Charge separation and charge transfer to solvent in NaCl–water clusters S.S.M.C. Godinho a , P. Cabral do Couto a,b , B.J. Costa Cabral a,b, * a Grupo de Fı ´sica Matema ´ tica da Universidade de Lisboa, Av. Professor Gama Pinto 2, 1649-003 Lisboa, Portugal b Departamento de Quı ´ mica e Bioquı ´ mica, Faculdade de Cie ˆncias da Universidade de Lisboa, 1749-016 Lisboa, Portugal Received 11 August 2004; in final form 1 October 2004 Available online 22 October 2004 Abstract The electronic structure of NaCl–w n (w ” H 2 O and n = 1–6, 8) clusters was investigated by ab initio and time dependent density functional theory calculations. Upon photoexcitation, when n < 5 the electronic density migrates from one of the Cl valence p orbit- als to the Na atom. When n P 5, the electronic density associated with one of these orbitals delocalizes over the nearby water mol- ecules indicating the formation of CTTS states in the clusters. When n P 6, contact ion-pair (CIP) and solvent separated ion-pair (SSIP) structures are found. The onset of charge separation in the ground state of small NaCl–water clusters seems to coincide with the formation of CTTS states upon photoexcitation. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Charge separation of salt ion pairs in solution is one of the most important processes in chemistry [1–6]. The classical example is the dissolution of NaCl in water. Although this process is empirically well known, it is not very well understood at the microscopic level. Sev- eral studies of NaCl in water clusters were reported [7–10]. One relevant issue concerns the minimum num- ber of solvent molecules that are necessary to character- ize charge separation of the ion pair in the aggregates [8–10]. Charge separation is characterized by the pres- ence of solvent molecules between the ions and by a large interionic distance in comparison with the isolated ion pair. These aggregates are named as solvent- separated ion pair (SSIP). Clusters where the interionic distance is similar to that of the isolated ion pair are contact ion pair (CIP) structures. Most of the quantum mechanical studies of small NaCl–water clusters pointed out the presence of CIP structures [8,10]. However, one ab initio study of Jungwirth [9] concluded that NaCl–w 6 is the first cluster size where a SSIP structure can be found. The presence of several SSIP structures in small AgCl–water and NaCl–water clusters was recently con- firmed [11]. On the other hand, experimental works on photodis- sociation of salt ion pairs in clusters of polar molecules show that charge separation, or the formation of SSIP structures, can be investigated through an excitation scheme involving, initially, the transition from the ionic ground state (GS) to a covalent dissociative state. The excitation scheme for the prototype NaI system was de- scribed in detail elsewhere [1,2]. For this particular case, the electronic oscillator strength associated with the transition from the GS X 1 R + to the excited A 1 R + state is proportional to the overlap of the electronic wave function of I  and Na (3s 1 S 1/2 ), which is dependent on the interionic distance. For CIP, this dependence should be weak. Only small variations of the interionic distance with the cluster size are observed in CIP structures. For SSIP structures, the increase of the interionic distance 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.016 * Corresponding author. E-mail address: ben@adonis.cii.fc.ul.pt (B.J. Costa Cabral). www.elsevier.com/locate/cplett Chemical Physics Letters 399 (2004) 200–205