The thermochemistry of TNAZ (1,3,3-trinitroazetidine) and related species: models for calculating heats of formation

The heats of formation of TNAZ(1,3,3-trinitroazetidine) and related compounds have been calculated with the B3LYP/6-31G(d,p) model to give a mean absolute deviation of 3.1 kcal/mol for 18 singlet species and 5.1 kcal/mol for 12 free radical intermediates. A minimal seven-parameter atom/group additivity scheme was developed that reduces these errors to 1.1 and 1.3 kcal/mol, respectively. Based on these calculations a set of structural enthalpy increments was compiled that allows the precise estimation of heats of formation of highly nitrated species from simpler ones via isodesmic reactions that are independent of atom/group additivity parameters. In a parallel analysis, an augmented Benson group-additivity scheme was developed that allows facile estimation of the heats of formation of the several nitroazetidines as well as related radical species with a precision that is comparable to that obtained via quantum mechanical calculations. In the final section measured activation energies for the initial fragmentation steps, for eight species, are compared with the computed enthalpies for dissociation. The computed lowest energy structures for eleven species are summarized in an appendix.