Reinterpretation of the UV Spectrum of Cytosine: Only Two Electronic Transitions?

UV vibronic spectrum for cytosine has been calculated using high‐level quantum chemistry. Coupled with a careful analysis of the available experimental spectra, it is suggested that the spectrum contains only two π–π* transitions; the other two, less resolved bands cannot be type π–π*, and are probably not even n–π* but rather vibrational subbands (see figure). magnified image Theoretical vertical excitation energies and simulated UV spectrum are reported for the canonical (amino‐oxo) form of cytosine. The calculations were performed by the EOM‐CCSD and EOM‐CC3 methods with basis sets up to triple‐zeta quality. Beyond vertical excitations, the complete vibronic spectrum has been determined the first time, using the linear vibronic coupling (LVC) method. On the basis of the computed data, a critical review of the available experimental information (UV spectra, linear dichroism, Resonance Raman and REMPI measurements) has led to the surprising conclusion that cytosine has only two, rather than four, observable electronic transitions in the UV range up to 60 000 cm −1 . The result may be of crucial importance for future photochemical studies.