An ab initio Investigation of the Equilibrium Structures of Hydrazine and of the Transition Structures Connecting Them

Abstract

An ab initio investigation of the various equilibrium and transition structures of hydrazine has been carried out with full geometry optimization and use of the 3-21G, 3-21G(N*), and 6-31G* basis sets. Best estimates of relative energies (as quoted below) were obtained by evaluation of total electronic energies at the MP3/6-31G**//6-31G* level. By vibrational analysis, it was confirmed that the gauche form is an equilibrium species and that the syn form, which lies 39.8 kJ mol^-1 above the gauche form, is a transition structure connecting enantiomeric gauche forms by internal rotation. The anti form, characterized by vibrational analysis as an equilibrium species on the STO-3G, 3-21G, and 6-31G* potential-energy surfaces, lies in a shallow minimum 7.7 kJ mol^-1 above the gauche form. A transition structure connecting gauche and anti forms by internal rotation lies, however, only 0.4 kJ mol^-1 higher on the electronic-energy surface and, after zero-point vibrational -energy corrections, 0.3 kJ mol^-1 below the anti form which is, therefore, the true transition structure. The transition structure for inversion at one nitrogen atom of the gauche form is shown to be a C_S structure lying 26.2 kJ mol^-1 above the gauche form itself. Various other structures, including those of D_2h and D_2d symmetry, are discussed briefly and shown by vibrational analysis to be neither equilibrium species nor true transition structures on the 3-21G potential-energy surface. Finally, the complete potential function for internal rotation has been evaluated at levels up to MP3/6-31G**//HF/6- 31G*, and the coefficients of the corresponding four-term Fourier cosine series are presented and briefly discussed.