Rotation barrier of the azide group in azidopyridines: Molecular orbital treatment

The height of the rotational barriers around C(pyridine)N(azide) single bond in azidopyridines has been determined via ab initio molecular orbital calculations. The optimized geometry obtained from RHF/6-31G∗∗ results was used as an input for a single-point MP2/6-31G∗∗ calculation, the results of which are reported in this work. The potential energy function of rotation was subjected to Fourier analysis and terminated nicely at V3 as is shown from the results of least square treatment. The s-cis conformer of 2-azidopyridine is slightly more stable than the s-trans conformer and both are more stable than others obtained during rotation. A transition state is identified and confirmed, during rotation, via the location of a stationary point through a saddle-point calculations. A Hessian-type run is carried out to calculate the frequency of vibration, only one imaginary, negative, frequency was obtained. The height of the rotational barrier of the azide group in 2-azidopyridine is the largest, about 7 kcal/mol, whereas it amounts 3.32 and 4.04 kcal/mol for 3-azido and 4-azidopyridine, respectively.