Thermochemical Processes During the Degradation of Nitroguanidine in Water: A Density Functional Theory Investigation

In the present study, thermochemical mechanisms are proposed for the decomposition of nitroguanidine (NQ) in aqueous solution. Minnesota density functional M06-2X was employed for depicting the pathways for the NQ decomposition with a conductor-like polarizable continuum model (CPCM) approach to consider the effect of bulk water solution. Followed by the formation of hydroxyguanidine and/or guanidine along with nitrite by eliminating the nitro group from NQ through photolysis, two main degradation steps through the thermochemical process were investigated. These thermochemical degradation steps include the formation of cyanamide from hydroxyguanidine and/or guanidine along with ammonia, and the formation of cyanoguanidine from the dimerization of cyanamide. Further, degradations of the fragments of cyanamide, cyanoguanidine, guanidine, and hydroxyguandine were also explored. The results show that melamine and urea may exist as degradation fragments while cyanide and nitrosoguanidine are unlikely to form through usual thermochemical processes due to the presence of high energy barriers. Water and hydroxide ions play important roles in the degradation process; in particular, hydroxide ions significantly lower the energy barriers demonstrating that the proposed pathway is energetically viable.