Adsorption of N2, O2, CO and CO2 Molecules on the Open Ended and Surface of Swcnts: A Computational NMR and NQR Study-A Review

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1 Adsorption of N2, O2, CO and CO2 Molecules on the Open Ended and Surface of Swcnts: A Computational NMR and NQR Study-A Review Ashraf Sadat Ghasemi, F Ashrafi Department of Chemistry, Payame Noor University(PNU), P.O. Box, Tehran, Iran Corresponding: Ashraf.Ghasemi@gmail.com ABSTRACT: Carbon nanotubes have important applications in gas adsorption. Therefore the study of their properties for this application is very important. Nano-tubes with small size, physical stability and sensitivity of their electric properties to adsorption of N 2, O 2, CO and CO 2 make them ideal materials for use in gas sensors. In this book the Density functional theory (DFT) method is utilized to study the adsorption of N 2, O 2, CO and CO 2 gas molecules on pristine Single-Walled Carbon Nanotubes (SWCNTs). Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) parameters consisting of isotropic and anisotropic chemical shielding parameters have been calculated based on DFT to investigate the properties of the electronic structures including bond lengths, bond angles, tip diameters, dipole moments, energies, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the HOMO-LUMO band gaps, and the electronic chemical potential (μ) for the lowest energy derived to estimate the structural stability of the composites have been investigated for the N 2, O 2, CO and CO 2 doped on Single-Walled Carbon Nanotube (SWCNT). Further, our theoretical results suggest that molecule-induced modification of the density of states close to the Fermi level might significantly affect the transport properties of nanotubes. The electronic properties of SWCNTs are sensitive to the adsorption of certain gases such as N 2, O 2, CO and CO 2. Charge transfer and gas-induced charge fluctuation might significantly affect the transport properties of SWCNTs. The aim of this study is investigating adsorption properties and NMR and NQR of the zigzag (5, 0) and armchair (4, 4) SWCNTs with the optimal length of 7.1 Å and 8.6 Å as a gas sensor and optimized adsorption rates by using DFT calculations. Geometry optimizations were carried out at the b3pw91/ g (d,p) level of theory using the Gaussian 98 program suite. Keywords: Adsorption, Single-Walled Carbon Nanotubes (SWCNTs), Density functional theory (DFT), Nuclear magnetic resonance (NMR), Nuclear Quadrupole Resonance (NQR) 1 INTRODUCTION Since the discovery of carbon nanotubes (CNTs), by Iijima (1991), SWCNTs have attracted great interest owing to their extraordinary structural, mechanical, chemical, physical, and electronic propertie (Kong et al., 2000). CNTs types are categorized into 2 types: a single cylinder known as SWCNTs and two or more concentric cylinders known as Multi Wall Carbon Nanotubes (MWCNTs). A SWCNT can conceptually be constructed from the rolling of a graphene sheet. Its structure is conveniently classified by a pair of numbers (n, m) indexing the two carbons that coincide upon rolling. The separation of these carbons on the planar graphene sheet is na 1+ ma 2, where a 1 and a 2 are the unit vectors of the two-dimensional hexagonal graphene lattice. This results in three classes of SWCNTs, armchair, zigzag, and chiral, characterized by (m, n), (m, 0), and (m, n) with n m, respectively. For both (n,0) and (n, n) tubes, the energy difference between possible isomers was found to decrease as the diameter of XXX-1

2 the tube increased, which has led to the conclusion that, in a sample with typical SWCNTs diameters, there is no particularly strong site preference for the bonding of the functional group. Depending on chirality, carbon nanotubes can either be conducting or semiconducting. Since the band structure is dependent upon the degree and position of functionalization. No covalent and covalent unctionalization of the tubes can render them soluble in aqueous media and, therefore, potentially useful in biotechnology and biomedical applications. Gas molecular adsorption in nanostructures is an important issue for both fundamental research and technical application (Lu et al., 2005). When a foreign atom is inserted in the nanotube lattice, the nanotube symmetry is altered structure and properties consequently change. This was first confirmed experimentally by Stephan et al. Studied the absorption of various gas molecules (NO2, O2, NH3, N2, CO2, CH4, H2O, H2 and Ar) on SWCNTs using first principles method (Peng and Cho, 2000; Babanejad et al., 2010; Ashrafi et al., 2010; Ghasemi et al., 2010; Lu et al., 2005; Liu et al., 2006; Ding et al., 2004; Zhao et al., 2005). The Self- Consistent Field (SCF) electronic structure calculations are performed based on Density Functional Theory (DFT) (Schimizu and Tsukada, 1993; Lynch and Hu, 2000; McClenaghan et al., 2000; Zhang and Hu, 2000; Noguera, 2001). Jhi et al. (2000) theoretically studied the effect of gases on the electronic and magnetic properties of SWCNTs (Meyyappan, 2005; Dae-Hwang et al., 2003), their calculation for the density of states shows that weak coupling between carbon and gas leads to conducting states near the band gap (Charlier and Lambin, 1998). One possible way to modify the electronic and vibronic properties is a charge transfer during their intercalation and fictionalization (Barros et al., 2007). Depending on their diameter and felicity it was predicted that they can be semiconductors or densities (Avouris et al., 2000) and their electrical properties can be modified by doping (Lee et al., 1997). Nuclear magnetic resonance (NMR) measurements reveal the effect of gases on density of state at the Fermi leve (Mirzaei andhad ipour, 2006; Mirzaei and Hadipour, 2007). The calculation of NMR parameters using DFT techniques has become a major and powerful tool in the investigation of molecular structur (Parr and Yang, 1994; Becke, 1993).Comparing the adsorption of gases on the surface, using computational methods substantially reduces costs and thus the Nuclear Quadrupole Resonance (NQR) was used in related investigations. All armchair nanotubes (4, 4) are semiconductors, the zigzag nanotubes (5, 0), also, are semiconductors. Impurity addition to semiconductor nanotubes with zigzag form (5, 0) causes energy gap (Charlier and Lambin, 1998). decrease and tends to quasimetallic state which results in increase of its conductivity. The purpose of our approach is studying in adsorption of N 2, O 2, CO and CO 2 molecules on single-walled carbon nanotube surface with both armchair (4, 4) and zigzag (5, 0) forms DFT method. This study consists of the structural forms are firstly optimized and then the calculated tensors in the optimized structures are converted to chemical shielding isotropic ( iso ) and chemical shielding anisotropic ( ) and asymmetric (µ j ) parameters, configuration of adsorption, determination of binding energy located on carbon nucleuses of nanotube which involve in chemical binding with N 2, O 2, CO and CO 2 molecules, the NQR, and determination of bond length after N 2, O 2, CO and CO 2 molecules adsorption on nanotube which will optimize by calculation methods (Saito et al., 1992; Mintmire et al., 1992). The NQR measurable asymmetry parameter ( Q ) is also reproduced by quantum chemical calculations of the Electric Field Gradient (EFG) tensors shown by (Ghasemi et al., 2010; Marian and Gastreich, 2001). 2. Materials and methods In present work, all computations are carried out via Gaussian98 package (M.J. Frisch, et al., 2002; Chang et al., 2006; Souza et al., 2007; Souza et al., 2006; Saito et al., 1992) at the level of DFT using the hybrid exchange-functional b3pw91 method. According to prior calculations, G (d, p) standard basis set is sufficient for structural optimization of XXX-2

