HEI AND HEII SPECTRA OF 3-HALOPYRIDINES

The ultraviolet photoelectron spectra (UPS) of 3-fluoro-, 3-chloro-, 3 -bromo- and 3-iodopyridine were recorded and interpreted using a compo site-molecule model. The sequence of the four lowest ionization energi es for 3-fluoro- and 3-chloropyridine is: pi(3) (1a(2)) < n(N) (11a(1) ) < pi(2) (2b(1)) < sigma(pyr) (7b(2)), whereas for 3-bromo- and 3-iod opyridine the assignment is: pi(3) (1a(2)) - pi(x) < n(N) (11a(1)) < s igma(x) < pi(2) (2b(1)) (where X represents a bromine or iodine lone-p air). Comparison of the HeI and HeII band intensities and correlations with the UPS assignment of the 2-halopyridines confirm this sequence. However, ab initio calculations using the STO-3G//STO-3G* and 6-31G* //STO-3G* models do not agree with either the composite-molecule mode l, simple UPS correlations and HeI/HeII cross-section ratios. For 3-fl uoropyridine, the HAM/3 model was in agreement with the proposed assig nment.