95 Acta Chim. Slov. 2013, 60, 95–104 Oftadeh et al.: A Study of Donor-Acceptor in the Charge Transfer ... Scientific paper A Study of Donor-Acceptor in the Charge Transfer Molecular Complexes of Some Thiacrown Ethers with Dihalogen Molecules by DFT Method Mohsen Oftadeh, 1, * Mitra Moghadary, 1 Mohammad Solimannejad 2 and Abolfazl Semnani 3 1 Chemistry Department, Payame Noor University, 19395-4697, Tehran, I. R. of IRAN 2 Quantum Chemistry Group, Department of Chemistry, Faculty of Sciences, Arak University, Arak 38156-8-8349, Iran 3 Faculty of Sciences, University of Shahrekord, P.O. Box 115, Shahrekord, IRAN * Corresponding author: E-mail: m_oftadeh@pnu.ac.ir Received: 04-07-2012 Abstract The molecular complexes of 1,3,5-trithiane, (TT), tetrathia–8–crown–4, (TT8C4), and trithia–9-crown–3 , (TT9C3) with dihalogens in the ground state were investigated in the gas and dicholoromethane phases using B3LYP method and 6-31G** and 6-31+G** bases sets. In both TT and TT8C4 complexes, it is predicted that charge transfer takes place from the dihalogen to the thiacrown ether molecule; the magnitude trend of the total CT was ICl > IBr > I 2 and Cl 2 > Br 2 > I 2 , respectively. There was not such a trend with TT9C3. The frequency analysis showed that all complexes in the ex- cited state were unstable. The analysis of natural bond orbitals and comparison of the calculated thermodynamic quan- tities of the complexes between the gas phase and tetrachloromethane solution confirmed the results. Keywords: Donor-acceptor; charge transfer; thiacrown ether; natural charges; density functional theory 1. Introduction There are many possible different experiments in or- der to investigate charge transfer (CT) phenomena in dif- ferent areas in chemistry. Considerable experimental evi- dence on CT complexes has been reported in the liquid phase. 1,2 The formation of the CT complexes involves adapted parallel planer geometry, and a HOMO-LUMO electron transition according to Mulliken’s theory. 3 This donor-acceptor interaction is characterised by an electro- nic absorption band with low energy. According to the theory, a chemical hunch is needed to define a šdonor’ and an šacceptor’ molecule in a CT complex. CT complexes are regarded as important materials for use as organic su- perconductors, 4,5 photo-catalysts, 6 dendrimers, 7 solar en- ergy storage, 8 mechanism of drug action, 9 and non-linear optical materials. 10 Among geometries, two electronic structures of dif- ferent connectivity and stability are possible for CT com- plexes: a linear arrangement and a hypervalent T-shaped arrangement, especially in complexes of methyl substitu- ted imidazoline and imidazolidine-2-thiane derivatives with dihalogens. 11 The many crystallographic investiga- tions and computational examinations have been carried out on CT complex systems. 12–16 Structural, frequency and energy features have been fully characterised and par- ticular emphasis has been placed on investigating the ef- fect of the electronegativity and polarization of dihalogens on this interaction. Directly, evaluation of electronic charge distribution in an adduct of two different molecules by the help of ab initio and DFT calculations enables us to characterise the donor and acceptor molecules. Thus, no pre-assumption is necessary for such cases. As a result of this, researchers have performed explicit theoretical calculations for vari- ous CT complexes. 17,18 Bhattacharya 19 carried out such calculations and estimated the CT transition energies by the theoretical calculations for complex of o-chloranil