3 nanotubes. In order to avoid the boundary effects, atoms at the open ends of the tube are saturated by hydrogen atoms. Calculations based on the DFT were performed in this investigation with the generalized gradient approximation (GGA) by Perdew et al. Therefore, structures of SWCNTs are allowed to fully relax during the b3pw91/ g (d, p) optimization process and Properties of the structures considered models of SWCNTs is (4,4) armchair (constructed of 64 C and 18 H atoms), and (5,0) zigzag (constructed of 40 C and 10H atoms), types (Fig. 1). (a) (b) Fig.1. The basic structure of carbon nanotubes(a) armchair (4, 4), (b) Zigzag (5,0). Vibration frequencies were also calculated at the same level to confirm that all the stationary points correspond to true minima on the potential energy surface. Geometry and DOS of (5,0) zigzag and (4,4) armchair SWCNTs the optimized tube is shown in Fig. 1, indicating that the tube is semi-conductive with HOMO/LUMO energy gap (Eg) of and ev, respectively. The diameter and the length of the optimized pure SWCNTs (5,0) zigzag and (4,4) armchair are computed to be about 4.07 and 7.08 A, 5.67 and 7.34A, respectively. The binding energy of N 2, O 2, CO and CO 2 on the SWCNTs are determined through the following equation: Ead E N2CNTS E CNTS E N2 (1) Ead E O2CNTS E CNTS E O2 (2) E E E E (3) ad COCNTS CNTS CO Ead E CO2CNTS E CNTS E CO2 (4) Where, E CNTS, E O2, E CNT +E O2, E N2, E CNT + E N2, E CO,E CNT + E CO and E CO2,E CNT + E CO2 are the energies of the optimized tubes, which are adsorption systems, respectively. Ead 0 corresponds to exothermic adsorption which leads to local minima stable for adsorption of gas molecule on the surface and open-ended of nanotube. Then, the optimized structures were used to obtain shielding tensors at the sites of 17-O,15- N and 17O, 15N and 13-C nuclei are calculated based on the gauge included atomic orbital (GIAO) method (Wolinski et al., 1990) via b3pw91/ g (d,p) level of theory. London (London, 1937) initially suggested local gauge origins to define the vector potential of the external magnetic field in the study of molecular diamagnetism. The idea was then adapted by Ditchfield (1974) in the GIAO method for magnetic shielding calculations (Parr and Yang, 1994; Becke, 1993). Following Ditch field s work in which each atomic orbital has its own local gauge origin placed on its center Giessner- Prettre and coworkers and Fukui et al. implemented the GIAO method. In chemistry, one of the most versatile experimental tools to study the geometry and electronic structure of molecules and solids is NMR. So far, NMR has not been among the main tools for the characterization of carbon nanostructures. The name refers to spectroscopic studies of transitions between the Zeeman levels of an atomic nucleus in a magnetic field, and the subject was initially developed by physicists as a method for determining the size of nuclear magnetic moments, and thus testing models of nuclear structure. The calculated CS tensors in principal axes system (PAS) ( ) are converted to measurable NMR parameters, chemical shielding isotropic ( iso ) and chemical shielding anisotropic ( ) using (1) and (2), respectively (Duer, 2001). The evaluated NMR parameters at the sites of 17-O, 13-C and 15-N nuclei in the SWCNTs (5, 0) zigzag and (4, 4) armchair model are presented in (Tables 1 and 2), respectively. This shows a second-order change in the molecular energy: XXX-3

4 E E B XB B (5) i 0 The summation is taken over the O, N and C nuclei in the system. We are not interested in the magnetic susceptibility,, but only in the bilinear response property (Duer, 2002): 2 E ij B IJ B 0 i j (6) Where j the components of magnetic moment and B i are is external magnetic field. The principal components for specification of shielding are defined by this coordinate system as following equation (Marian and Gastreich, 2001): 3 ( 33 iso ) 2 ( ) iso ( ) In which, iso (7) and are isotropic, anisotropic and asymmetric parts of tensor, respectively and in certain cases vanishes. The NQR measurable asymmetry parameter (η Q ) is also reproduced by quantum chemical calculations of the electric field gradient (EFG) tensors. Geometry optimizations and EFG calculations were performed using G** basis set with B3PW91 functional. In quadrupolar spin system, the electric field gradient (EFG) tensor at 13-carbon nuclear sites has axial symmetry (asymmetry parameter 0). The existence of the zero asymmetry parameter was one of the reasons why this compound is considered to present such interest. The interaction between nuclear electric quadrupole moment and EFG at quadrupole nucleus is described with Hamiltonian: Abragam, 1961), e Qq zz H [3 I Z I ) Q ( I X IY ) (8) 4 I(2I 1) Where eq is the nuclear electric quadrupole moment, I is the nuclear spin and q zz is the largest component of EFG tensor. The principal axis system (PAS) components of the EFG tensor, qii, are computed in atomic unit 21 2 ( 1au Vm ), with qxx q yy qzz and qxx q yy qzz 0 these diagonal elements are related by a symmetry parameter / q q q Q yy xx zz and 0 1 measures the deviation of EFG Q tensor from axial symmetry (Marian and Gastreich, 2001). Cluster model is proved to be valid for nanotubes computed q zz component of EFG tensor is used to obtain nuclear quadrupole coupling constant from the equation 2 C e Qq / h. Q zz The aim of this study is investigating adsorption on carbon nanotubes modifies sensibly, their electronic properties proposed the use armchair (4, 4) and zigzag(5, 0) on the surface and open ended of SWCNTs as gas sensors or biosensor industry and optimizing adsorption rates by using DFT calculations. Difficulties in SWCNTs production and spectroscopic characterization have made computational studies of these systems indispensable. 3. Results and discussion In the present study, two models of zig-zag (5, 0) and armchair (4, 4) SWCNTs with specified tube lengths are studied using quantum chemical calculations (Fig.1 to12). Chemical shielding tensors of H-capped (5, 0) and (4, 4) SWCNTs interacted with N 2, O 2, CO and CO 2 molecules are obtained. The calculated geometry parameters and binding energies 17 O, 15 N and 13-C chemical shielding tensors and EFG tensor are presented in (Table 1 to 11). The molecular geometries and electronic properties and NMR chemical shielding and NQR tensors resulted from gases molecular adsorption are discussed in following section, separately. 3.1 Molecular geometries and Electronic properties: XXX-4

5 In this study, the efficacy on electronic properties of SWCNT by N 2, O 2, CO and CO 2 physisorption have been established to appear field of spin-electronics, a field that influences the electron s spin degree of freedom for transfer and storage of information and communication. The optimized geometries of calculated configurations of N 2, O 2, CO and CO 2 molecule adsorbed on zigzag (5, 0) and armchair (4, 4) SWCNT are schematically displayed in Fig 1. The characteristic behavior of a field effect transistor based on an individual reported in Fig. 2-12, electronic density of states for N 2 -SWCNT (5, 0) (Fig 1) and N 2 -SWCNT (4, 4) (Fig. 6-8), O 2 -SWCNT (5, 0) (Fig 2) and O 2 -SWCNT (4, 4) (Fig. 3-5), CO -SWCNT (5, 0) (Fig 11) and CO -SWCNT (4, 4) (Fig. 12), CO 2 -SWCNT (5, 0) (Fig 9) and CO 2 -SWCNT (4, 4) (Fig. 10) systems. Obtained values show that as the dipole moment gets bigger, the absolute value of adsorption energy increases. We can interpret this fact as following: the big dipole moment depends to the big distance between electron clouds, then, as the distance becomes bigger the absolute value of adsorption energy will become higher. Fig. 2. (A) The (5,0) SWCNT, (A 1 ) and (A 2 ) adsorption configurations of an N 2 molecule (sitesa1 and A 2, respectively) (A 3 ) and (A 4 ) adsorption configurations of an O 2 molecule (sites A 3 and A4, respectively). Fig. 3: Oxygen molecules chemisorption and physisorptionon Fig. 4: Oxygen molecules chemisorption and physisorptionon open ended of SWCNTs of armchair (4, 4). external surface of SWCNTs of armchair (4, 4). XXX-5

