The structure, thermodynamic instability and energetics of NI3, its specific impulse and a strategy for its stabilization

Abstract

The present work is dedicated to deepen the causes of the physicochemical instability of NI3. As a support study, a theoretical investigation is performed. The quantum chemical calculations were performed by using SE(PM6), HF/6-31G* and DFT/M06-2X/6-311G** approach. The calculated gas phase formation enthalpy to NH3.NI3 is 91.75 kJmol−1. The sublimation enthalpy for the adduct is calculated as 237.75 kJmol−1. By structure calculations, it is shown that the stabilization of NI3 in the NH3.NI3 adduct is consequence of the I-N-I angle increase. The enthalpy for the reaction NH3.NI3 (s) → NH3(g) + NI3 (s) is calculated as 46 kJmol−1. Pure NH3:NI3 detonated only three minutes after dried at room conditions (humidity = 65%, temperature = 39 ºC). On the other hand, the NH3:NI3 polyvinylchloride resin, polyvinyl acetate resin or polysiloxane resin “entrapped” samples, do not detonate after 30 h. By using the empirical equation Is = 17.562 η + 125.551, where Is is the specific impulse (s) and η is the absolute chemical hardness (eV), the specific impulse for NH3.NI3 can be calculated as 185.52 s (HF/6-31G* data) and 163.75 s (DFT/M06-2X/6-311G** data). To NI3the Is calculated values are 201.68s and 168.75 s, respectively. The decrease in the specific impulse values from NI3 to NI3:NH3 is explained as consequence of the increase in the polarizability of the entire system.