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 wat er clusters. In photodetachment-photoelectron spectra of the pyrimidine nuc leobases, sharp structures were found at 86 +/- 8 meV (uracil), 62 +/- 8 me V (thymine) and 85 +/- 8 meV (cytosine), which are due to photodetachment o f 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 ani sotropy of electron emission and their sharp photodetachment structures can be taken for dipole-bound state recognition. Water attachment to the nucle obases results in positive valence-bound electron affinity, s-wave detachme nt and broad spectra, implying that the electron now is trapped inside the pi* LUMO of the nucleobases, stabilized by the water dipole. The solvent sh ifts in dependence on water aggregation are linear and allow by extrapolati on an estimation of the monomer electron affinities. All three pyrimidine n ucleobases are estimated to have a very similar valence-bound electron affi nity in the range of 0-200 meV. In nucleobase . (H2O)(n) clusters, due to t he large total dipole moment, dipole-bound states also exist. Resonant exci tation of these dipole-bound states with a photon of 1064 nm wavelength cau ses dissociation of the anion cluster, leading to monomer anions in their d ipole-bound state. These monomer anions can be photodetached by a second IR photon. Whereas, for uracil and thymine, one dipole-bound state is detecte d, for cytosine we find two dipole-bound states (85 +/- 8, 230 +/- s meV) w hich are attributed to the dipole-bound states of the simultaneously presen t amino-hydroxy and amino-oxo cytosine tautomers. We also give a possible e xplanation why the formation of the dipole-bound state of the amino-oxo tau tomer at 230 meV is improbable in the supersonic expansion. (C) 1998 Elsevi er Science B.V. All rights reserved.