In the present investigation, the vertical ionization energies, excitation energies and oscillator strengths of cytosine and its methyl derivatives were calculated by using a semi-empircal SCF-MO-HAM/3 method proposed by Asbirink et al. Fig. 1 in the Chinese Text is a schematic diagram of some occupied frontier orbitals of cytosine and its methyl derivatives. Fig. 2-4 are correlation diagrams of the experimental and calculated vertical ionization energies of cytosine and its eight methyl derivatives. Fig. 5-7 give HAM/3 assignments of photoelectronic spectra of cytosine and its eight methyl derivatives respectively. Fig. 8-9 show the comparison of UV spectra of cytosine and 3-methyl cytosine with \l\'s calculated by HAM/3. From these figures, it may be seen that the agreement between experimental values and HAM/3 calculations is much better than the results of CNDO/S calculations. The reasons why we prefer the assignments of the first two bands of the UV photoelectron spectra of cytosine and its methyl derivatives by the HAM/3 method to those by the CNDO/S method were fully discussed. The excitation energies and oscillator strengths of four cytosine tautomers were calculated by the HAM/3 configuration- interaction method. The results were compared with UV absorption spectra as shown in Table 1.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, IONIZATION ENERGY, ELECTRONIC STRUCTURE, PHOTOELECTRON SPECTROSCOPY, MOLECULAR ORBITAL THEORY, CYTOSINE, HAM METHOD