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.