INSTITUTE OF PHYSICS POLISH ACADEMY OF SCIENCE

ON21 MOLECULAR PHOTOPHYSICS GROUP





AB INITIO STUDIES OF CLUSTER MODELS OF ELECTRON SOLVATION IN WATER

Andrzej L. Sobolewski





Contents

Experimental facts

Computational methods

Models of hydronium radical formation

Solvation of the hydronium radical

Conclusions

References


Experimental facts

Electrons solvated in water can be produced by UV excitation of some organic chromophores (e.g. indole, phenol) in water, liquid water itself, or by photodetachment of some anions in water.

The solvated (hydrated) electron in water is a species with the following distinctive properties:

- broad and intense absorption spectrum centered near 720 nm

- high rate of diffusion

- high reactivity

Despite many years of research on the production mechanism of the hydrated electron, its reaction kinetics and its spectroscopy, the microscopic structure of this species remains an enigma.


Computational methods

The ground-state structures were optimized with the DFT and/or the second-order Moller-Plesset (MP2 or UMP2) methods. The geometry optimization of excited states was performed at the CASSCF level. The potential-energy profiles are minimum-energy reaction-path profiles, that is, the values of all other coordinates were optimized for a given value of the reaction coordinate. Single-point energy calculations along the reaction path were performed with the CASPT2 method. In the calculation of the absorption spectra of clusters, both the CASPT2 and TDDFT methods were used.

The standard 6-31G** split-valence double-zeta Gaussian basis set was supplemented with an additional set of diffuse s and p Gaussian functions of exponent =0.02, located on the oxygens in order to provide additional flexibility for the description of the Rydberg electron. Details of the calculations are described in the original papers listed below.


Models of hydronium radical formation




Fig.1 Photoexcitation of an organic chromophore in water

Phenol-water clusters


Fig.2 Photoexcitation of liquid water

Water dimer





Fig.3 Photodetachment of a halide anion in water

HCl-water cluster


Solvation of the hydronium radical


n=0

n=1

n=3

n=6

n=9

Fig.4 DFT(MP2) geometry of clusters



n=0

n=1

n=3

n=6

n=9

Fig.5 Singly occupied molecular orbital of clusters





Fig.6 Electronic absorption spectra (ABS)
and vertical detachment energy (VDE)

Fig.7 Stability of (n=0,3) clusters

Conclusions

radicals can be formed by a barrierless hydrogen-transfer reaction in the lowest excited singlet state of water clusters or in the lowest state of an organic chromophore complexed with water.

Microsolvation of the radical leads to a charge-separated complex, consisting of a hydronium radical and a localized electron cloud, connected by a water network.

The vertical excitation energies of the s p electronic transitions of the clusters cover roughly the energy range of the absorption spectrum of the hydrated electron.

Microsolvation with water stabilizes considerably the (unstable) radical, but for n=3 it is still a metastable species which can decay by detaching the hydrogen atom.

clusters could be the carriers of the characteristic properties of the hydrated electron.


References

1. A. L. Sobolewski and W. Domcke, "Photoinduced electron and proton transfer in phenol and its clusters with water and ammonia",J. Phys. Chem. A 105 (2001) 9275

2. A. L. Sobolewski and W. Domcke, "Hydrated hydronium: a cluster model of the solvated electron?", Phys. Chem. Chem. Phys., 4 (2002) 4

3. A. L. Sobolewski, W. Domcke, "Photochemistry of : a cluster model of the photodetachment of the chloride anion in water", J. Phys. Chem. A, submitted



Contents

Experimental
facts

Computational
methods

Models of hydronium
radical formation

Solvation of
the hydronium radical

Conclusions

References