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Formations of CNT modified 5-(halogen)uracil hybrids: DFT studies
Superlattices and Microstructures 65 (2014) 375–379
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Formations of CNT modified 5-(halogen)uracil hybrids: DFT studies Mahmoud Mirzaei a,⇑, Rahman Salamat Ahangari b a Laboratory of Nano Computations (LNC), Department of Medical Nanotechnology, Pharmaceutical Sciences Branch, Islamic Azad University, Tehran, Iran b Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 12 November 2013 Accepted 13 November 2013 Available online 20 November 2013 Keywords: Carbon nanotube Uracil Density functional theory Quadrupole coupling constant
a b s t r a c t Density functional theory (DFT) calculations have been performed to investigate the formations of carbon nanotubes (CNTs) modified 5-(halogen)uracil hybrids (CNT–U–X). The structures of individual counterparts and hybrids have been optimized and the molecular properties have been evaluated. Moreover, the atomic scale properties have been evaluated by computing quadrupole coupling constants (CQ) for the atoms of the optimized structures. The molecular properties indicated that the binding energies for the CNT–U–F hybrid could be more proper than other CNT–U–X hybrids. Lower values of energy gaps and higher values of dipole moments could be seen for the CNT–U–X hybrids in contrast with the individual counterparts. The atomic scale CQ properties indicated that the properties of X atoms could be seen unchanged during the hybridization processes whereas those of oxygen and nitrogen atoms detect the effects especially close to the chemical attachment regions. Finally, changes and similarities could be seen for the properties of CNT–U–X hybrids in comparison with the individual counterparts. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Since the days of carbon nanotubes (CNTs) discovery by Iijima, several efforts have been dedicated to investigate the properties and applications of the novel material [1–5]. Loading functional groups or other molecules on the CNTs could yield new hybrid compounds helpful for so many applications in ⇑ Corresponding author. Fax: +98 21 22602059. E-mail address: [email protected] (M. Mirzaei). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.11.013
376
M. Mirzaei, R. Salamat Ahangari / Superlattices and Microstructures 65 (2014) 375–379
various fields [6]. In the targeted drug delivery systems, the CNTs and other nanostructures are expected to show their unique role of carriers in the living systems to reduce the side effects [7,8]. The edges of CNTs are seen as proper sites for contributing to covalent attachments to other atoms of molecules [9–12], in which a molecule of 5-(halogen)uracil has been attached to the mentioned site in this work (Fig. 1). Earlier works indicated that the 5-halogen derivatives of uracil, the characteristic nucleobase of RNA, could play significant roles in the pharmaceutical applications [13]. However, several side effects are arisen for the users under treatments by 5-(halogen)uracil drug compounds [14]. Therefore, efforts on reducing the side effects and also increasing the targeting characters are important in this manner. Based on the benefits of computations in the molecular levels, the formations of molecular systems of CNT modified 5-(halogen)uracil hybrids (CNT–U–X) have been investigated in this work. Density functional theory (DFT) calculations have been performed to optimize the investigated structures and to evaluate the desired properties to achieve the purpose of study (Tables 1 and 2 and Fig. 1). The molecular and atomic levels properties have been evaluated for the optimized structures to better characterize the formations and effects of hybridizations on the individual counterparts. 2. Computational details The B3LYP exchange–correlation functional and the 6-31G(d) standard basis set implemented in the Gaussian 98 program have been used for the computations of this work [15]. The molecular systems include the individual CNT, the individual uracil derivatives (U–X), and the CNT–U–X hybrids, in which X could be one hydrogen atom or one halogen atom: fluorine, chlorine, bromine, or iodine (Fig. 1). The representative CNT of this work is a hydrogen capped (6,0) model with the stoichiometry of C48H12. The hydrogen atoms are used to saturate the edges of nanotubes to avoid the dangling ef-
Fig. 1. Individual CNT and U–X models. The connecting atomic sites are C7 and N1 atoms in the CNT–U–X hybrids.
