Electrostatics-guided ab initio studies on weakly bonded complexes of substituted naphthalenes Shridhar R. Gadre a, * , Milind M. Deshmukh a , Tapas Chakraborty b a Department of Chemistry, University of Pune, Ganeshkhind, Pune 411 007, India b Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India Received 25 November 2003 Published online: 9 January 2004 Abstract Electrostatic guidelines have been employed to explore weak complexes of naphthalene and its methoxy- and cyano-derivatives. The initial geometries of these complexes are obtained from the electrostatic potential for intermolecular complexation (EPIC) model, with a follow-up by ab initio calculations at the Hartree–Fock, density functional and second-order-Møller–Plesset (MP2) levels of theory. The EPIC model is seen to provide starting structures and energetic trends in good agreement with the ab initio ones. Further, the results of ab initio calculations are in conformity with the semi-qualitative conclusions regarding the geometries reached from spectroscopic experiments. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction The complexes of substituted aromatic molecules are of interest to chemists, physicists as well as biologists [1–6]. Recently, such complexes have been subjected to several theoretical and experimental investigations. The intermolecular interactions prevalent in these complexes are also fundamental to many photochemical reactions [6]. Further, there has been a sharp increase in the studies of such complexes for obtaining insights into crystal packing and molecular recognition [7]. Many studies have reported geometrical investigation of sim- ple molecular complexes, viz., the T-shaped dimer of benzene [8,9], parallel-displaced face-to-face geometry of naphthalene pair [10] etc. A set of rules for the stable geometry of the system interacting through p–p inter- actions, has been proposed by Hunter and Sanders [11] based on their study of porphyrin systems. Calculations of interaction energies for molecules of chemical as well as biological importance have recently been reported [12–14]. It is concluded that the inclusion of dispersion interaction is necessary for a meaningful treatment of these interactions. A majority of these studies are re- stricted to MP2 level, barring a few cases of smaller systems, wherein CCSD(T) computations are reported. It is also reported that density functional theory (DFT) is not fully adequate for studying such interactions [14]. The van der Waals (vdW) complexes of small aromatic molecules are important in many physico-chemical problems, e.g., the naphthalene dimer is known to form an excimer through exciton resonance [15–20]. Murrell and Tanaka [19] proposed that, for excimer formation, an overlap between the molecular orbitals of the two moieties in face-to-face sandwich geometry is essential. Recently, Das et al. [21–23] have employed the fluorescence as well as the hole-burning spectroscopy to investigate the excimer/exciplex formation in homo- as well as mixed vdW dimers formed by naphthalene (NP) and substituted naphthalenes viz., 1-methoxynaphtha- lene (1-MNP), 2-methoxynaphthalene (2-MNP) and 2-cyanonaphthalene (2-CNP). They have observed a variety of geometries for these complexes, for example: face-to-face configuration in case of both homo- as well as mixed dimers of 1-MNP, non-overlapping geometry for 2-MNP homodimer [21], distorted parallel-displaced geometry for 2-MNPNP dimer [22], planar geome- try of 2-CNP homodimer and T-shaped geometry of * Corresponding author. Fax: +91-020-25601728. E-mail address: gadre@chem.unipune.ernet.in (S.R. Gadre). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.11.099 Chemical Physics Letters 384 (2004) 350–356 www.elsevier.com/locate/cplett