Hindered Internal Rotation in Boron Halide Complexes of N,n-Dimethylformamide

The barriers to internal rotation about the CN partial double bond in N,N-dimethyl-formamide (DMF) and its complex with BF 3 have been determined in tetrachloroethane (TCE) by nuclear magnetic resonance measurements. The activation energy for rotation E a , increased from 6.2 ± 0.3 kcal mole −1 for DMF to 14.5 ± 1.1kcal mole −1 for DMF•BF 3 . Values of ΔH ≠ and ΔS ≠ were also derived. Low field shifts of the methyl resonances and an increase in the relative shifts between the two methyl groups were observed on complex formation. The formyl coupling constant to the methyl groups was larger for the complex than for pure DMF. In addition, for DMF—BCl 3 evidence for the presence of two types of complex was found. The present results are compatible with BF 3 and BCl 3 complexing to the amide oxygen atom. The energy changes for internal rotation and J variation are attributed to changes in the π character of the CN bond due to stabilization of the charged resonance from [Formula: see text] upon complex formation.