Table 1 Optimized properties for the individual and hybrid models.a Property
CNT–H
Stoichiometry
C48H12
CNT–U–H (U–H) CNT–U–F (U–F) CNT–U–Cl (U–Cl) CNT–U–Br (U–Br) CNT–U–I (U–I)
C52H14N2O2 (C4H4N2O2) Total energy/keV 49.682 60.875 ( 11.226) Binding energy/eV 578.356 653.235 (80.470) Energy gap/eV 0.426 0.415 (5.726) Dipole moment/Debye 0.001 3.546 (4.128)
a
See Fig. 1 for the models.
C52H13FN2O2 (C4H3FN2O2) 63.561 ( 13.911) 653.284 (80.504) 0.413 (5.429) 4.383 (3.845)
C52H13ClN2O2 (C4H3ClN2O2) 73.325 ( 23.675) 651.876 (79.121) 0.413 (5.335) 5.114 (3.934)
C52H13BrN2O2 (C4H3BrN2O2) 130.577 ( 80.927) 651.741 (78.983) 0.413 (5.208) 4.491 (3.818)
C52H13IN2O2 (C4H3IN2O2) 248.367 ( 198.717) 651.154 (78.398) 0.413 (5.053) 4.541 (3.720)
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M. Mirzaei, R. Salamat Ahangari / Superlattices and Microstructures 65 (2014) 375–379 Table 2 NQR properties for the individual and hybrid models (CQ /MHz).a
a
Atom
CNT–H
CNT–U–H (U–H)
CNT–U–F (U–F)
CNT–U–Cl (U–Cl)
CNT–U–Br (U–Br)
CNT–U–I (U–I)
X
–
O2
–
O4
–
N1
–
N3
–
C2
–
C4
–
C5
–
C6
–
C7 C8 C9 C10 C11 C12 C13 C14 H1
1.036 1.316 1.022 1.105 1.105 1.022 1.315 1.037 –
H3
–
H6
–
H7
0.209
0.216 (0.218) 8.379 (8.255) 9.503 (9.433) 3.147 (3.958) 3.720 (3.812) 1.050 (1.115) 1.942 (1.961) 0.727 (0.482) 2.340 (2.147) 1.798 0.869 1.047 0.844 1.186 0.962 1.310 1.145 – (0.278) 0.271 (0.273) 0.207 (0.211) –
65.710 (65.877) 8.342 (8.200) 9.474 (9.448) 3.278 (4.148) 3.765 (3.846) 0.987 (1.081) 1.653 (1.025) 2.725 (2.828) 1.822 (1.651) 1.898 0.863 1.057 0.834 1.193 0.953 1.306 1.182 – (0.278) 0.270 (0.272) 0.208 (0.212) –
72.109 (72.036) 8.370 (8.262) 9.342 (9.346) 3.175 (4.002) 3.776 (3.860) 1.019 (1.097) 1.687 (1.699) 2.180 (2.286) 2.013 (1.865) 1.940 0.874 1.062 0.829 1.200 0.948 1.302 1.242 – (0.277) 0.270 (0.272) 0.206 (0.209) –
564.810 (563.641) 8.372 (8.258) 9.350 (9.341) 3.161 (3.993) 3.776 (3.860) 1.020 (1.097) 1.731 (1.741) 1.837 (1.960) 2.057 (1.904) 1.906 0.872 1.060 0.832 1.198 0.951 1.304 1.215 – (0.277) 0.270 (0.272) 0.206 (0.210) –
1708.796 (1700.996) 8.385 (8.276) 9.215 (9.210) 3.132 (3.948) 3.782 (3.868) 1.024 (1.101) 1.774 (1.784) 1.533 (1.661) 1.278 (1.997) 1.915 0.877 1.061 0.833 1.198 0.952 1.303 1.232 – (0.277) 0.270 (0.272) 0.206 (0.210) –
H14
0.209
0.209
0.209
0.209
0.209
0.209
See Fig. 1 for the models and the atoms numbers. X is replaced by H5, F, Cl, Br, or I.
