Insight into Hydrazinium Nitrates, Azides, Dicyanamide, and 5-Azidotetrazolate Ionic Materials from Simulations and Experiments

Abstract

A transferrable, polarizable, quantum chemistry (QC) based force field has been developed for hydrazinium (N(2)H(5)(+)), monomethylhydrazinium ((CH(3))N(2)H(4)(+)), and dimethylhydrazinium ((CH(3))(2)N(2)H(3)(+)) cations in combination with the nitrate (NO(3)(-)), azide (N(3)(-)), dicyanamide (N(CN)(2)(-)), and 5-azidotetrazolate (CN(7)(-)) anions. Inclusion of the off-atom charge center to represent a lone pair on the hydrazinium-based cations significantly improved the electrostatic potential description around cations and led to overall a more accurate prediction of ionic crystal cell parameters in molecular dynamics (MD) simulations. Seven different ionic systems have been investigated: [N(2)H(5)][NO(3)], [(CH(3))N(2)H(4)][NO(3)], [(CH(3))(2)N(2)H(3)][NO(3)], [N(2)H(5)][CN(7)], [(CH(3))N(2)H(4)][N(3)], [(CH(3))(2)N(2)H(3)][N(3)], [N(2)H(5)][N(CN)(2)]. For all but [(CH(3))(2)N(2)H(3)][NO(3)] and [N(2)H(5)][N(CN)(2)], QC calculations of a single, gas-phase ion pair predicts spontaneous deprotonation of the cation. Crystal lattice parameters obtained from MD simulations for these seven ionic crystals were compared with the previously published experimental data as well as the crystal structure of [N(2)H(5)][N(CN)(2)] determined in this work from X-ray data. In general, MD simulations predicted crystal lattice vectors/angles (volumes) within a 5% (3%) absolute margin of error from experiments, with outlying volume deviations of 5-6.6% for three crystals [(CH(3))N(2)H(4)][N(3)], [N(2)H(5)][NO(3)], and [(CH(3))N(2)H(4)][NO(3)] with combinations of particularly small anions and/or cations. Structural comparisons between ionic materials in the liquid and crystalline states are made, including the observation of two crystalline systems where the crystalline state induces conformational changes in the methylated hydrazinium cations between the gas-phase and liquid states. Elastic constants and estimated shear and bulk moduli were extracted from MD simulations for all seven ionic crystals and correlated with the structural motifs of ion interactions in the crystals.