Electron correlation and basis set effects in unimolecular reactions. A study of the model rearrangement system N2H2

As a model for simple 1, 2 shift rearrangements, N2H2 has been studied with ab initio wave functions at several levels of theory. Equilibrium structures for aminonitrene, trans-diimide, and cis-diimide and transition state structures for the two rearrangement reactions that interconvert the isomers were carefully determined so as to isolate basis set and electron correlation effects on the structure predictions. An examination of the changes in the calculated electron density between wave functions from different levels of treatment is presented. The self-consistent electron pairs (SCEP) method was used to handle the large correlated wave functions obtained in this study. With the largest basis set (TZ+2P) used, and with SCEP wave functions spanning a space of up to 176 000 configurations, the following energies relative to trans-diimide were found: 7.2 kcal for cis-diimide, 55.1 kcal for the trans to cis-diimide barrier, 24.5 kcal for aminonitrene, and 82.6 kcal for the trans-diimide to aminonitrene barrier. The system was found to be planar in all structures.