Development of the charge-transfer-to-solvent process with increasing solvent fluid density: the effect of ion pairing

GERMÁN SCIAINI; ERNESTO MARCECA; ROBERTO FERNÁNDEZ PRINI

PHYSICAL CHEMISTRY CHEMICAL PHYSICS

Elsevier

Año: 2006 vol. 8 p. 4839 - 4848

1463-9076

The study of the UV spectroscopic behaviour of alkali metal iodides dissolved in supercritical ammonia showed that two absorb ing species contributed to the UV absorption.  The two species differed in the type of interaction of iodide with the cation, i.e. going contact ion pairs to free iodide ion, the observed absorption band varied according to the species that prevailed as the solvent density changed.  This experimental evidence was supplemented with molecular dynamics simulations and electronic structure calcultions which showed that at very low solvent density when the contact ion pair is the dominant species, a sudden change from the internal charge transfer photexcitation route to a charge-transfer-to-solvent transitions occurred. This finding empahsized the importance of solvation at very low density not only for the photoexcitation process, it also allows connecting the thermodynamci behaviour of the soltues in solution with that observed in their vapour pahse.  We have tried to draw a consistent picture of the available information of UV photoexcitation for iodides in vapour, in solution either forming contact ion pairs or present as free iodide ions, including their behaviour in small clusters of highly polar molecules.  The importance of the cation has been clearly shown in this investigation.  A relation betwqeen the phtoexcited electron in contact ion pairs and the solvated electron of alkali metals in small NH3 clusters has been conjectured.