In the u.v. absorption spectra, main absorption bands of 5-oxo isomers were always found at around 300 nm, whereas 7-oxo isomers showed no band there but at 260～270nm. This suggested that n-electrons would more be delocalized on the ring system of 5-oxo series than those in 7-oxo series. In accord with the u.v. spectra, the i.r. absorption spectra indicated that the carbonyl group of 7-oxo isomer considerably polarized than that of 5-oxo isomer. The absorption band due to carbonyl group appeared around 1690 cm- in the spectra of 5-oxo isomers, whereas the band always below 1650 cm^-1 for 7-oxo isomers. Proton magnetic resonance spectra showed that the hydrogen on pyrimidine ring(6-H) of a 5-oxo isomer resonated at lower magnetic field than that of the corresponding 7-oxo isomer. This was indicative of higher density of electron on the 5-oxo-pyrimidine ring and consistent with the results from u.v. and i.r. spectra. In the mass spectrometry, the most of the predominant ions from 5-oxo isomers were explained by a manner, in which the fragmentation began with CO-elimination. A process similar to retro Diels-Alder reaction and fragmentation accompanying hydrogen transfer produced some important ions. In addition to CO-elimination, electron impact to 7-oxo isomers caused predominant loss of thiocyanato radical, leading to important ions. Retro Diels-Alder reaction followed by cyanide elimination furnished the characteristic ion peak at m/e 100 for all of the 7-oxo isomers tested.
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