A new allotrope of nitrogen in which the atoms are connected to form a novel N6 molecule is predicted to exist at ambient conditions. The N6 molecule is a charge-transfer complex with an open-chain structure containing both single and triple bonds. The charge transfer induces ionic characteristics in the intermolecular interactions and leads to a much higher cohesive energy for the predicted crystal compared to solid N2. The N6 solid is also more stable than a previously reported polymeric solid of nitrogen. Because of the kinetic stability of the molecules and strong intermolecular interactions, the N6 crystal is shown by metadynamics simulations to be dynamically stable around room temperature and to only dissociate to N2 molecules above 700 K. The N6 crystal can likely be synthesized under high-pressure high-temperature conditions, and the considerable metastability may allow for an ambient-pressure recovery of the crystal. Because of the large energy difference between the single and triple bonds, the dissociation of the N6 crystal is expected to release a large amount of energy, placing it among the most efficient energy materials known today.

Theoretical structure prediction calculations have revealed that the conformation of ammonia borane (NH 3 BH 3 ) in the crystalline state can be modified by pressure, changing from the staggered configuration at low pressure to an eclipsed geometry at high pressure. At low pressure, the crystalline structure is stabilized by the charge transfer N H δ+ ··· δ− HB dihydrogen interactions. In the high pressure polymorphs, the NH 3 BH 3 is predicted to form a layered structure. The NH δ+ ··· δ− HB bonding is still predominant within the layer. The stacking of the layers, however, is determined by the occurrence of additional homopolar B H δ− ··· δ− HB interaction unprecedented in NH 3 BH 3 and facilitated by the eclipsed conformation. This bonding is shown to be the result of secondary interactions between BH 3 groups from molecules of adjacent layers. Topological analysis of the charge density and perturbation calculations on the molecule fragments show unambiguously that the BH δ− ··· δ− HB interaction is covalent in nature with the bond strength comparable to a conventional hydrogen bond. ■ INTRODUCTIONHydrogen, the most abundant element in the universe, is a promising candidate to eventually replace petroleum as the fuel of choice. In the context of hydrogen storage research, ammonia borane (NH 3 BH 3 ) has received continuous attention for decades due to its high storage capacity and moderate dehydrogenation temperature. Molecular NH 3 BH 3 is a prototypical electron donor−acceptor complex formed between NH 3 and BH 3 molecules (1) and arranged in a staggered conformation similar to the geometry of the isoelectronic ethane (C 2 H 6 ). In solid state, however, NH 3 BH 3 and C 2 H 6 have very different physical properties. For example, the melting temperature of NH 3 BH 3 is higher than that of C 2 H 6 by 285 K. This suggests a strong intermolecular interaction, often referred to as "dihydrogen bonding" (2), 1−3 to present in NH 3 BH 3 . The dihydrogen bonding originates from the N H δ+ ··· δ− HB charge-transfer interaction which usually occurs when the intermolecular distance d H···H is shorter than the sum of the van der Waals (vdW) radii. A survey of the Cambridge Structural Database (CSD) carried out by Richardson et al. 2 shows that dihydrogen bonding has a preference for a bent B H···HN angle θ and a nearly linear NH···HB angle ψ, arranged such that the NH vector points toward the middle of the BH vector. This geometry suggests that the electron donor of the dihydrogen bond is the BH σ bond, rather than an individual atom. This is an extraordinary illustration of the versatility of the hydrogen bonding; in the past, we have seen π electrons of a multiple bond or aromatic ring, or a transition metal center, act as electron donors. 4,5 Clearly, dihydrogen bonding, to a great extent, determines the crystal structures of NH 3 BH 3 . At ambient conditions, NH 3 BH 3 adopts a dynamical disordered structure (I4mm), which exhibits halos of hydrogen atom occupancy surrounding the N and B atoms. 6,7 Be...

An allotrope of nitrogen formed solely by N−N single bonds is predicted to exist between 100 and 150 GPa. The crystal structure has the Pccn symmetry and is characterized by a distorted tetrahedral network consisting of fused N 8 , N 10 , and N 12 rings. Stability of this structure is established by phonon and vibrational free energy calculations at 0 K and finite temperatures. Simulated x-ray diffraction pattern of the Pccn phase is compared to the pattern of a recently synthesized nitrogen phase at the same P-T conditions, which suggests that the Pccn phase is likely a minor component of the latter. The Pccn phase is expected to form above the stability field of cubic gauche (cg) phase. The outstanding metastability of this phase is attributed to the intrinsic stability of the sp 3 bonding as well as the energetically favorable dihedral angles between N−N single bonds, in either gauche or trans conformation. The prediction of another single-bonded phase of nitrogen after the lab-synthesized cg phase will stimulate research on metastable phases of nitrogen and their applications as high-energy-density materials.