C−H Bond Dissociation Energies of Alkyl Amines: Radical Structures and Stabilization Energies§

A previous experimental study of the αC−H bond dissociation energies (BDEs) of amines indicated a significant decrease in BDE (or increase in radical stabilization energy, Es) in the series primary, secondary, and tertiary. However, this was not supported by theoretical investigations. The αC−H BDEs of trimethylamine ((CH3)3NH), triethylamine ((C2H5)3NH), and tri-n-butylamine ((C4H9)3NH) and of the cyclic secondary amines piperidine, piperazine, morpholine, and pyrrolidine were therefore determined by photoacoustic calorimetry in benzene solvent. Ab initio procedures, which incorporated isodesmic reactions to minimize residual correlation errors, were used to obtain the BDEs of several of these for direct comparisons. Also the BDEs of methylamine (CH3NH2), ethylamine (C2H5NH2), isopropylamine ((CH3)2CHNH2)), and dimethylamine ((CH3)2NH) were calculated as a check on the earlier results. The experimental BDEs in kJ mol-1 at 298 K (±10 kJ mol-1), estimated from the photoacoustic calorimetric measurements, w...