Kinetics and mechanism of thermolysis of energetic polynitro glycolurils: thermoanalytical and quantum chemical study.

Polynitro glycoluril derivatives, viz., 1,4-dinitroglycoluril (DINGU) and 1,3,4,6-tetranitroglycoluril (sorguyl, TNGU), are very promising components of energetic formulations because of their good detonation performance, high density, and low sensitivity to mechanical stimuli. However, the data available on the kinetics and mechanism of their thermal decomposition remain very limited. In the present contribution, we employed mutually complementing advanced experimental techniques (DSC and TGA in the solid state both under linear heating and isothermal conditions along with advanced thermokinetic models, optical microscopy, and gas product detection) and predictive quantum chemical calculations (DLPNO-CCSD(T)) to study the thermal stability of the title species. The experimental thermolysis data of DINGU and TNGU including both gravimetric (TGA) and caloric (DSC) datasets were used for building two-step kinetic models that universally describe all DSC and TGA data. More specifically, the first step for both nitroglycolurils is a nucleation-growth reaction described by the Kolmogorov-Johnson-Mehl-Avrami-Erofeev equation (KJMAE), while the second consecutive step obeys flexible Prout-Tompkins and third-order reaction models for DINGU and TNGU, respectively. The experimental findings were complemented by the mechanistic details from DLPNO-CCSD(T) quantum chemical calculations. The radical N-NO2 bond cleavage is the dominant primary decomposition channel with the kinetic parameters Ea = 185.8 kJ mol-1 and log(A/s-1) = 18.6 for DINGU, and Ea = 166.1 kJ mol-1 and log(A/s-1) = 18.7 for TNGU, respectively. Apart from the primary reactions, we also considered a number of uni- and bimolecular secondary decomposition channels. We found that the "bridge" C-C bond unzipping followed by the ˙NO2 radical elimination with activation barriers of ∼120-130 kJ mol-1 are the most energetically favorable unimolecular secondary channels. At the same time, the hydrogen abstraction from an initial reagent molecule by a primary nitramine radical product is the most important bimolecular secondary channel. The reaction mechanism switches from bimolecular to unimolecular C-C bond unzipping at the isokinetic temperatures of 860 K for DINGU and 610 K for TNGU. The reported secondary reactions might also be important in the thermolysis mechanisms of the related energetic secondary nitramines (e.g., RDX, HMX, and CL-20). Apart from this, we also determined a mutually consistent set of thermochemical and phase change data for a series of polynitro glycoluril derivatives.