Short-range and long-range solvent effects on charge-transfer-to-solventtransitions of I- and K+ contact ion pair dissolved in supercriticalammonia

G. SCIAINI; R. FERNANDEZ PRINI; D.A. ESTRIN; E. MARCECA

JOURNAL OF CHEMICAL PHYSICS

American Institute of Physics

Año: 2007 vol. 126 p. 174505 - 174517

0021-9606

Vertical excitation and electron detachment energies associated with the optical absorption of iodideions dissolved in supercritical ammonia at 420 K have been calculated in two limiting scenarios: asa solvated free I ion and forming a KI contact ion pair CIP . The evolution of the transition energies as a result of the gradual building up of the solvation structure was studied for eachabsorbing species as the solvent’s density increased, i.e., changing the NH3 supercriticalthermodynamic state. In both cases, if the solvent density is sufficiently high, photon absorptionproduces a spatially extended electron charge beyond the volume occupied by the solvated solutecore; this excited state resembles a typical charge-transfer-to-solvent CTTS state. A combinationof classical molecular dynamics simulations followed by quantum mechanical calculations for theground, first-excited, and electron-detached electronic states have been carried out for the systemconsisting of one donor species free I ion or KI CIP surrounded by ammonia molecules.Vertical excitation and electron detachment energies were obtained by averaging 100 randomlychosen microconfigurations along the molecular dynamics trajectory computed for eachthermodynamic condition fluid density . Short- and long-range contributions of the solvent donorinteraction upon the CTTS states of I and KI were identified by performing additional electronicstructure calculations where only the solvent interaction due to the first neighbor molecules wastaken into account. These computations, together with previous experimental evidence that wecollected for the system, have been used to analyze the solvent effects on the CTTS transition. Inthis paper we have established the following: i) the CTTS electron of free I ion or KI CIP presents similar features, and it gradually localizes in close proximity of the iodine parent atomwhen the ammonia density is increased; ii) for the free I ion, the short-range solvent interactioncontributes to the stabilization of the ground state more than it does for the CTTS excited state,which is evidenced experimentally as a blueshift in the maximum absorption of the CTTS transitionwhen the density is increased; iii this effect is less noticeable for the KI ion pair, because in thiscase a tight solvation structure, formed by four NH3 molecules wedged between the ions, appearsat very low density and is very little affected by changes in the density; iv the long-rangecontribution to the solvent stabilization can be neglected for the KI CIP, since the main featuresof its electronic transition can be explained on the basis of the vicinity of the cation; v) however,the long-range solvent field contribution 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 andin clusters.