fects [16,17]. The uracil derivatives have the stoichiometry of C4H3XN2O2; X implies for the hydrogen or halogen atoms. In the hybrids, one hydrogen atom from the tubular edge and the hydrogen atom of N1 of uracil derivatives have been removed to make possible the chemical attachment of uracil to the CNT. The models of study have been allowed to relax during the optimization processes to yield the stabilized structures. Subsequently, the properties have been evaluated; total energies, binding energies, energy gaps, and dipole moments (Table 1). To better achieve the purpose of study, quadrupole coupling constants (CQ) have been also calculated for the atoms of the optimized structures (Table 2). The value of CQ could be experimentally measured by nuclear quadupole resonance (NQR) spectroscopy technique [18]. The CQ is defined as the interaction energy between the nuclear quadrupole moment (Q) and the electric field gradient (EFG) tensors, in which it could be generated by computations [18]. We calculated the EFG tensors (qii; |qzz|>|qyy|>|qxx|) for the atoms and then we used the standard values of Q to evaluate the values of CQ in the equation of CQ (MHz) = e2Qqzzh 1 [19]. The EFG tensors are originated from the electronic sites of atoms; therefore, they could well describe the electronic properties of matters at the atomic levels [20–22]. Due to electronic complexity of nanostructures for doing MQR experiments, evaluation of the values of CQ is a benefit of computational studies [23].
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M. Mirzaei, R. Salamat Ahangari / Superlattices and Microstructures 65 (2014) 375–379
3. Results and discussion 3.1. Optimization processes Doing the processes of optimizations yielded the stabilized structures of individual and hybrid models (Fig. 1) of this work. The individual models are the CNT and U–X structures, in which X could be replaced by one atom of H, F, Cl, Br, or I. The hybrid structures are constructed by chemical attachments of U–X to the CNT by removing one H atom from the edge of CNT and the H atom of N1 of U–X. In the uracil nucleobase, N1 is important due to its contribution to glycoside bonding in the uridine nucleotide; therefore, we used this atomic site for chemical attachment to the CNT [24]. The optimized properties of Table 1 indicate that the most proper binding energy belongs to the individual U–F model and the least proper one is for the U–I structure. The values of binding energies refer to the required energy to break a bond, in which they could be compared in the parallel structures to detect the better possibilities of formations. It is noted that the binding energies are evaluated by differences of energies between the structure and the atomic components. For the hybrid structures, the ordinal positions are remained unchanged in comparison with the individual U–X structures; the CNT–U–F and CNT–U–I are highlighted in the hybrids. The values of energy gaps, which are evaluated by differences of energies between the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO), indicate that the magnitudes for hybrids are lower than the individual structures. Since the distances between HOMO and LUMO are related to the conductivity properties, lower distances could be proposed for better conductions what are seen for the hybrids. Moreover, although the magnitudes of energy gaps are different for the individual U–X structures, they are interestingly similar for the CNT–U–X structures. The values of dipole moments indicate that the structures show different polarities; the smallest value belongs to CNT–U–H and the largest one belongs to CNT–U–Cl among the hybrids. For the individual ones, the CNT is a non-polar structure but the U–X structures show polarities by their dipole moments. After hybridizations, the U–X counterpart significantly changes the polarity for the CNT counterpart in the new compound. Polarity is important especially for dispersing a matter in the aqua systems, in which the CNT modified U–X hybrids could be seen more proper for dispersion than the individual CNT.
