A UV photoelectron spectroscopic study of addition compounds of boron trifluoride with methyl-substituted pyridines

Abstract

Vertical ionization energies of the 1 : 1 molecular complexes of boron trifluoride with the three picolines and 3,5-lutidine are reported along with assignments based on their HeI photoelectron spectra and semi-empirical molecular orbital calculations. The shift in the lone pair (nN) orbital of the donor molecules towards a higher ionization energy (IE) upon complexation is found to be essentially constant in all the complexes (nearly 2.8 eV) except in 3,5-lutidine·BF3 where the shift is 3.3 eV. Such shifts in the IEs of the π orbital are about 1.1 eV for the 2- and 3-picoline complexes and 1.3 eV for the complexes of 4-picoline and 3,5-lutidine. These shifts reflect the relative stabilities of the complexes. The complex 2,6-lutidine·BF3 undergoes dissociation into the free donor and acceptor molecules under the conditions of our experiment. A comparison of the shifts is made with the experimental heats of reaction of the pyridine bases with boron trifluoride. Relative stabilities of the complexes as deduced from ab initio and AM1 calculations are compared with predictions of gas-phase basicities of the bases. © 1991.