Geometry Change on Ionization. Calculation of I_V-I_A Relaxation Energies for NH₃, N₂H₄ and CH₃NH₂

Abstract

Ab initio calculations on the NH₃, N₂H₄ and CH₃NH₂ molecules and their respective ground- state cations represent the experimentally observed adiabatic and vertical ionization energies with reasonable accuracy. The unusually large I_V - I_A difference energy for N₂H₄ is a measure of the considerable change in geometry accompanying ionization. The calculated value of this relaxation energy at the MP3/6-31G** level is 2.3 eV , rather higher than the two experimental measurements of 1.55 eV , based on the band profile of the ultraviolet photoelectron spectrum, and 1.6-1.8 eV , determined from ion-molecule equilibrium measurements. The smaller I_V-I_A values for NH₃ and CH₃NH₂ are calculated more accurately. Theoretical vibration frequencies are also obtained for the molecular cations.