Gas Phase and Aqueous Thermochemistry of Hydrazine and Related Radicals and the Energy Profiles of Reactions with H• and OH•: An ab Initio Study

Ab initio calculations by a modified G2(MP2) procedure have been used to obtain bond dissociation energies, proton affinities, and heats of formation of hydrazine (H 2 NNH 2 ) and the related radicals and radical ions. Bond dissociation energies and proton affinities in these species are strongly influenced by the three-electron stabilization, which occurs in HNNH 2 • and H 2 NNH 2 •+ . Free energies of formation in solution were estimated and used to examine relative stabilities of the radicals and the overall energetics of their reactions. For example, H 2 NNH 2 •+ is 100 kJ mol -1 more stable than HNNH 3 •+, and this explains the rearrangement which has been observed experimentally. As a guide to the relative importance of possible reactions, the energy profiles of the reactions of H 2 NNH 2 and H 2 NNH 3 + with H • and OH • in the gas phase were also studied. With H 2 NNH 2, barrier heights above reactants are generally low for the H abstraction reactions of both OH • (−5 kJ mol -1 ) and H • (25 kJ mol -1 ), which is consistent with the low activation energies observed experimentally. For H 2 NNH 3 +, reaction profiles for attack at both the H 2 N and NH 3 + sites were examined. In the case of OH •, abstraction of H from the −NH 3 + end to form H 2 NNH 2 •+ is preferred over production of HNNH 3 •+ by attack at H 2 N−. However, in solution the former mode will be inhibited by the strong bonding of water to the charged NH 3 + end, and attack at both centers can be expected. For H • attack on H 2 NNH 3 +, formation of H 2 NNH 2 •+ by H abstraction from the NH 3 + end has a very low gas phase E a, but in solution solvation effects will again interfere. Thus production of H 3 NNH 3 •+ by H addition at H 2 N is likely to be a competitive process. This product is expected to decompose to H 2 N • + NH 4 + ( = −13 kJ mol -1 ), and complexation of water with H 3 NNH 3 •+ was shown to lower the barrier height for that process. In the gas phase prereaction complexes of OH • are seen with both H 2 NNH 2 and H 2 NNH 3 +, the latter being quite strongly bound. However, competition with solvent water molecules would probably reduce the role of these in solution.