GAS-PHASE AND AQUEOUS THERMOCHEMISTRY OF HYDRAZINE AND RELATED RADICALS AND THE ENERGY PROFILES OF REACTIONS WITH H-. AND OH - AN AB-INITIOSTUDY

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 i ons. Bond dissociation energies and proton affinities in these species are strongly influenced by the three-electron stabilization, which oc curs in HNNH2. and H2NNH2.+. Free energies of formation in solution we re estimated and used to examine relative stabilities of the radicals and the overall energetics of their reactions. For example, H2NNH2.+ i s 100 kJ mol(-1) more stable than HNNH3.+, and this explains the rearr angement which has been observed experimentally. As a guide to the rel ative importance of possible reactions, the energy profiles of the rea ctions of H2NNH2, and H2NNH3+ with H-. and OH. in the gas phase were a lso studied. With H2NNH2, barrier heights above reactants are generall y 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 energ ies observed experimentally. For H2NNH3+, reaction profiles for attack at both the H2N and NH3+ sites were examined. In the case of OH., abs traction of H from the -NH3+ end to form H2NNH2.+ is preferred over pr oduction of HNNN3.+ 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 o n H2NNH3+ formation of H2NNH2.+ by H abstraction from the NH3+ end has a very low gas phase E-a, but in solution solvation effects will agai n interfere. Thus production of H3NNH3.+ by H addition at H2N is likel y to be a competitive process. This product is expected to decompose t o H2N. + NH4+ (Delta G degrees((react)) = -13 kJ mol(-1)), and complex ation 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. H owever, competition with solvent water molecules would probably reduce the role of these in solution.