3.2. NQR computations The values of CQ for the atoms of optimized structures are listed in Table 2 for both individual and hybrid models of study (Fig. 1). For the individual CNT, the properties for representative atoms are reported showing slight changes for their values of CQ. Comparing the magnitudes among the models shows that the most significant changes of properties for atoms of the CNTs belongs to C7 and C8 atoms, which are close to the chemical bonding region with U–X. For other atoms of CNTs among the individual and hybrid models, only slight changes are seen. It means that the atomic scale properties of CNTs are almost kept unchanged during the hybridization with the U–X counterparts. The halogen X atom plays dominant role in the pharmaceutical application of the U–X structures [13]; the results indicate that the properties for X atoms in the individual and hybrid structures are remained almost unchanged. For the oxygen atoms, the changes of properties for O2 are more significant than the changes for O4 because of placing closer to the attached CNT in the hybrid structures. Comparing the properties for nitrogen atoms in the individual and hybrid models also indicates that the properties for N1, which connects to the CNT, detect more changes than N3, which is farther than the attachment region. Among the individual and hybrid models, the values of CQ for each atom of oxygen or nitrogen are almost similar in the sets of structures. The trend means that similarities of properties for structures in the individual form are remained in the hybrid structures. The carbon atoms of U–X structures detect the effects of CNT attachment by comparing the values of CQ in the individual and hybrid models. The EFG tensors are very sensitive to the electronic properties at the atomic sites and they could properly detect any changes employed at the sites [20–22]. Therefore, the values of CQ parameters could reveal the magnitudes of effects on the atomic scale properties for each individual
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CNT and U–X models during the hybridizations. For the hydrogen atoms, the magnitudes of changes for the values of CQ are almost negligible due to low electronic densities at the atomic sites. 4. Conclusions By doing DFT calculations to investigate the formations of CNT–U–X hybrids, some trend could be concluded. First; the formation of CNT–U–F hybrid could be proposed more than other hybrids based on the binding energies whereas that of CNT–U–I could be less proposed. Second; better conductivity properties could be proposed for the CNT–U–X hybrids than the individual counterparts based on the energy gaps. Third, higher polarities could be expected for the CNT–U–X hybrids especially in comparison with the individual CNT based on the dipole moments. Fourth, the atomic scale properties of X atoms do not detect notable changes during the hybridizations in comparison with the individual U–X structures based on the values of CQ. Fifth, the most changes of properties for the oxygen and nitrogen atoms could be seen for O2 and N1 which are close to the attachment region in the hybrids based on comparing the values of CQ in the hybrids and individual counterparts. Sixth, the changes of properties for C7 and C8 atoms of CNT during the hybridization processes are significant whereas those of other carbon atoms are almost negligible. Finally, changes and similarities could be seen for the properties for hybrids in comparison with the individual counterparts based on the evaluated molecular and atomic properties. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24]