6 Fig. 5: Oxygen molecules chemisorption and physisorption on Fig. 6: Nitrogen molecules chemisorption and Physisorption on external surface of SWCNTs of armchair (4, 4). open ended of SWCNTs of armchair (4, 4). Fig. 7: Nitrogen molecules chemisorption and physisorption on Fig. 8: Nitrogen molecules chemisorption and physisorption on external surface of SWCNTs of armchair (4, 4). external surface of SWCNTs of armchair (4, 4). A 1 A 2 Fig.9. (A) The (5, 0) SWCNT, (A 1 ) and (A 2 ) adsorption configurations of a CO 2 molecule (sitesa 1 and A 2, respectively). XXX-6

7 A 3 A 4 Fig. 10. (A) The (4, 4) SWCNT, (A 3 ) and (A 4 ) adsorption configurations of a CO 2 molecule (sites A 3 and A 4, respectively). A 1 A 2 Fig. 11. (A1) and (A 2 ) adsorption configurations of a carbon monoxide molecule on the SWCNTs (5, 0). A 1 A 2 Fig. 12. (A 1 ) and (A 2 ) adsorption configurations of a carbon monoxide molecule on the SWCNTs (4, 4). The electron configuration of N 2 is KK ( ) ( ) ( ) ( ) ( ), and the 2s 2s 2 px 2 py 2 pz binding orbital of N 2 molecule is filled, so that the transferred electron can't enter into this binding orbital. The electron configuration of O 2 is KK ( ) ( ) ( ) ( ) ( ) ( ) ( ) s 2s 2pz 2px 2py 2px 2py and the transferred electron, certainly will occupy the half-filled anti-bonding orbitals of O 2 hence will weaken the O O bond. The groundstate ( 1 Σ + ) electronic configuration of CO can be XXX-7

8 written as, (1 ) (2 ) (3 ) (4 ) (1 ) (5 ) in terms of a single configuration. The O 2 -tube and N 2 -tube equilibrium distances for adsorption sites A 1, A 2, A 3 and A 4 illuminate the interaction of O 2 and N 2 with CNT belongs to physisorption. The C C distance increases from(c C) 1 =1.401A to (C C) 1 =1.50 and (C C) 2 =1.465A to (C C) 2 =1.49A and hence C O bind in A 2 site shows better adsorption, which is very similar to literature. It was found that O 2 binding energy decreases as the diameter of (n, 0) CNT increases. It is well known that the tendency for sp 2 sp 3 rehybridization upon O 2 adsorption is strong for thin nanotubes, because highly bent sp 2 bonding of thin nanotubes is favoured for the transition to sp 3 bonding. When two carbon atom substituted by one N 2 molecular in a super cell, we find the geometric structures of the N-doped (4,4) and (5,0) SWNTs present dramatic changes, as schematically displayed in (Table 1&2), before and after the doping of N atoms, the bond length of in SWNT-A 1 (4,4) from (C-C) 1,3 =1.421 A and (C-C) 2,4 =1.422 A decreased to 1.334A and bond length of in SWNT-A 2 (4,4) from (C- C) 1 =1.451A, (C-C) 2 =1.419A, (C- C) 3 =1.422A and (C-C) 4 =1.452A decreased to before and after the doping of N atoms, the bond length of in SWNT-A 1 (5,0) from (C-C) 1,3 =1.437A and (C-C) 2,4 =1.451A increased to A and A bond length of in SWNT-A 2 (5,0) from (C-C) 1 =1.426 A, (C- C) 2 =1.451A, (C-C) 3 =1.408A and (C- C) 4 =1.437A decreased to Density functional calculations of SWNT tips with substituted nitrogen show the nitrogen related donor levels reduce the work function of the tip and significantly enhance the localized density of states. Nitrogen doping can also sensitize nano-tubes for gas chemisorption, which may suggest another way to improve field emission properties of N - doped nano-tubes. More recent DFT calculations by Nevidomskyy et al. (2003) in much larger super cells confirm the presence of a donor state associated with substituted nitrogen at around 0.2 ev below the conduction band in zig-zag (8,0) nanotubes, and around the same energy in armchair (5,5) nanotubes. Whether these will be electron-donor or electron acceptor state or none of these two, depends crucially on the way the hetero atoms are substituted into the lattice. In the following, we will focus on the electronics most relevant case of CNTs substitution of nitrogen atoms into the carbon lattice of a SWNT. For nitrogen atoms we have considered distinct chemisorptions sites, marked as CNT (A, A1, A2, A3 and A4). Performed calculation on the values of nitrogen molecule chemisorption energy over zig-zag (5,0) and armchair, (4,4) nanotubes with determined length and diameter, by DFT and HF methods show the difference amounts twice grater. Although (4, 4) carbon nano-tubes have more Chemisorption energies of nitrogen, compared to N2-CNTs-A3 ( ev) and N2-CNTs-A4 ( ev) to (5, 0) carbon nano-tubes have Chemisorption energies of nitrogen, compared to N2-CNTs-A3 ( ev) and N2-CNTs- A4 ( ev) for a nanotube with a diameter of 0. 6 nm and 0.4 nm depending on the nanotube and nitrogen molecules, also HF method has proven this fact, respectively (Table 1&2). For oxygen molecule stable configurations chemisorption at the surface of SWCNT are discussed. Armchair (4, 4) and zigzag (5, 0) tube has different C-C bonds thus offers two distinct chemisorption sites (Table 1&2) before and after the doping of O atoms, the bond length of SWNT-A 1 (4, 4) from (C-C) 1,3 = Aº and (C-C) 2,4 = Aº decreased to Aº and bond length of in SWNTA 2 (4, 4) from (C- C) 1 = Aº, (C-C) 2 = Aº, (C-C) 3 = Aº and (C-C) 4 = Aº decreased to before and after the doping of O atoms, the bond length of SWNT-A 3 (5, 0) from (C-C) 1,3 = Aº and (C-C) 2,4 = Aº decreased to Aº bond length of SWNT-A 4 (5, 0) from (C-C) 1 = Aº, (C-C) 2 = Aº, (C-C) 3 = Aº and (C-C) 4 = Aº increased to Density functional calculations of SWNT, efficient process of charge transfer between the oxygen molecule and the nano-tube is found to substantially reduce the susceptibility of the π- electrons of the nano-tube to modification by oxygen while maintaining stable doping. XXX-8

