Enthalpies of Formation of Gas‐Phase N3, N‐3, N+5, and N‐5 from ab initio Molecular Orbital Theory, Stability Predictions for N+5N‐3 and N+5N‐5, and Experimental Evidence for the Instability of N+5N‐3.

Abstract

Abstract : Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3(-), N5(+), and N5(-) from total atomization energies. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, permit for the first time reliable stability predictions for the hypothetical N5(+) N3(-) and N5(+)N5(-) salts. Contrary to previous predictions, it is shown that neither salt can be stabilized, decomposing spontaneously to N3 radicals and N2. This conclusion was experimentally confirmed by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is also shown that the vertical ionization potentials and electron affinities, used in previous stability predictions, must not be used in stability predictions involving species, such as N5 ions, because the resulting N5 radicals are not vibrationally stable and undergo spontaneous further decomposition. The use of the vertical values, that neglects this highly exothermic secondary decomposition of N5 to N3 and N2, results in large errors of about 100 kcal/mol for each N5 unit. It is also shown that a previous comparison of the density and energy density of hydrazine with those estimated for N5(+) N5(-) is significantly in error.