Dinitramide Ion: Robust Molecular Charge Topology Accompanies an Enhanced Dipole Moment in Its Ammonium Salt

Delocalization quantitatively mapped for prototypic organic nitroanions as well as azidoform anions

Delocalization of occupied orbitals impacts the chemical bonding in the simplest known pernitroanions [(NO2)3C]- (1) and [(NO2)2N]- (2) as well as other functionalized organic anions. By quantitatively mapping it onto molecular backbones of 1, 2, [CH2NO2]- (3), [CH3NNO2]- (4) and [C(N3)]- (6) anions (all modeled by QM calculations), the Weinhold's NBO analysis refines their chemical structure, enabling to explain and even predict their essential chemical behaviour. In detail, the HOMO of 1 and 2 is associated with the central atom to the degree of 70.7% and 80.4%, respectively, while the HOMO localization on O atoms for 3 and 4 is 85.3% and 81.1%, respectively. Predomination of C-alkylation for 1 and that of O-alkylation for 3 in non-coordinating solvents thus becomes clear. The important news is that the easiness of homolytically disrupting the N-N bond in 2, a constituent of inexpensive powerful explosives, is because of the occupancy of the related σ*orbital increases with stretching this bond. The same is true for electrocyclic extrusion of NO3- from this molecule. This antibonding effect may be assumed to be the common cause of the proneness of aliphatic nitro compounds to decompose. Pyramidal anion 6 is a highly localized carbanion. Its isomer of molecular symmetry CS has a unique chemical structure of its azido substituents: each of them is represented by one high-weight resonance structure, e.g., N-N[triple bond, length as m-dash]N. The prediction is that the dinitrogen-eliminating decomposition of this isomer is more facile than of the isomer of C3 symmetry. In summary, this study affords three novel particular insights into the chemical structure and reactivity of these anions: chemically telling delocalization-augmented molecular structures, a reasonable hypothesis of the common cause of thermally triggered instability of aliphatic nitro compounds, and discovered one-resonance structure azido groups.

QTAIM Assessment of the Intra- and Intermolecular Bonding in a Bis(nitramido-oxadiazolate) Energetic Ionic Salt at 20 K

Accurate experimental determination of the electron density distribution for the energetic ionic salt bis(ammonium) 2,2'-dinitramido-5,5'-bis(1-oxa-3,4-diazolate) dihydrate (1) is obtained from multipole modeling of single-crystal X-ray diffraction data collected at 20 K. The intra- and intermolecular bonding is assessed in terms of the quantum theory of atoms in molecules (QTAIM) with a view to better understanding the physicochemical properties in relation to chemical bonding. Topological analysis reveals stronger bonding for the N-NO₂ bond relative to energetic nitramines RDX and HMX and the indication of a trend between this and impact sensitivity of nitro-containing energetic materials is noted. The intermolecular bonding of 1 is dominated by classical H-bonds but includes multiple π-bonding interactions and interactions between H-bond donor and acceptor atoms where bond paths are deflected by H atoms. There also exists a weak O···O interaction between end-on nitro groups, as well as an intramolecular ring-forming 1,5-type interaction. An anharmonic description of thermal motion was required to obtain the best fitting model, despite the low temperature of the study. The experimental study was complemented by periodic boundary DFT calculations at the experimental geometry as well as gas phase calculations on the isolated dianion.

An investigation of KS-DFT electron densities used in atoms-in-molecules studies of energetic molecules

The atoms-in-molecules (AIM) theory has been proposed as a method to understand chemical stability through stationary properties of the electron density. To assess the applicability of this method for establishing such correlations with various performance and vulnerability properties of energetic materials, we calculated the Kohn-Sham density functional theory (KS-DFT) wavefunctions and their subsequent AIM data for representative materials, including hexanitrobenzene (HNB), pentaerythritol tetranitrate (PETN), pentanitroaniline (PNA), 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), ethylenedinitramine (EDNA), 1,1-diamino-2,2-dinitroethylene (FOX-7), 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and the TATB dimer using the B3LYP, PBE, and PW91 potentials as well as Hartree-Fock (HF). For the HNB and HMX molecules and the TATB dimer, the number of critical points in the low-density regions of the density gradient vector field varied, sometimes dramatically, with basis set and potential even at their individually optimized geometries. Adding ghost atoms in the low-density regions also affected the existence of critical points. The variation was seen in results generated with three separate AIM software packages, AIMPAC, AIMAll, and InteGriTy. This inconsistency implies that KS-DFT wave-functions can have significant variation in the topology of the electron density to such an extent that these calculations cannot be used to justify the existence or absence of low-density critical points. Therefore, predictions of the stability of a molecule based solely on properties of low-density bond critical points generated from a single DFT calculation are questionable.

First structural characterization of solvate-free silver dinitramide, Ag[N(NO(2))(2)]

The crystal structure of solvate-free silver dinitramide (Ag[N(NO(2))(2)]) was determined by X-ray diffraction for the first time and displays secondary contacts between silver and oxygen as well as between silver and nitrogen, furnishing different coordination modes of the dinitramide moiety.