Non-covalent intermolecular interactions are the primary factors controlling the arrangement of ‎molecules in the crystal packing structures [1]. A very large number of intermolecular ‎interactions have been used in the crystal engineering. In this regard, the understanding and ‎control of these forces in the context of crystal packing, is a prerequisite for the progress of ‎crystal engineering [2-3]. Theoretical investigations‏ ‏on these interactions can‏ ‏provide many ‎important facts for revealing the nature of‏ ‏interactions in molecular crystals [4].‎ In this work, theoretical study‏ ‏of mercury (II) halide‏ ‏complexes, [HgCl2(L)2], [HgBr2(L)2]‎‏ ‏and ‎‎[Hg2I4(L)2], including the flexible‏ ‏N-(2-pyrazine)-thiophene-3-acetamide ligand (L) were ‎performed by density functional theory (DFT) using the hybrid density functional B3LYP ‎method. The LanL2DZ basis set with quasi-relativistic effective core potentials (LanL2) was ‎used to represent the atomic cores and double-zeta quality (DZ) to describe the valence ‎electrons of mercury atom. The various basis sets such as 6-311+G (d, p) and aug-cc-pVDZ ‎were used to describe the electrons of the other atoms. The reliability of the optimized ‎geometries was analyzed by calculating the differences between the experimental [5] and ‎theoretical data. The electronic properties of these structures were studied based on the natural ‎bond orbital (NBO) analysis. The vibrational frequencies and infrared intensities of title ‎complexes were calculated at the same level of theory. Results from theoretical calculations ‎were performed to gain insight into the nature of the intermolecular interactions and their ‎importance‏ ‏in the crystal structure‏ ‏of title compounds. The effect of various types of interactions ‎and coordination to metal ion on the structure of flexible ligand also was investigated. ‎

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