9 Oxygen chemisorption can be achieved with O 2 + ion implantation (Kamimura et al. 2005). For CO (C-C) 1&2 = Aº, (C-C) 3&4 = Aº, (C-C) 5 = Aº and (C-C) 6 = Aº to CO SWCNT- A 1 (5, 0) nanotube has two different C-C bonds (C-C) 1&2 = Aº and (C- C) 3&4 = Aº to increased SWCNT-A 2 (5, 0) nanotube has four different C-C bonds (C-C) 1 = Aº, (C-C) 2 = Aº, (C-C) 3 = Aº and (C-C) 4 = Aº thus suggests two distinct adsorption sites. Armchair (4, 4) and zigzag (5, 0) tubes have different C-C bonds thus offers two distinct adsorption sites (Table 1) before and after the doping of C-O atoms, the bond length of in SWCNT (4, 4) from (C-C) 1 = Aº and (C-C) 2 = Aº (C-C) 3 = Aº, (C-C) 4 = Aº and (C-C) 5&6 = Aº increased to Aº in the SWCNT-A 1 (4, 4) and bond length of in SWCNT-A 2 (4, 4) increased to from (C-C) 1&2 = Aº, (C-C) 3 = Aº and (C-C) 4 = Aº(Table 1&2). Density functional calculations of SWCNTs, efficient process of charge transfer between the CO molecule and the nanotube is found to substantially reduce the susceptibility of the π- electrons of the nano-tube to modification by CO while maintaining stable doping. A diagrammatic view of this form is showed in Fig. 11 and 12 SWCNTs and COSWCNTs- A 1&2. Such a structure has also been observed for other SWCNTs (Sorescu et al., 2001; Ghasemi et al. 2010). For the molecular CO- SWCNTS systems, CO seemed to place parallel to the outer surface of the tube. Geometry calculations of distortion caused by the carbon monoxide molecule on the (C 1 - ) bond of zigzag (5, 0) and armchair (4, 4) SWCNTs are changed partly. Two different types of adsorbed CO molecules were recognized (Fig. 11 and 12. SWCNTs (5, 0) and (4, 4), COSWCNT- A 1, CO-SWCNT-A 2 model (5, 0) and (4, 4)). The calculated adsorption energies were predicted to be and ev for CO-SWCNTs- A1&2(5, 0) and and ev for CO- SWCNTs-A 1&2 (4, 4), respectively. The length of nanotube have selected with regard to the length of unit cell of nanotube. Such adsorptions of CO molecule are known as cycloaddition which is very similar to those found for larger diameter tubes (Walch, 2003; Krstiƒ et al., 2002). The geometry of (5, 0) and (4, 4) tubes are considerably modified when such oxidation occurs and physisorbed product is formed. The electron can't enter into CO molecule binding orbital because the binding orbital is filled. This arrives to either sp3 hybridization for two carbon atoms or breaking of one C-C bond. Two different types of adsorbed CO species were identified (Table 1&2). Also, the dipole moments were calculated by Gaussian software and have shown in (Table 1&2). Obtained values demonstrate that as the dipole moment increases, the absolute value of bond energy increases too. We can explain this reality as following: the big dipole moment relies to the large distance between electron clouds, then, as the distance becomes larger the absolute value of bond energy will become higher. By comparing the obtained results with Jordan's one (Collins et al., 2000), It is well known that the tendency for sp 2 -sp 3 rehybridization upon CO adsorption is strong for thin nanotubes, because highly bent sp 2 bonding of thin nanotubes is favored for the transition to sp 3 bonding. According to adsorption energy and dipole moment parameters in (Table 1&2), CO-SWCNT (5, 0) molecule shows the highest adsorption rate. This is a general reason for the binding in the performed studies (Dae-Hwang et al., 2003).which shows that CO molecules energy values of adsorption on zig-zag (5, 0) and armchair (4, 4) SWCNTs models with determined diameter and length have about twice differences in grandeur. XXX-9

10 Table. 1. Calculated adsorption energies, bond energies (A ) and dipole momentum (Debye) of the O2 and N2 and CO adsorbed on surface armchair (n, n), n = 4 nanotube. Model RC (A ) RC C(A ) X RO O (X O, N,C) (A ) a Dipole RC O(A ) RN N (A ) E ad ( ev ) momentum (Debye) CNT (5, 0) (C-C)1 = (C-C)2 = CNTs-N2- A1 (C-C)1 = 1.51 (C-C)2 = 1.51 (C-N)1 = (C-N)2 = CNTs-N2- A2 CNTs-O2- A1 (C-C)1 = 1.50 (C-C)2 = 1.49 (C-C)1 = 1.51 (C-C)2 = 1.51 (C-N)1 = (C-N)2 = (C-O)1 = (C-O)2 = CNTs-O2- A2 (C-C)1 = 1.50 (C-C)2 = 1.49 (C-O)1 = (C-O)2 = CNTs-CO- A1 (C-C)1 = (C-C)2 = (C-C)3 = (C-C)1 = (C-O)2 = (C-C)4 = CNTs-CO- A2 (C-C)1 = (C-C)2 = (C-C)3 = (C-C)4 = (C-C)1 = (C-O)2 = a: These values of E ( ev ) are calculated for entire complex; The related values adsorptions on nanotube are indicated inside parenthesis ad Table 2: Calculated adsorption energies, bond energies (A ) and dipole momentum (Debye) of the O2 and N2 and CO adsorbed on surface armchair (5, 0) nanotube. Model RC (A ) RC C(A ) X RO O (X O, N,C) (A ) Dipole a RC O(A ) RN N (A ) Ead ( ev ) momentum (Debye) (C-C)1 = CNT (5, 0) (C-C)2 = CNTs-N2- A1 CNTs-N2- A2 CNTs-O2- A1 (C-C)1 = 1.51 (C-C)2 = 1.51 (C-C)1 = 1.50 (C-C)2 = 1.49 (C-C)1 = 1.51 (C-C)2 = 1.51 (C-N)1 = (C-N)2 = (C-N)1 = (C-N)2 = (C-O)1 = (C-O)2 = CNTs-O2- (C-C)1 = 1.50 (C-O)1 = A2 (C-C)2 = 1.49 (C-O)2 = (C-C)1 = CNTs-CO- (C-C)2 = (C-C)1 = A1 (C-C)3 = (C-O)2 = (C-C)4 = (C-C)1 = CNTs-CO- (C-C)2 = (C-C)1 = A2 (C-C)3 = (C-O)2 = (C-C)4 = a: These values of E ( ev ) are calculated for entire complex; The related values adsorptions on nanotube are indicated inside parenthesis. ad For CO 2, nanotube (5, 0) has (C-C 1 ) S = Aº, (C 1 -O) D = 4.77 Aº, and CO 2 SWCNT- D and S (4,4) nanotube has two different CNT- CO 2 bonds (C-C 1 ) D = Aº and (C-C 1 ) S = 4.60 Aº, that show increased adsorption of CO 2 molecule on the surface SWCNT-D (5, 0) nanotube and has different C-C bond length on the open ended (5,0) (C 1 -C 6 ) = Aº, on the XXX-10

11 open ended (4,4) (C 1 - )= Aº, and on the surface(5,0) (C 1 - ) = Aº and (C 1 -C 6 ) = Aº and on the surface(4,4) (C 1 - ) = 1.41 Aº and (C 1 -C 6 ) =1.43 Aº thus suggests two distinct adsorption sites, Respectively. The increased bond length (C 1 - ) and diameter SWCNT-(S) (5,0) on the surface after adsorbed the molecular CO 2 on the (5,0) SWCNT systems, CO 2 seemed to place parallel to the outer surface (5,0) tube. Density functional calculations of SWCNTs, efficient process of charge transfer between the CO 2 molecule and the nanotube is found to substantially reduce the susceptibility of the π- electrons of the nano-tube to modification by CO 2 while maintaining stable doping. Diagrammatic view of this form is showed in (Fig. 9 and 10) SWCNTs and CO 2 -SWCNTs-(S) and (D). Geometry calculations of distortion caused by the carbon dioxide molecule on surface and open ended the (C-C) bond of zigzag (5, 0) and armchair (4, 4) SWCNTs are changed partly. Two different types of adsorbed CO 2 molecules were recognized (Fig. 9 and 10) SWCNTs (5, 0) and (4, 4). The calculated adsorption energies were predicted to be and ev on surface and open ended CO 2 - SWCNTs-S&D(4, 4) and and ev for CO 2 -SWCNTs-S and D(5, 0), respectively. The length of nanotube have selected with regard to the length of unit cell of nanotube. The geometry of zig-zag (5, 0) on the open ended and armchair (4, 4) on the surface and open ended tubes are considerably modified when such oxidation occurs and physisorbed product is formed and the geometry of zig-zag (5, 0) on the surface tubes chemisorbed product is formed. The electron can't enter into CO 2 molecule binding orbital because the binding orbital is filled. This arrives to either sp 3 hybridization for two carbon atoms or breaking of one C-C bond. Two different types of adsorbed CO 2 species were identified (Fig. 9and 10 and Table 3and 4). The dipole moments were calculated by Gaussian software and have shown in Table 3and 4. The results of the dipole moment for single species and most stable configurations of considered complexes demonstrate that during CO2 adsorption for all systems, total dipole moment increases. We considered that dipole moment for (5, 0) CO2- CNTs and (4, 4) CO2-CNTs on the surface and open ended are 9.55, 1.37 and 0.05 Debye, respectively (Table 3&4). Table. 3. Calculated adsorption energies ΔE ads (ev), Bond gap(ev), Charge(DFT) and dipole moment (Debye) of the CO2 adsorbed surface and open-ended zigzag (5,0) nanotube. Model SWCNT(5,0) CO2-SWCNT(5,0) D1 CO2-SWCNT(5,0) D2 CO2-SWCNT(5,0) A1 CO2-SWCNT(5,0) A2 Atoms C4 C5 HOMO(eV) LUMO(eV) Bond gap (ev) Charge (DFT) ΔEads(eV) Dipole moment (Debye) XXX-11

