Effect of the range of solvent interactions upon the charge-transfer-to-solvent of the donors I− and K+I− contact ion pair dissolved in supercritical ammonia

G. SCIAINI, R. FERNÁNDEZ PRINI, D. ESTRIN Y E. MARCECA

JOURNAL OF CHEMICAL PHYSICS

American Institute of Physics

Lugar: Chicago, E.UU.; Año: 2006

0021-9606

Vertical excitation and electron detachment energies associeated to the optical absorption of iodide ions dissolved in supercritical ammonia at 420 K have been calculated in two limting scenarios: as a solvated free I- and forming K+I- contanct ion pair (CIP).  The evolution of the transitionenergies as a result of the gradual buiding up of the solvation structure was astudied for each absorbing species as the solvent's density increased, i.e. changing the NH2 supercritical thermodynamic state. In both cases, if the solvent density is sufficiently high, photon absorption produces a spatially extended electron charge beyond the volume occupied by the solvated solute core; this excited state resembles a typical charge-transfer-to-solvent (CTTS) state. A combination of classicla molecular dynamics simulations followed by quantum mechanical calculations for the ground, first-excited and electron-detached electronc states, have been carried out for the systema consisting o one donor species (free I- ion or CIP) surrounded by ammonia molecules.  Vertical excitation and electron detachment energies were obtained by averaging one hundred randomly chosen microconfigurations along the molecular dynamics trajectory computed for each termodynamic condition (fluid density). Short- and long-range contributions of the sonvent-donor interaction upon the CTTS states of I- and KI were identifed bt performing additional electronic structure calculations where only the solvent interaction due to the first neighbor molecules was taken into account.  These computations, together with previous experimental evidence that we collected for the system, have been used to analyze the solvent effects on the CTTS transition. In this paper we established that: i) the CTTS electron fo free I- ion or KI CIP presents similar features, and it fradually localizes in close proximity of the iodine parent atom when the ammonia density is increased; ii)for the free I- ion, the short-range solvent interaction contributes to the stabilization of the ground state more than it does for the CTTS excited statre, which is evidenced experimentally as a blue shift in the maximum absorption of the CTTS transition when the density is increased; iii) this effect is less noticeable for the KI ion pair because in this case a tight solveation structure, formed by four NH3 molecules wedged between the ions, appears at very low density and is very little affected by changes in the density; iv) the long-range solvent field contribution can be neglected for the KI CIP,s ince the main features of its electronic transition can be explained on the basis of the visinity of the cation; v) however, the long- range solvent field contribuition is is essential for the free I- ion to become an efficient CTTS donor upon photoexcitation, and this establishes a difference in the CTTS behavior of I- in bulk and in clusters.