Interstellar Carbodiimide (HNCNH): A New Astronomical Detection from the GBT PRIMOS Survey via Maser Emission Features
Astrophys. J. 758, Art. No. L33 (2012).
The molecule displays tunneling splitting into two components separated by slightly less than 0.5 GHz. Four and a half pairs of transitions of the type J+10,J+1 ↔ J1,J with J between 15 and 19 would fall into the range of the 146 GHz line survey of Sagittarius B2(N), the frequencies of one pair (J = 16), however, fall into the 1617 GHz gap. Both components were seen for J = 17 and 18, one for J = 19. The maser activity was explained by the energy level structure and the Einstein A values. No maser activity was expected and seen for the J = 15, and no maser activity is expected for J = 16. The authors also suggest that HNCNH is chemically enhanced with respect to the gas phase equilibrium with H2NCN either via evaporation from water ice or via non-LTE formation in the gas phase (possibly via dissociative recombination of cations such as H2NCNH+).