HE-I AND HE-II SPECTRA OF 2-HALOPYRIDINES

The He I and He II ultraviolet photoelectron spectra of 2-fluoro-, 2-c hloro-, 2-bromo-, and 2-iodopyridine have been recorded and interprete d in terms of a composite-molecule model. The sequence of the four low est ionization energies for 2-fluoro- and 2-chloropyridine is: pi3 (1a 2) < n(N) (11a1) < pi2 (2b1) < sigma(pyr) (7b2), whereas for 2-bromo- and 2-iodopyridine the assignment is: pi3 (1a2) - pi(X) < n(N) (11a1) < sigma(X) < pi2 (2b1), where X represents a bromine and iodine lone-p air. Comparison of the He I and He II band intensities confirmed this assignment. However, ab initio calculations at the STO-3G//STO-3G* an d 6-31G*//STO-3G* levels did not agree with the sequence predicted by either the composite-molecule model, simple correlations and the He I /He II cross-section ratios. For the 2-fluoropyridine, a comparison us ing the HAM/3 model was found to be in agreement with this assignment.