Bond dissociation energy (BDE) has
been calculated for a series
of compounds that contain N–O bonds. These structures encompass
model N,N,O-trisubstituted
hydroxylamines that include O-methoxy, O-acyl, and O-phenyl hydroxylamines. The calculations
used three accurate composite methods, CBS-QB3, CBS-APNO, and G4 methods
and the computationally more affordable M06-2X/6-311+G(3df,2p) density
functional theory (DFT) functional. The calculated N–O single-bond
BDEs are 5–15 kcal/mol higher than a generic N–O BDE
of 48 kcal/mol quoted in the literature and in textbooks. The M06-2X
DFT functional provides BDEs that are in excellent agreement with
the higher-level composite methods. We also provided a comparison
of the N–O BDE for pyridine-N-oxide to simple
trialkylamine oxides. Based on an experimental BDE of 63.3 ±
0.5 kcal/mol for pyridine-N-oxide, our best estimate
gives 56.7 ± 0.9 kcal/mol N–O BDE for trimethylamine-N-oxide and 59.0 ± 0.8 kcal/mol for triethylamine-N-oxide.