PREDICTION OF STRUCTURE AND DENSITY FOR ORGANIC NITRAMINES

An approach to ab initio crystal structure prediction by packing optim ization is developed for organic nitramines, an important class of ene rgetic materials. The principal features of the search method are: use of statistical data on the organic crystal structural classes to sele ct typical space groups and site symmetries for further search; accoun ting for the energy-hypersurface symmetry to determine the unique sear ch region; and use of an automated similarity-search procedure to reco gnize non-unique minima and determine the symmetry of optimized packin gs. The wide convergence properties of the local search procedure perm it one to start optimization from an arbitrary point, so that no preli minary screening of the starting models is necessary. The numerical ca lculations were first carried out on the known crystal structures of t hree polymorphs of HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane ). In this step, the force-held parameters for the nitramine fragment have been improved to obtain the best correspondence between the predi cted and observed molecular geometries. The predicted packings were fo und to be in reasonably good agreement with the X-ray structural data, while the computed lattice energies were not accurate enough to predi ct the observed heats of sublimation and the trend of polymorph stabil ities. Secondly, the method was employed to predict the possible cryst al structures of eight isomeric azanitroadamantanes and wurtzitanes, w hose molecular structures were proposed earlier on the basis of a comp utational study (T.S. Pivina et al., Propellants, Explosives, Pyrotech nics, 20 (1995) 91). As a result, the energy-minimized structures with densities up to 2.08 and 2.04 g cm(-3) have been predicted for the ad amantane and wurtzitane series, respectively, as the possible crystal polymorphs. Due to the interaction between the conformational and pack ing forces giving rise to some gain in the total energy at the expense of at least partial loss in molecular symmetry, the predicted densiti es are expected to be lower estimates than the actual ones.