In the recent years, the calixarenes are of particular interest as metal ion receptors [1-4]. The use of calix[n]arenes in analytical chemistry and separation chemical technology has been discussed . It was founded that the cavity size, the position and kind of donor groups and the molecular flexibility have a pronounced impact on the complexation properties as well as the extraction power and selectivity [6, 7]. Calix[n]arenes have generated considerable interest due to their basket-shaped structure and as useful building blocks to synthesize selective receptors for the guest species, notably alkali, alkaline earth and transition metal cations [8, 9]. Supermolecular interaction is the vital initial process triggering biological and chemical events. Studies on binding between converging sites of synthetic hosts with diverging sites of guest molecules, atoms or ions have progressively provided valuable information towards an understanding of the supermolecular interaction of complicated natural elements [10-12]. Much attention has recently been paid to achieve the binding selectivity between the host and the guest atoms by controlling the size and shape of the binding cleft of the host molecule . A host molecule that can controllably switch its binding selectivity as desired is rare  but extremely desirable for various applications including imitating biological events. Owing to its pre-organized structure, calixarene is one of the most established molecular platforms for constructing three-dimensional hosts for atoms or ions . The structure of calixarene consists of eight phenol rings linked together with four methylene units in a circular manner producing a basket like architecture . A high number of calculations have recently reported in the literature concerning the determination of NMR chemical shift (c.s.) by quantum chemistry methods [17-22]. Moreover, it is known that the density functional theory (DFT) method [23-25] takes into account electron correlation contributions, which are especially important in systems contained extensive electron conjugation and/or electron lone pairs . Recently, DFT has been accepted by the quantum chemistry community as a cost-effective approach for the computation of molecular structure, vibration frequencies, and energies of chemical reactions. Many studies have shown that molecular structures and vibration frequencies calculated by DFT methods are more trustable than MP2 methods [26, 27]. While there is sufficient evidence that DFT provides an accurate description of the electronic and structural properties of interfaces and small molecules, relatively little is known about the symmetric performance of DFT applications to molecular associates. To further access the reliability of DFT methods applied to this field of chemistry, in this paper, the structure and bonding of the calixarene as obtained by high-level ab initio calculations will be discussed. The role of basis set size and basis set superposition effects will be analyzed in detail. In the present study, we will perform a high-level of calculations and systematic analysis of the theoretical result will be obtained [28, 29].
The geometry optimization of the azobenzene-bridged calixarene has been carried out using the GAUSSIAN 2003 programs package [29, 30]. The computational model consists of Geometries for calixarene were fully optimized by Restricted Hartree-Fock (RHF) with STO-3G, 3-21G and 6-31G levels . In this study, the NMR parameters of the title compound in the ground state have been calculated to compare with the experimental geometric parameters using the HF method. These calculations are valuable for providing insight into molecular parameters and NMR spectrum. To confirm the superiority of the DFT methods, HF method at the STO-3G, 3-21G and 6-31G basis set along with analytic NMR shielding tensors calculations were simultaneously adopted .
RESULTS AND DISCUSSION
This study deals with azobenzene-bridged calixarene molecule as a nanostructure compound for complexed by alkali or earth alkali cations. Before and after connecting the metal ions to calixarene NMR calculations were performed in the electric fieldof charges. NMR parameters are listed in Table 1 in different levels and different basis sets. σiso (isotropic chemical shift) and δ (chemical shift) curves versus atomic charges for calixarene and calix arene/alkali or earth alkali cations. These curves are drawn for different levels and different basic sets. Generally, all curves are similar. To assess the quality of the theoretical data, geometrical parameters available for calixarene. In the compound, the C-H stretch is decreasing total charge, which indicates the increasing acidity of the CH hydrogen from CH-N to CH-O. This phenomenon may be attributed to the induced effect of the electronegative element. In addition, the charge distribution in this compound is of primary importance from the point of view of the CH-Y. One stable structure of the azobenzene-bridged calixarene is shown in Fig. 1a, in addition, the structure with a metal ion is shown in Fig. 1b. Taking the calculated result of the seven complexes compared together.
As shown in Fig. 2a the most of the negative charges are attached to oxygen atoms and then it attached to nitrogen atoms. Similarly, the most of the positive charges are attached to hydrogen atoms that linked to oxygen atoms and then it attached to carbon atoms that linked to C-O bond. Initially, molecular structures of the title compound and 13C NMR, 1H and 14N calculations have been made by HF method. These results are shown in Table 1. As it is elaborated in Fig. 2b the most of the chemical shift attached to nitrogen atoms; for find reason notice to Fig. 1a although compound conjugated bonds must be aromatic theoretically but the aromaticity becomes not stable because of the spherical prohibition so that the configuration charges and the resonance occur between non-planar sheets; hence the chemical shift on the nitrogen atoms are under such more strong electrostatic field. As shown in Table 1 and Fig. 2c the least σiso is related to nitrogen atoms; that is because of conjugated bonds and aromaticity of the nitrogen bonds configuration and the resonance occur between non-planar sheets; hence the σiso on the nitrogen atoms is lower than another atom.
As a notice, to Fig. 3a the total charge computed for Li-calix is higher than of Na-calix and K-calix complexes. That is due to the nuclear effective charge which is decreased from Li to K atom. For more detailed analysis, the role of metal ions effects, Table 2 gives a detailed analysis of the chemical shifts obtained with metal ions. NMR parameters calculation using ab initio techniques has become a major and powerful tool in the investigation of variation in the molecular structure.
The ability to quickly evaluate and correlate the magnitude and orientation of the chemical shielding anisotropy tensor with variations in bond length, bond angles and local coordination and nearest neighbor interactions has seen in a high number of recent applications in the investigation of molecular structure. The chemical shifts of calixarene atoms principal values in the available method were obtained. Moreover, diffuse and polarizable functions effects in basis sets are investigated on NMR shielding tensors. As expected, the chemical shift computed for Li-calix is higher than of Na-calix then K-calix complexes as shown in Figs. 3b and 3c. As shown in Fig. 3, all parameters computed for Li-calix is higher than of Na-calix then K-calix complex.
In addition, shown in Table 3 for Ba-calix, Mg-calix and Ca-calix complexes, total charge, chemical shift and σiso computed for Ba-calix are higher than of Mg-calix then Ca-calix complex. In addition, the data were displayed in Figs. 4a, 4b and 4c. On the other hand, as shown in Table 4 and Figs. 5a, 5b and 5c, the study of Li-calix, Be-calix and B-calix complexes indicated total charge; chemical shift and σiso computed for Li-calix are higher than of Be-calix then B-calix complexes. As it is elaborated in Table 5, the least of the energy is attached to Li-calix complex. This complex is the most stable among the other complexes as shown in Fig. 6a. The most dipole moment is related to Li-calix complex (see Fig. 6b) that is because of nuclear effective charge which is decreased from Li to B atoms.
This article presents an HF study on calixarene which investigated the hydrogen, oxygen and nitrogen atoms as active sites of an organic structure. The most chemical shift and the least isotropic chemical shift is related to nitrogen atoms but the total charge decrease for them. That is because of conjugated bonds and aromaticity of the nitrogen bonds configuration and the resonance occur between non-planar sheets. In addition, the most of the total charge attached to nitrogen atoms and then metal atoms is due to of electronegativity of oxygen atoms that linked to them.
The authors would like to thank Islamshahr Branch of Islamic Azad University for all supports.
CONFLICT OF INTEREST
All authors declare no conflicts of interest in this paper.