12 Table 4. Calculated adsorption energies (Eads), Bond gap(eg), Q T (DFT) and dipole moment (Debye) of the CO 2 adsorbed surface and open-ended zigzag (5,0) and armchair (4,4) nanotubes. All energies are in units of ev. Dipole moment Bond gap System atoms E ads (ev) E (Debye) HOMO(eV) E LUMO(eV) (ev) CNT (4, 4) C Q T CO 2-CNT (4, 4)-D C CO 2-CNT (4, 4)-S C CO 2-CNT (5, 0)- S C NMR chemical shielding a) 15 N NMR chemical shielding Tables 5-7 and Fig 6, 7, 8, 13 shows the calculated NMR parameters for the nitrogen nuclei in the two models (4, 4) and (5, 0) of the CNTs, N2 molecule chemisorption of the SWCNTs has markable influence on 15 N NMR tensors, which is in complete accordance with the facts mentioned above. Previously, it has been indicated that for the H- capped SWCNTs, the calculated 15N chemical shielding values at the ends are smaller than in the tube s center if the carbon is directly bound to a hydrogen; otherwise it is large (Liu et al., 2007). It is also depicted that chemical shielding components converge in a way similar to that of the chemical shifts when increasing the tube length albeit not as smoothly as the isotropic shielding. Chemical shielding tensors and chemical shifts are efficient parameters for characterization of carbon nano-tubes. Calculation of these shielding tensors for nitrogen nuclei reveals that increasing length and diameter of SWCNTs A1 (5, 0) chemical shielding will cause N nuclei converge on the nano tube surface; Results are consistent with strong interaction between the tube and N2 molecule in SWCNTs A1 (5, 0). This is consistent with previous results derived from band structure calculations (Rubio et al., 1994; Balase et al., 1994). On the other hand, the calculated 15 N chemical shielding values in the middle of the CNT (4,4) and CNT (5,0) seem to approach values ppm, ppm and ppm, ppm, respectively (Tables 5-7). The NMR chemical shielding of finite SWCNTs were found to converge very slowly, if at all, to the infinite limit, indicating that hydrogen capped tube fragments are not necessarily good models of infinite systems. As the length of the fragment increases, these orbitals do not yield a contribution to the electron density along the tube (except at the ends) and must therefore be regarded as artifacts due to treating the finite-sized systems. More recently, this group indicated that Also, the introduction of nitrogen atoms is theoretically predicted to give rise to chiral current flow along the nano-tube (Zurek et al., 2008) due to symmetry breaking (Liu and Guo, 2004; Miyamoto, 1996). Due to N2 chemisorption the calculated 15 N NMR parameters of those interacted carbon atoms are also modified. As understood by comparison of sites (A1, A2, A3 and A4), the carbon atoms included in N2 chemisorption become more shielded. Among the four NMR principal components, intermediate shielding component, -tube to the N2 CNT system. The discrepancy between the 15 N chemical shielding tensor for the sites (A1, A2, A3 and A4) systems must be attributed to the different nature of the frontier orbitals which will have an influence on the 15 N chemical shielding. However, this theoretical considerations and predictions are undermined by recent experimentally investigations where chiral currents have been observed in undoped singlewalled carbon nano-tubes (Jang et al., 2004; Cantalini et al., 2004; Wang et al., 2004b). The XXX-12

13 interest in nitrogen-doped CNTs in terms of application is the control of the type of charge carriers within the carbon nano-tubes (Ashrafi et al., 2010; Ghasemi et al., 2010). This control is one key-issue for a successful implementation of CNTs in nano- and molecular-electronic (Wang et al., 2004a; Kong et al., 2000). N2-CNTs should show significant advantages over nanotubes for gas sensor applications, due to their reactive tube surfaces, and the sensitivity of their transport characteristics to the presence, distribution and chemistry of nitrogen. Peng and Cho first suggested N2-CNT for use in gas sensors, due to the ability of nitrogen dopants to bind to incoming gas species. The nitrogen in the nano-tubes can be seen as regular defects which change the chemical behavior of the tubes. Model CNT(5,0) (A1) - CNT(5,0) (A2) - CNT(5,0)- N2(A3) CNT(5,0)- N2(A4) A 3 A 4 Fig. 13. N 2 molecule Adsorption on external surface of SWCNT of zigzag (5, 0). Table 5: Calculated 15 N NMR parameters for CNT, N2 CNT (5, 0) systems a 15 N Atom ii( 11, 22, 33) iso N1 N2 N1 N2 C4 C4 C4 C4 ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; 102.2; ) ( ; 96.89; 96.89) ( ; 103.5; 157.3) ( ; 98.39; 98.39) (6.4895; ; ) ( ; ; ) ( ; ; ) (7.2977; ; ) a.calculated ii, iso, values in ppm. b : In each raw, the first number is for 11, the second number is for 22, and the third number Table 6: Comparison of chemical shielding and chemical shift tensors physisorption on the surface and open ended 15 N, 17 O NMR parameters for CNTs, Nitrogen-CNTs (4, 4) and Oxcygen-CNTs (4, 4) systems a Model 15 N, 17 O Atom ii( 11, 22, 33) b iso CNT4,4-O 2D 1 O 1 O 2 ( ; ; ) ( ;0.8351; ) NT4,4-N 2D 2 N 2 ( ; ; ) N 1 ( ; ; ) CNT4,4-O2A O 2 ( ; ; ) O 1 ( ; ; ) CNT4,4-O 2A 2 O 1 ( ; ; ) O 2 ( ; ; ) CNT4,4-N 2A 1 N 1 ( ; ; ) XXX-13