S. Iijima, Nature 354 (1991) 56. S. Peng, K. Cho, P. Qi, H. Dai, Chem. Phys. Lett. 387 (2004) 271. R. Chegel, S. Behzad, E. Ahmadi, Solid State Sci. 14 (2012) 456. J. Beheshtian, M.T. Baei, Z. Bagheri, A.A. Peyghan, Appl. Surf. Sci. 264 (2013) 699. M.G. Ahangari, A. Fereidoon, M. Jahanshahi, M.D. Ganji, Physica E 48 (2013) 148. S. Wijewardane, Solar Energy 83 (2009) 1379. M. Foldvari, M. Bagonluri, Nanomed. Nanotechnol. Biol. Med. 4 (2008) 173. S.K. Vashist, D. Zheng, G. Pastorin, K. Al-Rubeaan, J.H.T. Luong, F.S. Sheu, Carbon 49 (2011) 4077. S. Hou, Z. Shen, J. Zhang, X. Zhao, Z. Xue, Chem. Phys. Lett. 393 (2004) 179. M. Mirzaei, H.R. Kalhor, N.L. Hadipour, IET Nanobiotechnol. 5 (2011) 32. M. Mirzaei, M. Yousefi, Superlattices Microstruct. 55 (2013) 1. M. Mirzaei, M. Yousefi, M. Mirzaei, Mod Phys. Lett. B 25 (2011) 1335. U.P. Singh, R. Ghose, A.K. Ghose, A. Sodhi, S.M. Singh, R.K. Singh, J. Inorg. Biochem. 37 (1989) 325. A. Polk, M. Vaage-Nilsen, K. Vistisen, D.L. Nielsen, Cancer Treat. Rev. (in press). doi:10.1016/j.ctrv.2013.03.005. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese- man, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W.Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Gaussian, Inc., Pittsburgh, PA, 1998 . Z. Bagheri, M. Mirzaei, N.L. Hadipour, M.R. Abolhassani, J. Comput. Theor. Nanosci. 5 (2008) 614. A. Bodaghi, M. Mirzaei, A. Seif, M. Giahi, Physica E 41 (2008) 209. T.P. Das, E.L. Han, Nuclear Quadrupole Resonance Spectroscopy, Academic Press, New York, 1958. P. Pyykkö, Mol. Phys. 99 (2001) 1617. G. Wu, S. Dong, R. Ida, N. Reen, J. Am. Chem. Soc. 124 (2002) 1768. M. Mirzaei, N.L. Hadipour, Physica E 40 (2008) 800. M. Mirzaei, N.L. Hadipour, J. Comput. Chem. 29 (2008) 832. E. Zurek, J. Autschbach, J. Am. Chem. Soc. 126 (2004) 13079. M. Mirzaei, H.R. Kalhor, N.L. Hadipour, J. Mol. Model. 17 (2011) 695.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Formations of CNT modified 5-(halogen)uracil hybrids: DFT studies Mahmoud Mirzaei a,⇑, Rahman Salamat Ahangari b a Laboratory of Nano Computations (LNC), Department of Medical Nanotechnology, Pharmaceutical Sciences Branch, Islamic Azad University, Tehran, Iran b Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 2 September 2013 Received in revised form 12 November 2013 Accepted 13 November 2013 Available online 20 November 2013 Keywords: Carbon nanotube Uracil Density functional theory Quadrupole coupling constant
a b s t r a c t Density functional theory (DFT) calculations have been performed to investigate the formations of carbon nanotubes (CNTs) modified 5-(halogen)uracil hybrids (CNT–U–X). The structures of individual counterparts and hybrids have been optimized and the molecular properties have been evaluated. Moreover, the atomic scale properties have been evaluated by computing quadrupole coupling constants (CQ) for the atoms of the optimized structures. The molecular properties indicated that the binding energies for the CNT–U–F hybrid could be more proper than other CNT–U–X hybrids. Lower values of energy gaps and higher values of dipole moments could be seen for the CNT–U–X hybrids in contrast with the individual counterparts. The atomic scale CQ properties indicated that the properties of X atoms could be seen unchanged during the hybridization processes whereas those of oxygen and nitrogen atoms detect the effects especially close to the chemical attachment regions. Finally, changes and similarities could be seen for the properties of CNT–U–X hybrids in comparison with the individual counterparts. