The micro-solvation of Na þ : theoretical study of bonding characteristics in weakly bonded Ar n Na þ (n ¼ 1–8) clusters Kalathingal T. Giju a , Szczepan Roszak a,b , Robert W. Gora a,b , Jerzy Leszczynski a, * a Computational Center for Molecular Structure and Interactions, Department of Chemistry, Jackson State University, 1400 JR Lynch Street, P.O. Box 17910, Jackson, MS 39217, USA b Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27 50-370 Wroclaw, Poland Received 18 February 2004; in final form 19 April 2004 Available online 18 May 2004 Abstract New features of Ar n Na þ clusters are revealed using reliable, accurate post-Hartree–Fock computations. Molecular geometries and harmonic vibrational frequencies of Ar n Na þ (n ¼ 1–8) complexes are predicted at the MP2/6-311+G(3df) level of theory. Successive values of dissociation energy for all clusters were additionally calculated within the CCSD(T) approach. The nature of bonding in these structures is discussed based on natural population analysis and interaction energy decomposition scheme. A comprehensive discussion on the relevance of an appropriate treatment of core-valence electron correlation is provided for the ArNa þ ion. The presented data agrees well with the available experimental values. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Inert gas atoms are the simplest of solvents because of their formally filled valence shell. A term of ‘shellvation’ has been used to represent the conventional solvation process of solute–solvent interaction in such clustered medium [1]. There are many experimental and theoret- ical studies that have been performed to delineate the nature of ion–inert gas interactions [2,3]. We have pre- viously investigated the Ar n H þ [4] and Ar n Li þ clusters in order to substantiate the molecular properties and bonding interactions in Group I–inert gas complexes. It was found that a minimum of seven Ar atoms for H þ and six Ar atoms for Li þ is necessary to effectively form a closed shell structure in the first coordination sphere. Moving down the group, our goal in this study is to investigate the molecular geometries of different isomers and the nature of alkali ion interaction with argon ligands in the Ar n Na þ cluster. Experimentally, the di- meric ion, ArNa þ , is usually encountered in inductively coupled plasma-mass spectrometry as interferences [5–7]. It is also observed in the gas phase with a disso- ciation enthalpy in the range of 3.70–4.88 kcal/mol [8–11]. Such a range of experimental values points to difficulties in providing accurate data for weak com- plexes. The results for larger clusters are available from the time-of-flight (TOF) mass spectrometry [12]. The reported magic numbers for Ar n Na þ start from six and eight. There are a large number of theoretical studies on the ArNa þ cation [13,14]. The complex with two argon atoms was studied by Bauschlicher et al. [15]. The study of larger clusters (n ¼ 2–10) has also appeared recently for the most stable geometrical configuration [16]. The clusters of different size are formed as unsystematic and sometimes unusual geometries. In order to derive struc- tural conclusions this feature deserves more attention including studies of other isomers being higher on the potential energy surface. Studies on ArNa þ were mainly focused on derivation of interatomic potentials and properties along with structural details and investiga- tions of accuracy of correlation methods and basis sets. Potential energy curves of Ar 2 Na þ were studied along with other Ar 2 M nþ ions to gain understanding of these complexes. The lack of a definitive experimental and theoretical comparison and the absence of a dependable * Corresponding author. Fax: +1-601-979-7823. E-mail address: jerzy@ccmsi.us (J. Leszczynski). 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.04.085 Chemical Physics Letters 391 (2004) 112–119 www.elsevier.com/locate/cplett