Ab initio Studies on Hydrazines: 2H-1,2,3-Triazol-2-amine With Special Reference to Its Equilibrium Structure and Vibrational Spectrum

Abstract

The geometries of four stationary structures of 2H-1,2,3-triazol-2-amine have been optimized with the 3-21G and 3-21G(N*) basis sets. The lowest-energy and only equilibrium structure predicted by these calculations is the 'perpendicular' C_s form (3), whereas infrared studies on benzo-annelated analogues had suggested it might be the 'parallel' C_s form (2) stabilized by 'double hydrogen-bonding' of the amino-hydrogen atoms to the flanking ring-nitrogen atoms. The latter form (2) is here characterized as the transition structure for rotation about the N-NH₂ bond and, after zero-point vibrational-energy corrections, is calculated to lie 8.7 kJ mol^-1 above the equilibrium structure (3) at HF/3-21G(N*) level or only 3.8 kJ mol^-1 at MP4/6-31G** level. This very low barrier to internal rotation (cf. 26.5 kJ mol^-1 for the analogous 1H-pyrrol-1-amine) may be due to double hydrogen-bonding of the kind suggested by the experimental study mentioned above. The transition structure for inversion at the NH₂ centre is, as for 1H-pyrrol-1-amine, the perpendicular C_2v structure (5), the barrier being 25.8 kJ mol^-1 (cf. 24.5 kJ mol^-1 for 1H-pyrrol-1-amine), and the planar C_2v structure (4) is a second-order saddle point lying 41.7 kJ mol^-1 above the equilibrium structure (3). Calculated NH-stretching frequencies, their separation, and relative intensities as compared with experimental values for benzo-annelated analogues offer broad support for the assignments above based on relative energies.