Anion spectroscopy of uracil, thymine and the amino-oxo and amino-hydroxy tautomers of cytosine and their water clusters

In this work we investigate different forms of electron binding in the mass-selected and cooled nucleobases uracil, thymine and cytosine and their water clusters. In photodetachment-photoelectron spectra of the pyrimidine nucleobases, sharp structures were found at 86 +/- 8 meV (uracil), 62 +/- 8 meV (thymine) and 85 +/- 8 meV (cytosine), which are due to photodetachment of dipole-bound states. The photodetachment angle dependance of these states shows mostly p-wave detachment, which confirms the predicted predominant s-character of the electronic wave function of dipole-bound states. This anisotropy of electron emission and their sharp photodetachment structures can be taken for dipole-bound state recognition. Water attachment to the nucleobases results in positive valence-bound electron affinity, s-wave detachment and broad spectra, implying that the electron now is trapped inside the pi* LUMO of the nucleobases, stabilized by the water dipole. The solvent shifts in dependence on water aggregation are linear and allow by extrapolation an estimation of the monomer electron affinities. All three pyrimidine nucleobases are estimated to have a very similar valence-bound electron affinity in the range of 0-200 meV. In nucleobase . (H2O)(n) clusters, due to the large total dipole moment, dipole-bound states also exist. Resonant excitation of these dipole-bound states with a photon of 1064 nm wavelength causes dissociation of the anion cluster, leading to monomer anions in their dipole-bound state. These monomer anions can be photodetached by a second IR photon. Whereas, for uracil and thymine, one dipole-bound state is detected, for cytosine we find two dipole-bound states (85 +/- 8, 230 +/- s meV) which are attributed to the dipole-bound states of the simultaneously present amino-hydroxy and amino-oxo cytosine tautomers. We also give a possible explanation why the formation of the dipole-bound state of the amino-oxo tautomer at 230 meV is improbable in the supersonic expansion. (C) 1998 Elsevier Science B.V. All rights reserved.