He I and He II spectra of 2-halopyridines

The He I and He II ultraviolet photoelectron spectra of 2-fluoro-, 2-chloro-, 2-bromo-, and 2-iodopyridine have been recorded and interpreted in terms of a composite-molecule model. The sequence of the four lowest ionization energies for 2-fluoro- and 2-chloropyridine is: π₃ (1a₂) > n_N (11a₁) > π₂ (2b₁) > π_pyr (7b₂), whereas for 2-bromo- and 2-iodopyridine the assignment is: π₃ (1a₂) - π_X > n_N (11a₁) > π_X > π₂ (2b₁), where X represents a bromine and iodine lone-pair. Comparison of the He I and He II band intensities confirmed this assignment. However, ab initio calculations at the STO-3G*/STO-3G* and 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 using the HAM/3 model was found to be in agreement with this assignment.