Theoretical study on the transformation mechanism between dipole-bound and valence-bound anion states of small uracil-water clusters and their photoelectron spectra

Microhydration effects on the transformation from the dipole-bound to valence-bound anions of uracil have been investigated at the density-functional BH&HLYP level of theory. It was found that the transformation barrier height decreases with an increase in the number of water molecules although the calculated barrier height somewhat depends on the water-binding site. Photoelectron spectra of the hydrated uracil anions, U−·(H2O)n (n = 1 and 2), have been calculated by a simplified Franck-Condon model and by an on-the-fly direct molecular dynamics method and then compared to experimental results. We found that the broad feature of the measured photoelectron spectra is mainly determined by vibrational motions including out-of-plane bending of the uracil anion core and that water solvation structures do not largely affect spectral broadening.