New description of the substituent effect on electronic spectra by means of substituent constants—VI. Utraviolet spectra of 4-substituted pyridine N-oxides and blue shifted iodine bands of their EDA complexes with iodine

Abstract

Electronic spectra of 4-substituted pyridine N-oxides and their EDA complexes with iodine were studied. The substituent effect on the near u.v.1A1 intramolecular CT bands of the N-oxides and on the blue shifted iodine bands caused by CT complex formation are discussed in terms of a general equation, theoretically derived in order to describe the substituent effect on electronic spectra by means of substituent constants. The results are quite successful and supported by semi-empirical SCFMO-CI calculations. Based on the results mentioned above, the character of n-σ type N-oxide—iodine CT complexes is also examined. The complex formation constants (log K) and pKa values of the N-oxides correlate especially well, indicating that the CT interaction mechanism cannot be neglected in proton addition reactions such as hydrogen bonding and pKa values.