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Since the days of carbon nanotubes (CNTs) discovery by Iijima, several efforts have been dedicated to investigate the properties and applications of the novel material [1–5]. Loading functional groups or other molecules on the CNTs could yield new hybrid compounds helpful for so many applications in ⇑ Corresponding author. Fax: +98 21 22602059. E-mail address: [email protected] (M. Mirzaei). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.11.013
376
M. Mirzaei, R. Salamat Ahangari / Superlattices and Microstructures 65 (2014) 375–379
various fields [6]. In the targeted drug delivery systems, the CNTs and other nanostructures are expected to show their unique role of carriers in the living systems to reduce the side effects [7,8]. The edges of CNTs are seen as proper sites for contributing to covalent attachments to other atoms of molecules [9–12], in which a molecule of 5-(halogen)uracil has been attached to the mentioned site in this work (Fig. 1). Earlier works indicated that the 5-halogen derivatives of uracil, the characteristic nucleobase of RNA, could play significant roles in the pharmaceutical applications [13]. However, several side effects are arisen for the users under treatments by 5-(halogen)uracil drug compounds [14]. Therefore, efforts on reducing the side effects and also increasing the targeting characters are important in this manner. Based on the benefits of computations in the molecular levels, the formations of molecular systems of CNT modified 5-(halogen)uracil hybrids (CNT–U–X) have been investigated in this work. Density functional theory (DFT) calculations have been performed to optimize the investigated structures and to evaluate the desired properties to achieve the purpose of study (Tables 1 and 2 and Fig. 1). The molecular and atomic levels properties have been evaluated for the optimized structures to better characterize the formations and effects of hybridizations on the individual counterparts. 2. Computational details The B3LYP exchange–correlation functional and the 6-31G(d) standard basis set implemented in the Gaussian 98 program have been used for the computations of this work [15]. The molecular systems include the individual CNT, the individual uracil derivatives (U–X), and the CNT–U–X hybrids, in which X could be one hydrogen atom or one halogen atom: fluorine, chlorine, bromine, or iodine (Fig. 1). The representative CNT of this work is a hydrogen capped (6,0) model with the stoichiometry of C48H12. The hydrogen atoms are used to saturate the edges of nanotubes to avoid the dangling ef-
Fig. 1. Individual CNT and U–X models. The connecting atomic sites are C7 and N1 atoms in the CNT–U–X hybrids.
Table 1 Optimized properties for the individual and hybrid models.a Property
CNT–H
Stoichiometry
C48H12
CNT–U–H (U–H) CNT–U–F (U–F) CNT–U–Cl (U–Cl) CNT–U–Br (U–Br) CNT–U–I (U–I)
C52H14N2O2 (C4H4N2O2) Total energy/keV 49.682 60.875 ( 11.226) Binding energy/eV 578.356 653.235 (80.470) Energy gap/eV 0.426 0.415 (5.726) Dipole moment/Debye 0.001 3.546 (4.128)
a
See Fig. 1 for the models.
