Ab initio Studies of Hydrazines: Equilibrium Structures of 1,1-Dimethylhydrazine and Transition Structures Connecting Them

Abstract

Optimization of the geometries of various stationary structures of 1,1-dimethylhydrazine has been carried out with the 3-21G and 3-21G(N*) basis sets, and the energies of each of the latter optimized structures have been evaluated with the 6.31G* basis set. The gauche form with a (mean) internal rotational angle near 80º (hydrazine, approx. 90º) is the lowest-energy form. After zero-point vibrational -energy corrections, the anti form lies in a shallow well 14kJ mol^-1 higher on the potential-energy surface for internal rotation, and the transition structure connecting them lies approx. 1 kJ mol^-1 higher still. The anti form is thus a true equilibrium species, unlike the case for hydrazine, but constitutes less than 0.5% of the molecules present at room temperature. It is estimated to have a half-life of less than 10 ps , so its observation by present physical methods may prove to be difficult. The barrier to internal rotation via the syn form (41kJ mol^-1) coincides with that for hydrazine. gem-Dimethyl substitution in hydrazine leads to a small rise (1-2 kJ mol^-1) in the barrier to inversion at the unsubstituted nitrogen atom, but to an unexpectedly high (by 8-9 kJ mol^-1) calculated barrier to inversion at the substituted centre, whether the result be referred to the barrier in hydrazine or to that in dimethylamine. Calculated NH stretching frequencies for the gauche form show a spacing much larger than that for 'simple' primary amines, as is found experimentally.