The NB vibrational frequency in complexes of aromatic azines with BCl 3

Abstract

A comparative study of the infrared spectra for the 1:1 EDA complexes of pyridine, quinoline and acridine with BCl 3 yielded assignment of the NB frequency, which is identified as the shifted ring breathing mode of the azine. AM1 calculations for the interaction of BCl 3 with these molecules predict stable complexes at an NB intermolecular separation of 1.62–1.63 Å with the orientation of the non-planar BCl 3 being different for each azine donor. Predicted NB vibrational frequencies correlate well with the experimental values, the formation constant of the complex and the basicity of the donor. Results confirm an increasing n-electron pair donor strength in the order quinoline<pyridine<acridine. A normal coordinate analysis for the n→ σ type C 3 v EDA complex NH 3→BCl 3 is also included and is consistent with ab initio calculations and experimental data.