"Metastability" has drawn special attention as a means of energy storage, and much theoretical and experimental effort has been devoted to the study of the isoelectronic N 4 , N 2 CO, and C 2 O 2 molecules. [1][2][3] The largest amounts of energy released per mass unit by dissociation of tetraazatetrahedrane N 4 (T d ; 200 kcal mol À1 ), diazirinone N 2 CO (100 kcal mol À1 ), ethylenedione C 2 O 2 (70 kcal mol À1 ), and other theoretically predicted isomers prompted intense scrutiny of these 28-electron molecules as leading candidates for high-energydensity materials (HEDM). A renewed upsurge in interest springs from current missions to planetary atmospheres, where detection of reactive, even minor, species is one of the most attractive targets. After the recent experimental discovery of the open-chain N 4 molecule [4] and the ultimate answer to the long-standing challenge of synthesizing C 2 O 2 , [5] the question remains whether bound, high-energy N 2 CO species can be experimentally observed.[6] Ab initio calculations at different levels of theory predict that three N 2 CO species are potentially observable: the singlet C 2v diazirinone, which is the most stable N 2 CO isomer, the triplet open-chain NNCO, and a strained tetrahedrane-like structure, whose "accumulated" energy (about 220 kcal mol À1 ) would be higher than that of N 4 (T d ).[2] Despite the promising predictions, N 2 CO has so far eluded experimental detection and defied all attempts at its characterization as a bound species.Herein we report the preparation, positive detection, and characterization of N 2 CO by the one-electron reduction of the N 2 CO + cation, a result achieved by neutralizationreionization mass spectrometry (NRMS). [7] We have already utilized this approach for the detection of other metastable, elusive molecules [8] and for the detection of the related 28-electron molecule N 4 , [4] which was recently extensively studied by the same technique with various pressure regimes and neutralizing target gases. [9] The N 2 CO + ion was formed by chemical ionization (CI) of a CO/N 2 mixture according to Equation (1).The interfering isobaric N 4 + and C 2 O 2 + ions were separated by using 15 N-, 13 C-, and 18 O-labeled reagents. The remarkably high binding energy of the (CO) 2 + ion [3c, 10] required that CO and N 2 were introduced at a 1:20 pressure ratio, which typically ensures the N 4 + /N 2 CO + /C 2 O 2 + ratio displayed in the CI spectrum reported in Figure 1. Under such conditions the abundance of the N 2 CO + species was appreciable, yet the intensity of the N 4 + ion was far larger. To prevent any possible contamination from N 4 + isotopomers, which are liable to neutralization, mixtures containing the following combinations of labeled reagents were examined: N 2 /C The NR mass spectra of all the examined N 2 CO + ions display significant recovery peaks at the same m/z value as that of the precursor ion (Figure 2). This result positively proves the existence of a neutral N 2 CO species, which has survived at least for the ...
