Microsolvation of sodium ion in acetonitrile clusters: Structure and energetic trend by first principle study Sandeep Nigam * , Chiranjib Majumder Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India article info Article history: Received 12 December 2008 Received in revised form 11 April 2009 Accepted 11 April 2009 Available online 23 April 2009 Keywords: Acetonitrile clusters Sodium ion solvation DFT calculations abstract We report a systematic theoretical study on the growth pattern and energetics of Na + (CH 3 CN) n clusters (n = 1–8,12) using density functional approach at the B3LYP/6-31++G(d,p) level. Geometry optimizations for all these clusters were carried out with various possible initial guess structures without any symme- try restriction and finally the stability of the lowest energy isomer was verified from Hessian calculations. It is found that the incorporation of a sodium ion completely rearrange the equilibrium structure of the neat acetonitrile cluster. The solvated clusters favor multiple shell structure with higher symmetry. The first solvation shell is formed by six CH 3 CN molecules, where the nitrogen atoms of each molecule points towards the central sodium ion. Here, the nature of binding between the solute and solvent has been attributed as strong ion–dipole interactions. The onset of the second solvation shell occurs at n = 7 and thereafter additional acetonitrile molecules interact with the molecules of the first solvation shell through N...H interaction. Such interactions are weak compared to the ion–dipole interactions leading to minimal perturbation to the inner shell structure. In consistent with this conjecture, the ion-solvation energy is found to increase very sharply upto n = 6, and becomes less steep from n = 7 onwards. More- over, the calculated stepwise binding energies are found to be in good agreement with available exper- imental data, which provide confidence about the equilibrium geometries of the Na + (CH 3 CN) n clusters predicted in this study. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The study of microsolvation of ions has been an increasingly ac- tive area of research due to their significance in understanding sol- vation phenomena at the microscopic level [1]. Microsolvation of a solute ions alter the solvent network and the knowledge of struc- tural reorientation has immense importance not only in chemistry but also in physics, biology, and material science [2–4]. In recent years, numerous experiment and theoretical research efforts [5– 40] have concentrated on the microsolvation of various metal ions in variety of polar solvent such as water, ammonia, methanol, ace- tonitrile, acetone, etc. Acetonitrile is a good prototype to explore solvation processes in nonaqueous solvents. Acetonitrile is a polar molecule with a large dipole moment (3.92 D), greater than other typical polar solvents such as water, methanol and ammonia [41]. Compared to water, neat acetonitrile liquid is much less structured, the orientation of any given molecule being correlated only with those of its immediate neighbors [42–44]. The solvation of ions in water [12–20] is determined by both the large dipole moment of water and also the strong water structure; however the solva- tion in acetonitrile is expected to be different because acetonitrile have even higher dipole moment but strong internal structure is not there. Thus, it is worth to investigate solvation of ion in aceto- nitrile for its qualities as a solvent. Studies of the solvation of alkali metal ions in acetonitrile sol- vents are not as common as studies of water systems, however good numbers of reports are available in literature [21–40]. Using mass spectrometry, Megyes et al. [21] found up to six acetonitrile mole- cules surrounding the Li + ion in the gas phase. Both Fawcett et al. [22] and Barthel and Deser [23] found the coordination number to be four for both Li + and Na + , based on measured IR intensities for the acetonitrile CN stretching vibrations. The ionization potentials of the clusters of Cs atom solvated with acetonitrile, i.e. Cs(CH 3 CN) n were measured by Fuke and coworkers [24] and they found that in contrast to water and ammonia-solvated clusters, ionization poten- tial of Cs(CH 3 CN) n clusters show an anomalous dependence on size due to the large dipole moment of acetonitrile. Similarly, Ohshimo et al. investigated the ionization potentials (IPs) of the clusters of an alkali metal (Li, Na) atom solvated with acetonitrile molecules by photoionization mass spectroscopy [25] and they also found the size dependent irregular variation of ionization potentials with re- spect to size, originating from strong dipole–dipole interaction be- tween acetonitrile molecules. Valina et al. [29] have studied the collision-induced dissociation of Na + (CH 3 CN) n , n = 1–5, with xenon as a function of kinetic energy using guided ion beam mass spec- 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.04.013 * Corresponding author. Tel.: +91 2225590287; fax: +91 2225505050. E-mail address: snigam@barc.gov.in (S. Nigam). Journal of Molecular Structure: THEOCHEM 907 (2009) 22–28 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem