| Abstract
| - Bond dissociation enthalpies (BDEs) of a large series of aliphatic amines (21) were measuredby means of photoacoustic calorimetry. Despite the different structures studied in the primary, secondary,and tertiary amine series, the α(C−H) BDEs were found to be very similar for unconstrained amines withvalues very close to 91 kcal/mol. αC- and N-alkylation or introduction of an hydroxy group only slightlyaffect the BDEs, a fact in perfect agreement with calculations performed at different CBS levels. Thisdemonstrates the predominance of the two-orbital−three-electron interaction involving the N and αC• orbitals.On the other hand, the N−H BDE decreases when going from primary to secondary amines. This result isinterpreted in term of a hyperconjugation in σC−C bonds, which leads to a stabilization of the aminyl radical.For cyclized amines, the BDEs depend on the relative geometry of the singly occupied αC• orbital withrespect to that of the N atom, disfavoring the two-orbital−three-electron interaction. However, such structurescan exhibit through-bond interaction. For a crowded structure such as triisopropylamine, for which the αC•orbital is not coplanar with the nitrogen one, the relaxation of a strain energy allows the BDE to becomparable to flexible structures.
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