Ab initio vibrational analysis of three rotamers of 2‐propen‐1‐imine, H2CCHHCNH, and methanimine, H2CNH

The results of complete Hartree‐Fock (HF)/6–31G geometry optimization and HF/6–31G//HF/6–31G force fields evaluations of three rotamers of the H₂CCH–HCNH molecule (s‐trans–d‐trans–, s‐trans–d‐cis‐ and s‐cis–d‐trans‐2‐propen‐1‐imine) and the H₂C=N–H molecule are reported. All three conformers were found to be planar with the s‐trans–d‐trans conformer lowest in energy. This was corroborated by further complete optimizations of the geometries of the s‐trans–d‐trans and s‐trans–d‐cis conformers at the HF/6–31G*(5D) level as well as single‐point MP4/6–31G*//HF/6–31G*(5D) calculations. The assignment of the vibrational frequencies of the propenimine rotamers and some isotopomers of methanimine are also reported. The majority of the experimental frequencies of propenimine in the gaseous phase are found to belong to the s‐trans–d‐trans form, but a few frequencies are attributed to the high‐energy s‐trans–d‐cis conformer.