Ab initio Studies on Hydrazines: 4H-1,2,4-Triazol-4-amine

Abstract

The geometries of four stationary structures of 4H-1,2,4-triazol-4-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 (7). All its calculated vibrational frequencies are real and, after zero-point vibrational-energy corrections, it lies 26.9 kJ mol^-1 below the 'parallel' C, structure (6), here characterized as the transition structure for internal rotation about the N-NH₂ bond (cf. 26.5 kJ mol^-1 for the corresponding structures of 1H-pyrrol^-1-amine, but only 8.7 kJ mol^-1 for the corresponding structures of 2H-1,2,3-triazol-2-amine). The transition structure for inversion at the NH₂ centre is, as for 1H-pyrrol-1-amine and 2H-1,2,3-triazol-2-amine, the perpendicular C_2v   structure (5), the barrier being 21.4 kJ mol^-1 (cf. 24-26 kJ mol^-1 for the two reference azolamines ). The planar C_2v structure (4) is a second-order saddle point lying 66.6 kJ mol^-1 above the equilibrium structure (cf. 69.4 kJ mol^-1 for 1H-pyrrol-1-amine, but only 41 .7 kJ mol^-1 for 2H-1,2,3-triazol-2-amine). The calculated NH-stretching vibrational frequencies for 4H-1,2,4-triazol-4-amine are c. 20 cm^-1 higher than those of 1H-pyrrol-1-amine and their splitting is c. 8 cm^-1 greater but they show a very similar relative-intensity pattern, quite unlike that calculated for 2H-1,2,3-triazol-2-amine. ′