14 CNT4,4-N 2A 2 N 2 ( ; ; ) N 1 ( ; ; ) N 2 ( ; ; ) a.calculated ii, iso, values in ppm. b : In each raw, the first number is for 11, the second number is for 22, and the third number is for 33 Table 7: Comparison of chemical shielding and chemical shift tensors chemisorption on the surface and open ended 15 N, 17 O NMR parameters for CNTs, Nitrogen-CNTs and Oxcygen-CNTs armchair (4, 4) systems a Model 15 N, 17 O Atom ii ( 11, 22, 33 ) iso N1 ( ; ; ) CNT(4,4)-N2 (D1) CNT(4,4)-O2 (D2) CNT(4,4)-N2 (A1) CNT(4,4)-N2 (A2) CNT(4,4)-O2 (A1) CNT(4,4)-O2 (A2) N2 O1 O2 N1 N2 N1 N2 O1 O2 O1 O2 C4 C4 C4 C4 ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ;7.3599; ) ( ; ; ) a.calculated ii, iso, values in ppm. b : In each raw, the first number is for 11, the second number is for 22, and the third number 33. b) 17 O NMR parameters: New data and presentation of results are given here for O-doping computational NMR parameters of oxygen nuclei for two models (4, 4) and (5, 0) of CNTs (Tables 6-8 and Fig 3, 4, 5, 14). Oxygen molecule chemisorptions of SWCNTs have remarkable influence on NMR tensors, which is in complete accordance with the facts mentioned above. Consequently, it has been 17 O indicated that for the H-capped SWCNTs, the calculated 17 O chemical shielding values at the ends are smaller than in the tube s center if the carbon is directly bound to hydrogen; otherwise it is larger (Liu et al., 2007). It is also depicted that chemical shielding components converge in a way similar to that of the chemical shifts when increasing the tube length albeit not as smoothly as the isotropic shielding. Chemical shielding tensors and chemical shifts are efficient parameters for characterization of carbon nanotubes. Calculation of these shielding tensors for oxygen nuclei reveals that increasing length and diameter of SWCNTs-A 1 (5, 0) chemical shielding will cause O nuclei converge on nanotube surface. Results are consistent with strong interaction between the tube and O 2 molecule in SWCNTs-A 1 (5, 0). This is consistent with previous results derived from band structure calculations (Rubio et al., 1994; Balase et al., 1994). On the other hand, the calculated 17 O chemical shielding values in the middle of the CNT (4, 4) and CNT (5, 0) seem to approach values , , and ppm, respectively (Tables 6-8). The NMR chemical shielding of finite SWCNTs were found to converge very slowly, if at all, to the infinite limit, indicating that hydrogen capped tube fragments are not necessarily good models of infinite systems. As the length of the fragment increases, these orbitals do not yield a contribution to the electron density along the tube (except at the ends) and must therefore be regarded as artifacts due to treating the finitesized systems. More recently, this group indicated that Zurek et al. (2008) also, the introduction of oxygen atoms is theoretically predicted to give rise to chiral current flow along XXX-14

15 the nanotube due to symmetry breaking (Liu and Guo, 2004; Miyamoto, 1996). Due to O 2 chemisorptions the calculated 17 O NMR parameters of those interacted carbon atoms are also modified. As understood by comparison of sites (A 1, A 2, A 3 and A 4 ), the carbon atoms included in O 2 chemisorptions become more shielded. Among the four NMR principal components, intermediate shielding component, σ 22, shows more change from nanotube to the O 2 -CNT system. The discrepancy between the 17 O chemical shielding tensor for the sites (A 1, A 2, A 3 and A 4 ) systems must be attributed to the different nature of the frontier orbital's which will have an influence on the 17 O chemical shielding. However, this theoretical considerations and predictions are undermined by recent experimentally investigations where chiral currents have been observed in undoped singlewalled carbon nanotubes (Krstiƒ et al., 2002). The interest in oxygen-doped CNTs in terms of application is the control of the type of charge carriers within the carbon nanotubes. This control is one key-issue for a successful implementation of CNTs in nanotubes and molecular electronics. O 2 -CNTs should show significant advantages over nanotubes for gas sensor applications, due to their reactive tube surfaces and the sensitivity of their transport characteristics to the presence, distribution and chemistry of oxygen. (Peng and Cho, 2003) first suggested O 2 -CNT for use in gas sensors, due to the ability of oxygen dopants to bind to incoming gas species. The oxygen in the nanotubes can be seen as regular defects which change the chemical behavior of tubes. Model CNT(5,0)- (A 1) CNT(5,0)- (A 2) CNT(5,0)-O2(A 1) CNT(5,0)-O2(A 2) CNT(5, 0)(A1) CNT(5,0)(A 2 ) CNT(5, 0)-O2(A3) CNT(5, 0)-O2(A Fig.14. O 2 molecule Adsorption on external surface of SWCNT of zigzag (5, 0). Table 8: Calculated 17 O NMR parameters for CNT, O 2 -CNT (4, 4) systems a 17 O Atoms iso C C O 1 O 2 O 1 O 2 C 4 C 1 C 3 C 4 C 1 C 1 C 1 C a.calculated ii, iso, values in ppm. b In each raw, the first number is for 11, the second number is for 22, and the third number 33. XXX-15

16 c) 13 C NMR chemical shielding (5, 0) for O 2 and N 2 Table 9 exhibit the calculated 13 C chemical shielding for CNTs. O 2 and N 2 adsorption on the CNT has a remarkable in flounce on 13 C NMR tensors which is in complete accordance with the facts mentioned above Previously, it has been indicated that for the H-capped CNTs, the calculated 13 C chemical shielding value sat the ends are smaller than in the tube s center if the carbon is directly bound to a hydrogen; otherwise it is larger. It is also depicted that chemical shielding components converge in a way similar to that of the chemical shifts when increasing the tube length albeit not as smoothly as the isotropic shielding. On the other hand, the calculated 13 C chemical shielding values in the middle of The (5,0) CNT seem to approach values and ppm (Table 9).It may be noted that 13 C chemical shielding tensor a the carbon sites depends remarkably on the tube size and nature of frontier orbital's (Wu et al., 2002). DFT study of 13 Cchemical shielding tensors on small-to-medium diameter infinite SWCNTs revealed that chemical shielding decreases roughly inversely proportional to the tubes diameter. The NMR chemical shielding of finite SWCNTs were found to converge very slowly, to the infinite limit, indicating that hydrogen capped tube fragments are not necessarily good models of infinite systems. For the hydrogen capped (9, 0) tube case, all of the frontier orbital's have carbon p-s character, they are localized at each end of the tube. As the length of the fragment increases, these orbital's do not yield a contribution to the electron density along the tube (except at the ends) and must therefore be regarded as artifacts due to treating the finite sized systems. According to GIAO calculations performed after adsorption of O 13 2 C NMR parameters of those interacted carbon atoms are also modified. As understood by comparison of sites A 1, A 2, A 3 and A 4, the carbon atoms included in O 2 adsorption become more shielded. Among the two NMR principal components, intermediate shielding component, 22, shows more change from nanotube to the O 2 CNT system (Ghasemi et al., 2010; Duer, 2002). The results are consistent with strong interaction between the tube and O 2 molecule. The discrepancy between the 13 C chemical shielding tensor for the site CNT, A 1, A 2, A 3 and A 4 systems must be attributed to the different nature of the frontier orbitals. Table 9. Calculated 13 C NMR parameters for CNT, N 2 CNT, O 2 CNT system a Model Atoms iso CNT (5, 0) C 1 C 3 CNT(5,0)- O 2(A 1) C 1 CNT(5,0)- O 2(A 2) CNT(5,0)- N 2(A 3) CNT(5,0)- N 2(A 4) C 1 C 1 C 1 a. Calculated ii, iso values in ppm d) 13 C NMR chemical shielding (4, 4) for O 2 and N 2 Table 10 shows the calculated 13 C chemical shielding tensors for CNTs. O 2 and N 2 molecules adsorption on the CNT has a remarkable influence on 13 C NMR tensors, which is in complete accordance with the facts mentioned above. It is also explain that, chemical shielding components converge in a way similar to that of the chemical shifts which increasing the tube length however, not as monotonous as the isotropy shielding. On the other hand, the calculated 13C chemical XXX-16

