Computational methods
The structures of all complexes were optimized by the second-order Møller–Plesset perturbation theory (MP2) with the 6-311++G(d,p) basis set using Gaussian 09 suite of programs [11. Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G. A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H. P., Izmaylov, A. F., Bloino, J., Zheng, G., Sonnenberg, J. L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery, J. A., Jr., Peralta, J. E., Ogliaro, F., Bearpark, M., Heyd, J. J., Brothers, E., Kudin, K. N., Staroverov, V. N., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A., Burant, J. C., Iyengar, S. S., Tomasi, J., Cossi, M., Rega, N., Millam, J. M., Klene, M., Knox, J. E., Cross, J. B., Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R. E., Yazyev, O., Austin, A. J., Cammi, R., Pomelli, C., Ochterski, J. W., Martin, R. L., Morokuma, K., Zakrzewski, V. G., Voth, G. A., Salvador, P., Dannenberg, J. J., Dapprich, S., Daniels, A. D., Farkas, Ö., Foresman, J. B., Ortiz, J. V., Cioslowski, J., Fox, D. J. Gaussian, Inc., Wallingford CT, 2009 .]. All studied complexes have a nearly linear arrangement (Cs symmetric). It has previously been indicated that very little energy is required to make the nucleobases co-planar [22. Fonseca Guerra, C., Bickelhaupt, F. M., Snijders, J. G., Baerends, E. J. Journal of the American Chemical Society, 2000 , 122, 4117–4128.,33. Moroni, F., Famulari, A., Raimondi, M., Sabat, M. J Phys Chem ,2003 , 107, 4196–4202.].
The basis set superposition error (BSSE) has been considered by the Boys-Berrnardi counterpoise method [44. Boys, S. F., Bernardi, F. Mol. Phys . 1970 , 19, 553‒566.] in the geometry optimization.
The topological electron charge density has been analyzed by the quantum theory of atoms in molecules (QTAIM) approach [55. Bader, R. F. W. Atoms in molecules: a quantum theory, Oxford University Press, Oxford, 1990 .] using AIM 2000 program [66. Biegler König, F., Schönbohm, J. J. Comput. Chem .2002 , 23, 1489–1494.] on the obtained wave functions at MP2/6-311++G(d,p) level of theory. The features of bond critical points (BCPs) of the hydrogen bond were analyzed. It is well-known that characteristics of BCPs, such as the electron density (ρ(r)), their Laplacians (∇2ρ(r)), and that energy densities of BCPs (H(r)) allow us to categorize interactions and these topological parameters are also treated as measure of hydrogen bonding strength.
Moreover, the population analysis was performed by the natural bond orbital (NBO) method [77. Reed, A. E., Curtiss, L. A., Weinhold, F. Chem. Rev . 1988 , 88, 899–926.] at the HF/6-311++G(d,p) level on the optimized structures using NBO program under Gaussian 09 program package [88. Glendening, E. D., Reed, A. E., Carpenter, J. E., Weinhold, F. NBO Version 3.1. Department of chemistry, university of california-irvine, Irvine, 1995 .]. The NBO analysis is carried out by considering all possible interactions between filled donor and empty acceptor natural bond orbitals and estimating their energetic importance by second-order perturbation theory. Donor–acceptor interaction energy (E(2)) is above threshold (0.5 kcal) in all complexes.
Electrostatic Potential (ESP) generated by a chemical species is widely used as a tool for exploring its properties and locating potential sites for interaction with other moieties. To get a deeper insight into the nature of the interaction, the electrostatic potentials (ESP) have also been calculated using Multiwfn software [99. Lu, T., Chen, F.J. Comput. Chem . 2012 , 33, 580-592.] and visualized by VMD [1010. Humphrey, W., Dalke, A., Schulten, K. J. Mol. Graphics, 1996 , 14, 33–38.] software.