C52H13FN2O2 (C4H3FN2O2) 63.561 ( 13.911) 653.284 (80.504) 0.413 (5.429) 4.383 (3.845)
C52H13ClN2O2 (C4H3ClN2O2) 73.325 ( 23.675) 651.876 (79.121) 0.413 (5.335) 5.114 (3.934)
C52H13BrN2O2 (C4H3BrN2O2) 130.577 ( 80.927) 651.741 (78.983) 0.413 (5.208) 4.491 (3.818)
C52H13IN2O2 (C4H3IN2O2) 248.367 ( 198.717) 651.154 (78.398) 0.413 (5.053) 4.541 (3.720)
377
M. Mirzaei, R. Salamat Ahangari / Superlattices and Microstructures 65 (2014) 375–379 Table 2 NQR properties for the individual and hybrid models (CQ /MHz).a
a
Atom
CNT–H
CNT–U–H (U–H)
CNT–U–F (U–F)
CNT–U–Cl (U–Cl)
CNT–U–Br (U–Br)
CNT–U–I (U–I)
X
–
O2
–
O4
–
N1
–
N3
–
C2
–
C4
–
C5
–
C6
–
C7 C8 C9 C10 C11 C12 C13 C14 H1
1.036 1.316 1.022 1.105 1.105 1.022 1.315 1.037 –
H3
–
H6
–
H7
0.209
0.216 (0.218) 8.379 (8.255) 9.503 (9.433) 3.147 (3.958) 3.720 (3.812) 1.050 (1.115) 1.942 (1.961) 0.727 (0.482) 2.340 (2.147) 1.798 0.869 1.047 0.844 1.186 0.962 1.310 1.145 – (0.278) 0.271 (0.273) 0.207 (0.211) –
65.710 (65.877) 8.342 (8.200) 9.474 (9.448) 3.278 (4.148) 3.765 (3.846) 0.987 (1.081) 1.653 (1.025) 2.725 (2.828) 1.822 (1.651) 1.898 0.863 1.057 0.834 1.193 0.953 1.306 1.182 – (0.278) 0.270 (0.272) 0.208 (0.212) –
72.109 (72.036) 8.370 (8.262) 9.342 (9.346) 3.175 (4.002) 3.776 (3.860) 1.019 (1.097) 1.687 (1.699) 2.180 (2.286) 2.013 (1.865) 1.940 0.874 1.062 0.829 1.200 0.948 1.302 1.242 – (0.277) 0.270 (0.272) 0.206 (0.209) –
564.810 (563.641) 8.372 (8.258) 9.350 (9.341) 3.161 (3.993) 3.776 (3.860) 1.020 (1.097) 1.731 (1.741) 1.837 (1.960) 2.057 (1.904) 1.906 0.872 1.060 0.832 1.198 0.951 1.304 1.215 – (0.277) 0.270 (0.272) 0.206 (0.210) –
1708.796 (1700.996) 8.385 (8.276) 9.215 (9.210) 3.132 (3.948) 3.782 (3.868) 1.024 (1.101) 1.774 (1.784) 1.533 (1.661) 1.278 (1.997) 1.915 0.877 1.061 0.833 1.198 0.952 1.303 1.232 – (0.277) 0.270 (0.272) 0.206 (0.210) –
H14
0.209
0.209
0.209
0.209
0.209
0.209
See Fig. 1 for the models and the atoms numbers. X is replaced by H5, F, Cl, Br, or I.
fects [16,17]. The uracil derivatives have the stoichiometry of C4H3XN2O2; X implies for the hydrogen or halogen atoms. In the hybrids, one hydrogen atom from the tubular edge and the hydrogen atom of N1 of uracil derivatives have been removed to make possible the chemical attachment of uracil to the CNT. The models of study have been allowed to relax during the optimization processes to yield the stabilized structures. Subsequently, the properties have been evaluated; total energies, binding energies, energy gaps, and dipole moments (Table 1). To better achieve the purpose of study, quadrupole coupling constants (CQ) have been also calculated for the atoms of the optimized structures (Table 2). The value of CQ could be experimentally measured by nuclear quadupole resonance (NQR) spectroscopy technique [18]. The CQ is defined as the interaction energy between the nuclear quadrupole moment (Q) and the electric field gradient (EFG) tensors, in which it could be generated by computations [18]. We calculated the EFG tensors (qii; |qzz|>|qyy|>|qxx|) for the atoms and then we used the standard values of Q to evaluate the values of CQ in the equation of CQ (MHz) = e2Qqzzh 1 [19]. The EFG tensors are originated from the electronic sites of atoms; therefore, they could well describe the electronic properties of matters at the atomic levels [20–22]. Due to electronic complexity of nanostructures for doing MQR experiments, evaluation of the values of CQ is a benefit of computational studies [23].
