Proton affinity and heat of formation of vinylimine CH ₂ CHCHNH

The vinylimmonium cation [CH 2 CHCHNH 2 ] + , [1H] + , originating from cyclopentylamine has been studied by photoionization, ion cyclotron resonance mass spectrometry and ab initio molecular orbital calculations. A heat of formation, Δ H [CH 2 CHCHNH 2 ] + of 747±7 kJ/mol has been deduced from photoionization experiments, after correction for the thermal energy of the precursor molecules. The proton affinity of vinylimine, CH 2  CHCHNH, 1, (PA(1) = 908±4kJ/mol) has been deduced from proton exchange experiments using the ‘bracketing’ technique. This latter, combined with the previously determined Δ H [CH 2 CHCHNH 2 ] + leads to Δ H (CH 2 CHCHNH)=125kJ/mol with a possible error of ±11 kJ/mol. Molecular orbital calculations, conducted at the MP 2 /6‐311 ++G ** //MP2/6‐31G* + ZPE level, lead to comparable estimates Δ H (CH 2  CHCHNH 2 ) + = 746kJ/mol and Δ H (CH 2 CHCHNH) = 128–135 (kJ/mol).