The Absorption Spectrum of Cytosine Tautomers: Beyond the Static Approach.

The absorption spectrum of cytosine in water has been studied by combining Car-Parrinello molecular dynamics (MD) with a multiconfigurational perturbation theory treatment of the electronic structure. The MD simulations were performed for four different tautomeric forms of cytosine in a unit cell with 60 water molecules. The relative energies and transition dipole moments of a large number of excited states have been calculated on a representative sample of conformations along the MD trajectories. In this way, the broad experimental peaks can be decomposed, and the effect of the distortions on the nature of the excited states can be assessed. The loss of planarity of the molecule is significant, and hence, the excited states can no longer be defined as pure n → π* or π → π* excitations. We propose an analysis to assign the different transitions according to the main contribution. The keto N1H form turns out to be the most stable one, and the calculated spectra of this tautomer show good agreement with experimental measurements. The mixed nπ*/ππ* character of some states leads to a significant increase of intensity in spectral regions dominated by the dark nπ* transitions considering a planar structure.