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3. Results and discussion 3.1. Optimization processes Doing the processes of optimizations yielded the stabilized structures of individual and hybrid models (Fig. 1) of this work. The individual models are the CNT and U–X structures, in which X could be replaced by one atom of H, F, Cl, Br, or I. The hybrid structures are constructed by chemical attachments of U–X to the CNT by removing one H atom from the edge of CNT and the H atom of N1 of U–X. In the uracil nucleobase, N1 is important due to its contribution to glycoside bonding in the uridine nucleotide; therefore, we used this atomic site for chemical attachment to the CNT [24]. The optimized properties of Table 1 indicate that the most proper binding energy belongs to the individual U–F model and the least proper one is for the U–I structure. The values of binding energies refer to the required energy to break a bond, in which they could be compared in the parallel structures to detect the better possibilities of formations. It is noted that the binding energies are evaluated by differences of energies between the structure and the atomic components. For the hybrid structures, the ordinal positions are remained unchanged in comparison with the individual U–X structures; the CNT–U–F and CNT–U–I are highlighted in the hybrids. The values of energy gaps, which are evaluated by differences of energies between the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO), indicate that the magnitudes for hybrids are lower than the individual structures. Since the distances between HOMO and LUMO are related to the conductivity properties, lower distances could be proposed for better conductions what are seen for the hybrids. Moreover, although the magnitudes of energy gaps are different for the individual U–X structures, they are interestingly similar for the CNT–U–X structures. The values of dipole moments indicate that the structures show different polarities; the smallest value belongs to CNT–U–H and the largest one belongs to CNT–U–Cl among the hybrids. For the individual ones, the CNT is a non-polar structure but the U–X structures show polarities by their dipole moments. After hybridizations, the U–X counterpart significantly changes the polarity for the CNT counterpart in the new compound. Polarity is important especially for dispersing a matter in the aqua systems, in which the CNT modified U–X hybrids could be seen more proper for dispersion than the individual CNT.
3.2. NQR computations The values of CQ for the atoms of optimized structures are listed in Table 2 for both individual and hybrid models of study (Fig. 1). For the individual CNT, the properties for representative atoms are reported showing slight changes for their values of CQ. Comparing the magnitudes among the models shows that the most significant changes of properties for atoms of the CNTs belongs to C7 and C8 atoms, which are close to the chemical bonding region with U–X. For other atoms of CNTs among the individual and hybrid models, only slight changes are seen. It means that the atomic scale properties of CNTs are almost kept unchanged during the hybridization with the U–X counterparts. The halogen X atom plays dominant role in the pharmaceutical application of the U–X structures [13]; the results indicate that the properties for X atoms in the individual and hybrid structures are remained almost unchanged. For the oxygen atoms, the changes of properties for O2 are more significant than the changes for O4 because of placing closer to the attached CNT in the hybrid structures. Comparing the properties for nitrogen atoms in the individual and hybrid models also indicates that the properties for N1, which connects to the CNT, detect more changes than N3, which is farther than the attachment region. Among the individual and hybrid models, the values of CQ for each atom of oxygen or nitrogen are almost similar in the sets of structures. The trend means that similarities of properties for structures in the individual form are remained in the hybrid structures. The carbon atoms of U–X structures detect the effects of CNT attachment by comparing the values of CQ in the individual and hybrid models. The EFG tensors are very sensitive to the electronic properties at the atomic sites and they could properly detect any changes employed at the sites [20–22]. Therefore, the values of CQ parameters could reveal the magnitudes of effects on the atomic scale properties for each individual
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CNT and U–X models during the hybridizations. For the hydrogen atoms, the magnitudes of changes for the values of CQ are almost negligible due to low electronic densities at the atomic sites. 4. Conclusions By doing DFT calculations to investigate the formations of CNT–U–X hybrids, some trend could be concluded. First; the formation of CNT–U–F hybrid could be proposed more than other hybrids based on the binding energies whereas that of CNT–U–I could be less proposed. Second; better conductivity properties could be proposed for the CNT–U–X hybrids than the individual counterparts based on the energy gaps. Third, higher polarities could be expected for the CNT–U–X hybrids especially in comparison with the individual CNT based on the dipole moments. Fourth, the atomic scale properties of X atoms do not detect notable changes during the hybridizations in comparison with the individual U–X structures based on the values of CQ. Fifth, the most changes of properties for the oxygen and nitrogen atoms could be seen for O2 and N1 which are close to the attachment region in the hybrids based on comparing the values of CQ in the hybrids and individual counterparts. Sixth, the changes of properties for C7 and C8 atoms of CNT during the hybridization processes are significant whereas those of other carbon atoms are almost negligible. Finally, changes and similarities could be seen for the properties for hybrids in comparison with the individual counterparts based on the evaluated molecular and atomic properties. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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