Preparation of a neutral nitrogen allotrope hexanitrogen C2h-N6

As AgN 3 is an excellent reagent for the synthesis of polyazides35 and halogen azides both in the gas phase36 and in solution37,38, we suggest that the reaction of AgN 3 with XN 3 (X = halogen) is a viable route to N 6 (Fig. 1b). The reactions were conducted in either a quartz tube or a U-trap by flowing gaseous Cl 2 through solid AgN 3 under reduced pressure at room temperature (see the ‘Synthesis details’ section in Methods and Supplementary Fig. 1). Apart from the known bands of ClN 3 (ref. 39) and HN 3 (ref. 40), a distinct group of bands at 2,076.6, 2,049.0, 1,177.6 and 642.1 cm−1 was recorded (Supplementary Fig. 2). After irradiating the matrices with 436 nm light (Fig. 2a middle trace and Supplementary Fig. 3), all bands vanish. However, the rates of decomposition of the newly observed infrared bands differ from those attributed to ClN 3 (Supplementary Figs. 4 and 5). There were no discernible products other than chloronitrene (ClN) detected in the difference spectrum after irradiation. Furthermore, identical bands were detected when Br 2 was used instead of Cl 2 , indicating that the unidentified species does not contain halogens (Fig. 2a upper trace and Supplementary Fig. 6). Also, BrN 3 does not decompose on 436 nm irradiation, providing clean decomposition spectra of the yet unidentified species. Fig. 2: Infrared spectra of N 6 isotopomers and side products. a, Lower trace: computed anharmonic infrared spectrum of N 6 at B3LYP/def2-TZVP, including the ν 8 + ν 9 combination. Middle trace: difference spectrum showing the changes after 8 min of 436 nm irradiation of the products of the reaction of Cl 2 with AgN 3 . Upper trace: difference spectrum showing the changes after 6 min of 436 nm irradiation of the reaction products of Br 2 with AgN 3 . b, Difference spectrum of a neat N 6 film at 77 K showing the changes after 8 min of 436 nm irradiation. c, Bottom to top traces: computed anharmonic infrared spectrum of N 6 , 15NNNNN15N (1a), 15NNN15NNN (1b) and NN15N15NNN (1c) at B3LYP/def2-TZVP, including the ν 8 + ν 9 combination; difference spectrum showing the changes after 8 min of 436 nm irradiation of the reaction products of Br 2 with AgN 3 ; difference spectrum showing changes after 8 min of 436 nm irradiation of the reaction products of Br 2 with Ag15N14N14N. Matrix sites from natural abundance and isotope-labelled HN 3 (#) and H 2 O (*) are marked. Full size image The intensive vibrational band at 2,076.6 cm−1 compares favourably with the asymmetric stretching band of the azide moiety in isoelectronic N 3 –NCO (2,099.1 cm−1, Ar matrix)41. Compared with the computed harmonic vibrations at CCSD(T)/cc-pVTZ, the four bands noted above could be attributed to N 6 , except the band at 2,049.0 cm−1 of moderate intensity, although it disappeared together with the other bands following photolysis (Supplementary Figs. 4 and 7). To determine the origin of the band at 2,049.0 cm−1, anharmonic vibrational frequencies were computed at B3LYP/def2-TZVP (Supplementary Table 1). This analysis indicates that this band derives from a combination of fundamentals ν 8 (a g symmetric N3N4 stretching mode) and ν 9 (b u asymmetric N3N2N1 stretching mode). The substantial anharmonic intensity contribution (219 km mol−1; Supplementary Table 2) of the fundamental ν 11 at 2,143.5 cm−1 and the ν 8 + ν 9 combination is notably stronger than its fundamentals, suggesting that the combination ν 8 + ν 9 gains energy through Fermi resonance from the adjacent strong fundamental ν 11 (ref. 41). To confirm our assignments, isotope-labelling experiments were conducted using Ag15N14N14N. Three groups of distinct peaks can be discerned in the infrared spectra (Fig. 2c and Supplementary Fig. 8), indicating the presence of two N 3 moieties in the molecule, which can be attributed to three types of isotopomer (1a: 15NNNNN15N, 1b: 15NNN15NNN, 1c: NN15N15NNN), respectively. In particular, the unsymmetric isotopic substitutions in 1b lower its point group from C 2h to C s . Computations delineate that the terminal (N1 or N6) and internal (N3 or N4) 15N substitutions mainly influence the terminal (ν 11 ) and internal asymmetric stretching vibration (ν 9 ) of the N 3 moieties, respectively. This leads to a redshift of the ν 8 + ν 9 combination and a blueshift of the ν 11 fundamental in going from 1a to 1c, resulting in their gradual separation. The intensity ratio of the ν 8 + ν 9 combination and the ν 11 fundamental in 1a is nearly 1:1, which is much higher than that in 1c (about 1:17). These findings align well with the anharmonic infrared intensities computed by density functional theory (Supplementary Table 3), which are attributed to the closer proximity of the ν 8 + ν 9 combination to the strong ν 11 fundamental in 1a, resulting in an increase of the Fermi resonance and vice versa in 1c. Statistically, the anticipated ratio of the three isotopomers should be 1a:1b:1c = 1:2:1, which is reflected in the observed fundamental ν 7 in the experimental spectrum (Fig. 3). Furthermore, the computed intensity of ν 9 in 1c (107 km mol−1; Supplementary Table 3) is higher than that in 1a (92 km mol−1) and 1b (98 km mol−1), which matches the intensity ratios of ν 9 observed in 1a and 1b (approximately 1:2). The experimentally observed intensities agree with these findings and show a slightly higher intensity of ν 9 in 1c than in 1a. Fig. 3: Measured and computed UV-Vis spectrum of N 6 and molecular orbitals involved in the electronic transitions. Experimental difference UV-Vis spectrum reflecting changes following 4 min of 436 nm irradiation of the reaction products of Br 2 with AgN 3 in argon at 10 K. Inset, computed [TD-B3LYP/def2-TZVP] electronic transitions for N 6 and molecular orbitals involved. Full size image To explore the intrinsic stability of N 6 , we also prepared neat N 6 at room temperature and condensed it at liquid nitrogen temperature (77 K) as a film on the surface of the matrix window without using argon as a host gas. Irradiation of such N 6 films resulted in very similar spectral changes as those observed in argon matrices at 10 K (Fig. 2b and Supplementary Fig. 9). That is, neat N 6 is sufficiently stable at the temperature of liquid nitrogen to allow its direct identification.