Water dimer to pentamer with an excess electron:: Ab initio study

We have studied the electronic structures, energetics, electron vertical detachment energies (VDEs), and O-H vibrational spectra of various conformers of water clusters with an excess electron [e+(H2O)(n), n=2-5] or anionic water clusters [(H2O)(n)(-)] using comprehensive ab initio calculations. As noted in our preliminary work [J. Kim , Phys. Rev. A 59, 930 (1999)], the structure of the water dimer anion is characterized to be linear-like (slightly towards the cis conformer) but very floppy with large wide-ranging zero point vibration motion at 0 K. The lowest energy structures of the water trimer to pentamer anion are all cyclic with very small VDEs (< 0.05 eV). However, these cyclic structures which are metastable are prone to become the neutral species by releasing an excess electron because the transition barriers seem to be very small. Thus, observation of such cyclic structures would not be feasible. On the other hand, a linear water trimer structure which is 0.8 kcal/mol higher in energy than the cyclic form gives the VDE (0.14 eV) close to the experimentally observed value. A large VDE observed in the pentamer also corresponds to a slightly high energy conformer. This suggests that formation of anionic water clusters in experiments seems to be dynamically and kinetically driven. (C) 1999 American Institute of Physics. [S0021-9606(99)30140-9].