THERMOCHEMISTRY OF TETRAZETE AND TETRAAZATETRAHEDRANE - A HIGH-LEVEL COMPUTATIONAL STUDY

Tetraazatetrahedrane, N-4 (2), is 11.3 kJ mol(-1) lower in energy than tetrazete (1) at the G2 level, in contrast to its hydrocarbon analogu e, tetrahedrane (4), which has 108.9 kJ mol(-1) higher energy than tha t of cyclobutadiene (3). The open-chain C-s structure of tetranitrogen (5) having the triplet ground state is the most stable isomer of N-4, and its energy is 62.0 kJ mol(-1) lower than that of tetrazete. At th e G2 level, the enthalpies of formation, Delta H-f298, of tetraazatetr ahedrane, tetrazete, and the open-chain tetranitrogen are 732.5 +/- 8. 0, 746.5 +/- 7.6, and 686.6 +/- 7.6 kJ mol(-1), respectively. The high thermodynamical instability of tetraazatetrahedrane and tetrazete tow ard their dissociations into molecular nitrogen may be attributed to t he comparative weakness of the single N-N and double N=N bonds, the st rengths of which are only 29.0 and 54.2% of the strength of the N=N bo nd. For the C-C and C=C bonds, the corresponding ratios are 38.0 and 7 4.8%, respectively, and consequently, cyclobutadiene is stable with re spect to dissociation into two acetylene molecules. After correction f or the strain energy of the four-membered nitrogen ring, the antiaroma tic destabilization of tetrazete is 54.1 kJ mol(-1); this is considera bly less than the antiaromatic destabilization of cyclobutadiene (170 +/- 7 kJ mol(-1) at the G2 level). At the G2 level the strain energies of tetraazatetrahedrane N-4, tetrazetine, N4H2, and tetrazetidine, (N H)4, are 205.5, 156.9, and 131.4 kJ mol(-1), respectively. Azasubstitu ted cyclobutadienes have planar structures with bond-length alternatio n and, with the exception of tetrazete, are lower in energy than the c orresponding azasubstituted tetrahedranes. The energy difference decre ases with increasing number of the nitrogen atoms, and therefore, tetr aazatetrahedrane is eventually more stable than tetrazete. 1,3-Diazete is 42.2 kJ mol(-1) lower in energy than 1,2-diazete. The homodesmotic stabilization energies for the azetes show that the azasubstitution r esults in decreasing destabilization effects in these molecules. Howev er, the more nitrogen atoms in an azete, the greater its tendency to d issociate into triple-bond species, HC=CH, HC=N, or N=N.