THEORETICAL-STUDY OF THE ELECTRONIC-SPECTRUM OF CYTOSINE

Ab initio quantum chemical methods based on multiconfigurational secon d order perturbation theory have been employed to compute the electron ic excitation spectrum of cytosine in the gas phase. The computed vert ical pi --> pi transition energies for the four lowest singlet excite d valence states are 4.4, 5.4, 6.2, and 6.7 eV, in agreement with expe riment (measured energies are 4.6-4.7, 5.2-5.8, 6.1-6.4, and 6.7-7.1 e V, respectively). It is also suggested that the lowest n --> pi trans ition should be seen at energies close to 5.0 eV. The computed polariz ation directions for the three lowest pi --> pi transitions are appro ximately 60 degrees, -5 degrees, and -40 degrees and assign a large va lue for the first angle. This is in contradiction to experiment, which gives a small angle. The crystal field effects on the angles can, how ever, be expected to be large.