Electronic Spectra and Electronic Structures of Some Basic Heterocyclic N-Oxides

The ultraviolet absorption spectra of pyrazine mono-N-oxide, 4-methylpyrimidine N-oxide and pyrazine di-N-oxide have been measured under various conditions, and band analyses of those spectra have been carried out. In order to interpret the spectra and other physicochemical properties of pyrazine di-N-oxide, semi-empirical LCAO MO calculations including the treaments of configuration interactions, have been undertaken in a manner similar to previous calculations on pyridine N-oxide. On the other hand, the spectra of pyrazine mono-N-oxide and 4-methylpyrimidine N-oxide have been interpreted on the basis of the calculated results on pyridine N-oxide described in a previous paper. The high intensity bands (278, 274 and 323.6 mμ for 4-methylpyrimidine N-oxide, pyrazine mono- and di-N-oxides respectively) can reasonably be assigned to charge transfer bands such as the 282 mμ band of pyridine N-oxide. The above substances also exhibit weak bands in the longest wavelength region in non-polar solvents. These weak bands may possibly be due to the n→π* transitions. SMO calculations on isoquinoline N-oxide and acridine N-oxide have also been carried out with the same parameters as those in the case of pyridine N-oxide in non-polar solvents, and various physicochemical properties have been discussed on the basis of these calculations. As has been pointed out in a previous paper, the parameter for the oxygen atom used here is too small. To estimate the most reasonable value, SMO calculations were carried out many times on pyridine N-oxide, which was used as a model compound. As a result, it was found that ko (in Eq. 8) ≈0.8 is the best value.