17 shielding values in the middle of the (4, 4) CNT seem approach the values 53.8 and 57.3 mg/l. According to GIAO calculations performed after adsorption of oxygen and nitrogen molecules, the isotropy value of the 13 C NMR shielding tensor is decreased in A 1 and A 3 sites (about and mg/l at C 1 and of site A 1 and and mg/l at C 1 and of site A 3 ) and increased in A 2 and A 4 sites (about and mg/l at C 1 and of site A 2 and and mg/l at C 1 and of site A4) (Table 10). The associated anisotropy value decreases for both carbons. However, the effect is more significant for C 1 and nuclei. Results reveal that electronic charge distribution around the carbon atoms becomes more symmetric as a result of oxygen adsorption. The anisotropy value of the 13 C NMR shielding tensor is increased approximately about and mg/l, at C 1 and of site A 1. In this case, anisotropy values for both carbons decrease by adsorption, while the reduction is more evident for C 1 and. Due to O 2 adsorption, the calculated 13 C NMR parameters of those interacted carbon atoms are also modified. As deduced from comparison of sites A 1 and A 2, the carbon atoms contributed in O 2 adsorption, become more shielded. Among the two NMR principal components, intermediate shielding component, 22, shows more change from nano-tube than O 2 -CNT system, which is in contrast with N 2 -CNT. The results are consistent with fort interaction between nano-tube and O 2 molecule. The discrepancy between 13 C chemical shielding tensor for the site A 1 and A 2 systems and A 3 and A 4 systems must be attributed to the different nature of the frontier orbital. Table 10: Calculated 13 C NMR parameters for CNT, N 2 -CNT, O 2 -CNT systems a Model Atoms iso CNT(A) CNT(4,4)-O2(A1) CNT(4,4)-O2(A2) CNT(4,4)-N2(A3) CNT(4,4)-N2(A4) a. Calculated ii, iso, values in ppm e) The adsorption CO NMR parameter modeled of zigzag (5, 0) on external surface Table exhibits the calculated 13 C chemical shielding tensors for SWCNT. Carbon monoxide molecule adsorption on external surface of SWCNT has a significant influence on 13 C NMR tensors, which is in complete accordance with the facts mentioned above previously. Consequently, it has been CO indicated that for the H-capped SWCNT, the calculated 13 C chemical shielding values adsorption are different on the surface, if the carbon is directly bound to hydrogen, unless, it is larger. To assess the dependence of NMR results on carbon atom position, 13 C chemical shielding isotropy values of zigzag (5, 0) SWCNT have calculated on surface CO adsorption (Fig11). Two different parts of surface tube axis are considered. Interesting surface is evidenced: for zigzag (5, 0) SWCNT, the isotropy adsorption SWCNT (5, 0) - CO (A 1 ) shielding tensor is larger compared to the adsorption 15 N and 17 O at the surface. It is also show that the chemical shielding components converge in a way similar to that of the chemical shifts when increasing the tube length even though not as smoothly as the XXX-17

18 isotropic shielding. Chemical shielding tensors and chemical shifts are efficient parameters for characterization of single walled carbon nano tubes. Calculation of these shielding tensors for carbon monoxide nucleus reveals that increasing length and diameter of zigzag (5, 0) SWCNT chemical shielding will cause carbon monoxide nucleus converge on the single walled nano tube surface the results are consistent with strong interaction adsorption between the tube and carbon monoxide molecules in SWCNT (5, 0) CO (A 1 ). This is consistent with previous results derived from band structure; calculations. On the other hand, the calculated CO chemical shielding values in the middle of the surface zigzag (5, 0) SWCNT seem close to the values ppm (Table11). More recently, it is indicated that introduction of CO atoms is theoretically predicted to give rise to chiral current flow along the nanotube due to symmetry breaking. The results deduced from comparison of sites (A 1 and A 2 ), show that the carbon atoms included in carbon monoxide molecular adsorption become more shielded. Among the six NMR principal components, intermediate shielding component, σ 22, shows more change from SWCNT compared with surface the for CO-SWCNT system. The interest of carbon monoxide SWCNT in terms of application is the control of the type of charge carriers within the SWCNT. CO- SWCNT should show significant advantages over SWCNT for gas sensor applications, due to their reactive tube surfaces and the sensitivity of their transport characteristics in relation with the presence, distribution and chemistry of carbon monoxide. Peng et al (2003) first suggested CO- SWCNT for use in gas sensors, due to the ability of doped molecular carbon monoxide to bind to incoming gas species. The molecular carbon monoxide in the SWCNT can be seen as regular defects which change the adsorption behavior of the SWCNT. Table11: Calculated of chemical shielding and chemical shift tensors adsorption on the surface 13 C parameters for CNT (5,0), CNT(5,0) CO system a Model Atoms ii ( 11; 22; 33) b iso CNT(5,0) (A) CNT(5,0)- CO (A1) CNT(5,0)- CO (A2) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) ( ; ; ) a Calculated ii, iso, values in ppm. b In each raw, the first number is for 11, the second number is for 22, and the third number The 13 C NQR spectroscopic parameters a) The chemisorption and physisorption 17 O and 14 N NQR parameter of armchair (4, 4) model on the surface and open end: The calculated NQR parameters at the sites of 14 N and 17 O nuclei are shown in Table 12 for armchair (4, 4) models of oxygen-cnts and nitrogen- CNTs. The quadrupole moment is nonzero only for nucleuses with spin quantum number, I, greater than or equal to 1. This is a nuclear physics and chemistry parameter describing the distribution of charge in the nucleus. In CNT (4, 4) - O 2 (D 1 ), coupling to the EFG of the valence electrons is largely based on chemistry, although the local crystal packing also plays a role. The q yy values of those nucleuses which participate in the intermolecular H-bonding interactions decrease, but their Q values increase for chemisorption of molecular nitrogen and oxygen on open ended regarding to their physisorption on open ended. The magnitude of these changes for each nucleus depends on its contribution to the interactions. Therefore, more changes in each nucleus NQR parameter indicate its greater role among the other nucleuses in contributing to CNTs. The calculated parameter with the 6-311G* basis sets is in good consistent. In this section, the calculated 14 N and 17 O quadrupole coupling tensors, q ii, quadrupole coupling constant, C Q, and asymmetry parameter,, of the 14 N and 17 O molecular in surface and open ended of SWCNTs is discussed. As shown in (Fig. 3-8) Q XXX-18

19 due to this contribution to CNTs interaction, 14 N quadrupole coupling tensors for the target molecule in the CNTs deviate considerably from 17 O values. As shown in Table 12, both q xx and q yy tensor components increase from 17 O to 14 N, whereas q xx or C Q expose opposite tendency. This is also reflected in the calculated asymmetry parameter. As a general trend, from the monomer to the target molecule in the cluster, H bonding interactions reduce the calculated 14 N and 17 O quadrupole coupling constant values whereas increase the asymmetry parameter. This tensor becomes almost asymmetric for the target molecule in the CNTs cluster. The values for molecular nitrogen Q and oxygen change is 0.89MHz-0.05 MHz and C Q values for molecular nitrogen and oxygen change is MHz units from the monomer to the target molecule in the cluster, respectively. While EFG tensor at 14 N and 17 O sites is approximately axially symmetric, 0., from the gas phase isolated molecule Q to the monomer molecule in solid phase. Table 12: Calculated 13C EFG parameters in Nitrogen-CNTs (4, 4) and Oxygen-CNTs (4, 4) for surface and open ended systems a Model Atoms q xx q yy q C zz Q CNT(4,4)-O2 (D1) C CNT(4,4)-O2 (D2) C CNT(4,4)-N2 (D1) C CNT(4,4)-N2(D2) C CNT(4,4)-O2 (A1) C CNT(4,4)-O2(A2) C CNT(4,4)-N2 (A1) C CNT(4,4)-N2(A2) C a All q ii values in atomic units (1au = /Vm 2 ) b) The CO 2, CO and N 2 NQR spectroscopic parameters of zig-zag (5, 0) model The evaluated NQR parameters reveal that the EFG tensors of 13-Carbon are influenced and show particular trends from gas molecules in the SWCNT due to the contribution of CO 2, CO and N 2 gas molecules in SWCNT interactions. Semiconducting SWCNT are ballistic conductors with two and one spin degenerate conducting channel. The channels belong to the first π and π*-band of the delocalized π-electron system. The 13 C NQR parameter in the geometrically optimized Q SWCNT model zig-zag (5, 0) is estimated by EFG tensors calculations at the B3PW91 level of the DFT method and the 311++G** standard basis set. Since the electric field gradient (EFG) tensors are very sensitive to the electrostatic environment at the sites of quadrupole nuclei, the most possible interacting molecules with the target one were considered in a five carbon atoms in the open end of SWCNT (5,0) zig-zag model with the optimal diameter of 4.03 Å and the length of 7.07 Å. (Table 13) shows the calculated NQR and EFG tensors for SWCNT(5,0) parameter of CO 2, CO and N 2 Q adsorption on the SWCNT open ended has a remarkable effect on EFG tensors. A glimpse to values presented in (Table 13) reveals that Q for 13-carbon changes in EFG tensor for molecular adsorptions are quite significant which is in complete agreement with calculations. The B3PW91/311++G** calculations indicate that all three principal components of the EFG tensor (q ii ) and XXX-19

