From the Nonplanarity of the Amino Group to the Structural Nonrigidity of the Molecule: A Post-Hartree-Fock Ab Initio Study of 2-Aminoimidazole

摘要

The results of an ab initio post-Hartree-Fock study of the details of the molecular structure and the pathways of the interconversion of the 2-aminoimidazole molecule enantiomers are reported. The geometry of the local minimum and the transition states were optimized by the gradient procedure at the MP2 and DFT/B3LYP levels of theory and were verified by energy second derivative calculations. The medium and large size basis sets[(6-31 + G(d,p), 6-311 + + GG(df,pd), D95+ +(df,pd), and correlation consistent aug-cc-pVDZ)] have been used. Based on an analysis of the equilibrium geometry and the pathways of interconversion, we reveal that the phenomenon of a nonplanar geometry of the amino group in the amines has a more complex nature than previously expected. Our calculations show that this molecule is structurally nonrigid at least with respect to the torsion and umbrella vibration of the amino group. It results in a very complex picture of the amino group motion which includes tunneling of the barrier and an above barrier large amplitude motion. We have found that in contrast to the situation of aniline, the most probable pathway of the AIM interconversion is the cis-rotation of the amino group around the CN exocyclic bond.