In the first part of this research, using SIESTA code and DFT methods, adsorption of carbon monoxide, oxygen, and nitrogen on zinc oxide nano-cone with ۲۴۰ apex angle were studied. Using potential curve, the suitable distance for carbon monoxide adsorption was obtained about ۲.۴ Å from Zn atoms on nano-cone’s surface. And the other configurations are also investigated in the same circumstances. Furthermore, it was found that CO molecule tends to adsorb on zinc, from its carbon’s side. Adsorption, binding, deformation and BSSE correction energies were calculated for the considered configurations. Adsorption energy of carbon monoxide was obtained about -۲.۰۳ eV. A comparison of O۲ and N۲ adsorption energies with CO adsorption energy shows that zinc oxide nano-cone has a more tendency to adsorb and trap of carbon monoxide molecules. Investigation of DOS spectra, reveals that zinc oxide nano-cone can be used as a CO detector in the air; because these spectra show a lot of variation when ZnO adsorb carbon monoxide, while it remains almost intact after nitrogen or oxygen adsorption. Investigation of natural charges in Zn and O atoms shows that the bonds in the ZnO nano structures are also ionic and the charge difference in the middle part of nano-cone is more than the other parts. It seems that this matter causes to better adsorption of gas molecules in the middle part of nano-cone. Actually, the NBO results confirm the results from adsorption and binding energies and also DOS spectra. Given these results, it is suggested that when zinc oxide nano-cone is exposed to a mixture of carbon monoxide, oxygen, and nitrogen molecules, it can adsorb CO molecules among mixture. So, ZnO could be used as a selective adsorbent for carbon monoxide filtration and separation from the air. Moreover, zinc oxide nano-cone could be used as a detector for toxic CO gas in the air.

 

In the second part, gallium nitride nano-cage with Ga۲۴N۲۴ structure was optimized using DFT calculations (M۰۶-۲X/۶-۳۱۱++G(d,p) method) and then adsorption of hydrogen molecule on this structure was investigated. The obtained average adsorption energies (E_ad= -۱۹.۷ kJ.mol-۱) reveal that the molecular adsorption of H۲ at the Ga۲۴N۲۴ cluster surface is physisorption and nearly site independent. This adsorption is not dissociative and during the formation of complex no clustering for H۲ molecules is observed. In order to check the maximum ability of this structure, the ۲۰H۲@Ga۲۴N۲۴ complex was optimized and adsorption energy per H۲ molecules was calculated. It can be seen that Ead was not altered so much. These results show that the Ga۲۴N۲۴ nano-cage could adsorb more H۲ molecules due to polymerization. Therefore, it shows a good capacity for using as a hydrogen storage material. The evaluated NBO charges and hybridization refer to sigma-electron donation from H۲ bond to unfilled d orbital of gallium atoms and the other theoretical pieces of evidence (such as Fukui functions, DOS spectra, and RDG scatter graphs) confirm this mechanism. Therefore, all of the results suggest a quasi-molecular adsorption for nH2@Ga24N24 complexes which introduce this nanostructure as a good candidate material for hydrogen storage.

In the first part of this research, a new descriptor, called Atomic Zero Steric Potential (AZSP) is introduced based on the difference between the Atomic Zero Steric Potentials of a reaction to investigate the regioselectivity of some organic reactions. Some name organic reactions, included Diels-Alders reactions, Paternò–Büchi reactions and additions to alkenes are studied to check the ability of the new descriptor in prediction of regioselectivity. For this purpose, a quantity which is the summation of square of calculated AZSPs is defined. It is found that the lower value is belonging to the reaction in which the major product is obtained. It seems that the new introduced descriptor (AZSP), is successfully predict the regioselectivity in nucleophilic, electrophilic and radical reactions. Structural optimization is performed at B3LYP/6-31G** level of theory.
In the second part, using DFT calculations, the interaction between five nucleotides adenine, guanine, cytosine, thymine and uracil with aluminum nitride nanotubes (AlNNT) is studied. All calculations in this section are carried out with SIESTA package and using generalized gradient approximation (GGA) with ζ split double polarized basis set (DZP). The results show that the interaction between nucleobases and AlN nanotube is a weak chemical bond. All of the considered complexes are stable and tend to adsorption with considerable adsorption energy. For all nucleobases the binding energies are calculated. The adsorption and deformation energies are also calculated separately. The results show that the interaction of nanotubes with thymine and uracil molecules leads to high deformation in these nuclebases and it seems that this nanotube cannot be a good candidate for use as a carrier or sensor for these two nucleobases. Such changes were not observed for the other nocleobases. The following order is observed for binding energies of other nucleobases
G> C> A. The calculated electron density difference between molecules and AlN nanotube indicates that the charge transfers between nanotubes and molecules. Checking the obtained DOS and PDOS spectra shows that the band gap of nanotube is sensitive to adsorption of the nucleobases. So it seems that the AlN nanotubes could be a promising candidate for detection of considered nucleobases.
In the third part, natural bond orbital analysis, structural, electronic and topological properties of silicon-carbide nanoneedle are studied. To achieve this goal, nanoneedles with square, six, eight and ten atom in their cross sections are examined. All the calculations in this part are done using the Gaussian 03 software and B3LYP/ 6-31G * level of theory. The structures with 2 to 10 cells are considered. Binding energy per atom for all considered silicon-carbide nanostructures was calculated for investigation of the stability of the nanostructures. Calculated binding energies per atom show that the hexagonal cross-section nanoneedles have the most binding energies among the considered nanoneedles with square-, octagonal and Decagon cross sections. The results show that by increasing the number of cells, binding energy per atom and the stability of nanostructures is increase. The total density of states (DOS) introduce these nanoneedles as semiconductors which their conductivity properties did not change with the variation in length and diameter of compounds. The topological analysis indicates that the Si-C bonds in these nanostructures have ionic nature. The study of structural properties of Si-C nanoneedles shows that when the numbers of cells are increased, the bond lengths do not change significantly.

 

 In the first part of this research, the variation of Zero Steric Potential (ZSP(r)) through a C-C bond shows two maximums, which their values depend on the bond order. A Good relationship is observed between the maximum ZSP and the bond order of C-C bonds in ethane, ethylene and acetylene , as reference molecules (Ln BO=1.956 (ZSP) ̅_max -0.898). The obtained equation is used to predict the bond order of many aromatic and aliphatic hydrocarbons. The obtained bond orders are compared with those calculated from NBO and laplacian methods. The results show that the predicted bond orders from (ZSP) ̅_max are more reliable than those which are evaluated using two other methods.
In the second part Li inserting with dopping in cone structure have been proposed to be novel representatives of high- performance nonlinear optical (NLO) materials because of their large static first hyperpolarisabilities.(β_0). In this work, the electronic structure and second order nonlinear optical (NLO) properties of silicon carbide nanocones (SiCNC) with two different angles (240 and 180 degrees) and theirs Li-doped SiCNC derivatives have been calculated by density functional theory (DFT). Interestingly, we found that the β_tot values of their configurations are significantly larger than those of the non-doped SiCNC. To understand the optical transition and NLO response, time-dependent density functional theory (TD-DFT) calculations were carried out for these silicon carbide nanocones. By analyzing the molecular orbital transition feature and the excited state that is crucial to the NLO response, we found that the charge transfer from SiCNC to Li atom confirm that the increasing trend of the static first hyperpolarizability. It is also found that the frequency-dependent effect on the first hyperpolarizabilities is weak, which may be beneficial to experimentalist for designing Li-doped SiCNC molecules with large NLO responses. Our present work may be beneficial to development of high-performance inorganic NLO optical materials.