The N-B vibrational frequency in complexes of aromatic azines with BCl3

A comparative study of the infrared spectra for the 1:1 EDA complexes of pyridine, quinoline and acridine with BCl3 yielded assignment of the N-B frequency, which is identified as the shifted ring breathing mode of the azine. AMI calculations for the interaction of BCl3 with these molecules predict stable complexes at an N-B intermolecular separation of 1.67-1.63 Angstrom with the orientation of the non-planar BCl3 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 --> sigma type C-3 upsilon EDA complex NH3 --> BCl3 is also included and is consistent with ab initio calculations and experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.