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

Abstract

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 (H2NNH2) 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 HNNH2• and H2NNH2•+. 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, H2NNH2•+ is 100 kJ mol-1 more stable than HNNH3•+, 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 H2NNH2 and H2NNH3+ with H• and OH• in the gas phase were also studied. With H2NNH2, 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 H2NNH3+, reaction profiles for attack at both the H2N and NH3+ sites were examined. In the case of OH•, abstraction of H from the −NH3+ end to form H2NNH2•+ is preferred over production of HNNH3•+ by attack at H2N−. However, in solution the former mode will be inhibited by the strong bonding of water to the charged NH3+ end, and attack at both centers can be expected. For H• attack on H2NNH3+, formation of H2NNH2•+ by H abstraction from the NH3+ end has a very low gas phase Ea, but in solution solvation effects will again interfere. Thus production of H3NNH3•+ by H addition at H2N is likely to be a competitive process. This product is expected to decompose to H2N• + NH4+ ( = −13 kJ mol-1), and complexation of water with H3NNH3•+ was shown to lower the barrier height for that process. In the gas phase prereaction complexes of OH• are seen with both H2NNH2 and H2NNH3+, the latter being quite strongly bound. However, competition with solvent water molecules would probably reduce the role of these in solution.