He I and He II spectra of 2-halopyridines

• Published in: Journal of heterocyclic chemistry Vol. 32; no. 1; pp. 89 - 96
• Main Authors: Caine, Gavin R., Dunne, Simon J., Von Nagy-Felsobuki, Ellak I.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley-Blackwell 01-01-1995; Wiley‐Blackwell
• ISSN: 0022-152X; 1943-5193
• DOI: 10.1002/jhet.5570320116

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) > nN (11a1) > π2 (2b1) > πpyr (7b2), whereas for 2‐bromo‐ and 2‐iodopyridine the assignment is: π3 (1a2) ‐ πX > nN (11a1) > πX > 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.