20 associated asymmetry parameter are affected due to adsorption of CO 2, CO and N 2 molecules. For the (CO 2 SWCNT-D 2 ) systems, the EFG tensors of SWCNT (5, 0) is more significantly affected compared to CO 2 - SWCNT (5, 0) - D 1, CO-SWCNT (5, 0)-D 1&2 and N2-SWCNT (5, 0)- D, respectively. Fig.15. (D1) and (D2) adsorption configurations of dioxide carbon molecules. Fig.16. (D1) and (D2) adsorption configurations of monoxide carbon molecules. Fig.17. (D) adsorption configurations of nitrogen molecule. Table 13.Calculated Carbon-13 EFG parameters for the SWCNT, CO SWCNT (5, 0), CO2- SWCNT (5, 0) and N 2-SWCNT (5,0) and systems. Model Atoms q q xx yy q zz C SWCNT(5,0) - D C C 4 C CO2-SWCNT(5,0) - D1 C CO2-SWCNT(5,0) - D CO-SWCNT(5,0) - D1 C CO-SWCNT(5,0) - D N2-SWCNT(5,0) - D C C) The CO NQR parameters Semiconducting SWCNTs are ballistic conductors with two and one spin degenerate conducting channel(s) (Kang et al., 2005; Yeung et al., 2010). The channels belong to the first π and π*-band of the delocalized π -electron system. The C-13 NQR parameters (C Q and π) in the geometrically optimized SWCNTs models XXX-20

21 zig-zag (5, 0) and armchair (4, 4) were estimated by EFG tensors calculations at the B3LYP level of the DFT method and the 6-311G* standard basis set. Table 14 shows the calculated NQR and EFG tensors for SWCNTs π parameter of CO adsorption on the surface of zig-zag (5, 0) and armchair (4, 4) SWCNTs surface which has a remarkable effect on EFG tensors. A glimpse to π values presented in (Table 14) reveals that for 13-carbon, changes in EFG tensor for molecular adsorptions are quite significant which is in complete agreement with calculations. The B3LYP/6-311G* calculations indicate that all three principal components of the EFG tensor (q ii ) and associated asymmetry parameter are affected due to adsorption of CO molecule. For the (CO-SWCNTs) systems, the EFG tensors of SWCNTs (4, 4) and SWCNTs (5, 0) are more significantly affected compared to CO-SWCNTs (4, 4) -A 1&2 and CO-SWCNTs (5, 0) - A 1&2, respectively. As previously mentioned, CO molecule adsorption at the CO-SWCNTs (5, 0) - A 2 leads to the C-O bond cleavage and CO molecule adsorption at the CO-SWCNTs (5, 0) - A 2 breaks C 1 -O bond. CO adsorptions produce more EFG change at CO-SWCNT (5, 0) -A 2 which can be attributed to their hybridization effect (from sp 2 to sp 3 ). The principle components of EFG tensor change significantly after CO adsorption at C 1 and atoms in CO- SWCNTs (5, 0) -A 1&2. Model SWCNT (4, 4) SWCNT (5, 0) SWCNT (4, 4)- CO (A1) SWCNT (4, 4)- CO (A2) SWCNT (5, 0)- -CO (A1) SWCNT (5, 0)- -CO (A2) Table 14: Calculated carbon-13 EFG parameters for the SWCNTs, CO SWCNTs systems Atom q q xx yy zz d) The CO 2 NQR parameters The evaluated NQR parameters reveal that the EFG tensors of 13-Carbon are influenced and show particular trends from gas molecules in the SWCNT due to the contribution of CO 2 gas molecule in SWCNT interactions. Semiconducting SWCNT are ballistic conductors with two and one spin degenerate conducting channel. The channels belong to the first π and π*-band of the delocalized π-electron system. The C-13 NQR parameters (C Q and ) in the geometrically Q optimized SWCNT model zig-zag (5,0) is estimated by EFG tensors calculations at the B3PW91 level of the DFT method and the G** standard basis set. Since the electric field gradient (EFG) tensors are very sensitive to the electrostatic q environment at the sites of quadrupole nuclei, the most possible interacting molecules with the target one were considered in a five carbon atoms on the Surface and open ended of SWCNT (5,0) zig-zag model with the optimal diameter of 4.03 Å and the length of 7.07 Å. Tables 15 shows the calculated NQR and EFG tensors for SWCNT (5, 0) parameter of CO 2 adsorption on the Surface and open ended SWCNT has a remarkable effect on EFG tensors. A glimpse to values presented in (Tables 15) reveals that for 13-carbon changes in EFG tensor for molecular adsorptions are quite significant which is in complete agreement with calculations. The B3PW91/ G** calculations indicate that all three principal components of the EFG tensor (q ii ) and associated asymmetry parameter are affected due to adsorption of CO 2 molecule. For the Q Q XXX-21

22 (CO 2 SWCNT-D 2 ) systems, the EFG tensors of SWCNT (5, 0) is more significantly affected compared to CO 2 -SWCNT (5, 0) - D 1 and CO 2 - SWCNT (5, 0)-A 1&2, respectively. Fig 18. (D1) and (D2) adsorption configurations of dioxide carbon molecules. Fig 19. (A1) and (A2) adsorption configurations of dioxide carbon molecules Model SWCNT(5,0) Table 15.Calculated Carbon-13 EFG parameters for the SWCNT, CO2 SWCNT(5,0)-D1&2 and CO2-SWCNT (5,0)A1&2, systems. Atoms q q xx yy q zz C4 C CO2-SWCNT(5,0)- D CO2-SWCNT(5,0) D CO2-SWCNT(5,0)- A CO2-SWCNT(5,0) A a Calculated adsorption energies ΔEads(eV), Bond gap(ev), Charge(DFT) and dipole momentum(debye) of the CO2 adsorbed surface and openended zigzag (5,0) nanotube 4. Conclusion Summary, we studied the influence of substitutional N 2, O 2, CO and CO 2 on the single-walled carbon nanotubes conformation and a quantum-chemical calculation was performed. The calculations show that the combination of hexagons and N 2, O 2, CO and CO 2 molecules concentration produces kinks that include the regular shaped nanotubes. The GIAO calculations at the B3PW91/ G** level using DFT optimized geometries provided isotropic shielding tensors that correlated well with the observed chemical shift data. The calculated values provided the unambiguous definite assignment of the observed 17 O, 15 N and 13C-NMR calculative data and can be used in the prediction of the chemical shifts of known SWCNTs molecules. The present calculations can also be used to predict chemical shift data for species the formation of which has not yet been observed. For four N 2, O 2, CO and CO 2 adsorption model, we found band gaps above Fermi level become narrower and new local energy levels occur near the Fermi level, which result in the nearly continuous DOS peaks below Fermi level. In overall of our studies, it is worthwhile to replace the pure nanotubes by chemically doped nanotubes and exploit the new phenomena. According to DFT theory and XXX-22