ABINITIO INVESTIGATION OF DIHYDROGEN TRANSFER FROM CIS-1,2-DIAZENE

The reactions of cis-1,2-diazene with trans-1,2-diazene, cis-1,2-diazene, and ethylene have been investigated by ab initio methods. In all cases, hydrogen-bonded complexes were found for the reactants. Transition structures were located for the concerted dihydrogen transfer from cis-N2H2 at the HF/6-31G* level; the calculated activation enthalpies were in qualitative agreement with previous HF calculations, but were more that 20 kcal/mol greater than those estimated from experimental results. When electron correlation was included in the calculations (MP4/6-31G* level) with these geometries, the activation energies dropped dramatically to negligible values. Transition structures located at the MP2/6-31G* level were significantly earlier than those located at the HF/6-31G* level. Several reactions of known thermochemistry which are similar to the subject reaction were shown to need a MP4/6-31+G(2d,p)//MP2/6-31G* level calculation for acceptable accuracy. At this level, the values of DELTA-H double-ended dagger were negative with respect to the free reactants and less than 2.2 kcal/mol with respect to the complexes. Thus, when the effects of electron correlation are included, it is seen that the reactions have negligible activation enthalpies. The calculated free energies of activation are mostly entropic in nature and are in good agreement with those deduced from the experimental rates in solution.