Article pubs.acs.org/JPCC
Comparative Theoretical Study on the Positional Preference for Functionalization of Two OH and SH Groups with (5,5) Armchair SWCNT Tandabany C. Dinadayalane* and Jerzy Leszczynski* Interdisciplinary Center for Nanotoxicity, Department of Chemistry and Biochemistry, 1400 J. R. Lynch Street, P.O. Box 17910, Jackson State University, Jackson, Mississippi 39217, United States ABSTRACT: Covalent sidewall functionalization of (5,5) armchair single-walled carbon nanotube by OH and SH groups has been investigated using density functional theory. Two possibilities for each of 1,2- (adjacent), 1,3- (alternate), and 1,4-additions were considered. Strong intramolecular O− H···O hydrogen bond plays an important role in determining the positions for attachment of two OH groups with SWCNTs and reaction feasibility. The reactions of OH addition(s) are considerably more exothermic than the reactions of SH addition(s). The addition of two OH or SH functional groups prefers to attach at adjacent (1−2 and 1−1′) and 1,4-positions (1−4 and 3−6) rather than at alternate carbon sites (1−3 and 2−6). Owing to the strong intramolecular hydrogen bond, 1,2addition (1-2-OH) is slightly more favored (by 2.4 kcal/mol) than 1,4-addition (1-4-OH), while 1,4-addition of two SH groups (1-4-SH) is more feasible (by 10 kcal/mol) than 1,2-addition (1-2-SH). Interestingly, the addition reactions of two SH groups at alternate carbon sites (1-3-SH and 2-6-SH) are even less exothermic than the single SH addition with SWCNT (1-SH). We have observed changes in the bond lengths in the vicinity of the addition sites, and those changes depend on the functional groups and the positions of attachments. Covalent functionalization results in considerable deformation of SWCNT due to interruption of aromatic π conjugation and transformation of sp2 to sp3 hybridization of carbon atoms by attachment of functional groups. We have analyzed the variation of HOMO and LUMO energies and HOMO−LUMO energy gaps of SWCNTs by functionalization. The results of OH- and SH-functionalized tubes have been compared.
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desorption data.15 In our group, we have investigated, using density functional theory calculations, the covalent sidewall functionalization (by attaching H, F, Cl, and NH2 at different carbon sites) of defect-free and Stone-Wales defective SWCNTs.13,16−23 Very recently, computational studies have been reported on infrared (IR) vibrational spectra analyses for COOH tip-functionalized armchair and zigzag nanotubes.24,25 It is important to highlight that fluorinated nanotubes are used as precursors in many chemical reactions for further modification because fluorine could be easily substituted by other organic functional groups.26−28 Hamilton et al. have experimentally investigated the complexation of iron−molybdenum cluster with the pyridine-, thiol-, and phosphinefunctionalized SWCNTs and also with ultrashort SWCNTs. They revealed that the thiol-functionalized SWCNTs formed complexes with iron significantly compared with either the phosphine or pyridine derivatives.29 Thiol functional groups attached to carbon atoms of SWCNTs are known to stabilize gold clusters/gold nanoparticles. Hence, we are interested in
INTRODUCTION Functionalization of single-walled carbon nanotubes (SWCNTs) via covalent and noncovalent approaches is used to prepare smart nanomaterials.1−5 Chemical modification of SWCNTs is given importance due to the fundamental and technological aspects.6−9 Functionalized SWCNTs have applications in a wide range of areas including catalysis, biology, medicine, molecular electronics, molecular engineering, and sensors.1−4,9,10 In general, carbon nanotubes are insoluble in water or organic solvents. The appropriate functionalization is essential to make them soluble and to assist further investigations for practical applications.11,12 Functionalization of SWCNTs through noncovalent interactions occurs via π−π stacking, van der Waals forces, and hydrophobic interactions.10,13 The covalent sidewall functionalization by hydrophilic substituents, such as hydroxyl (−OH) and thiol (−SH) functional groups, is given importance in biomedical applications.10,14 Carboxylic acid functionalization with the sidewalls of SWCNTs was studied experimentally.14 Chakrapani et al. reported the combined theoretical and experimental study of the chemisorption of acetone on SWCNTs. They concluded that computed high chemisorption energies are in good agreement with the experimental thermal © 2013 American Chemical Society
Received: May 8, 2013 Revised: June 15, 2013 Published: June 19, 2013 14441
dx.doi.org/10.1021/jp404592u | J. Phys. Chem. C 2013, 117, 14441−14450
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Scheme 1. Single OH- and SH-Functionalized (5,5) Armchair SWCNT with Atom Numberinga
a
Two functional groups are attached at carbon atom sites of 1-2 (adjacent), 1-1′ (adjacent), 1-3 (alternate), 2-6 (alternate), 1-4 (1,4-position), and 3-6 (1,4-position).
shows the positions of carbon sites where we attached functional groups. Our main aim is to identify the site selectivity for the addition of two OH or SH functional groups in particular within a single six-membered ring. We also compare the results of addition reactions of OH and SH radicals to reveal any difference in the site selectivity. Both experimental and theoretical studies concluded that covalent functionalization has a strong effect on the electronic properties of the nanotubes.10,36−38 It was shown experimentally that OH functionalization of SWCNTs induces shrinking of the energy gap between the highest occupied and the lowest unoccupied molecular orbitals.39 Hence, we have also analyzed the change in HOMO−LUMO energy gap of (5,5) armchair SWCNT by functionalization using OH and SH radicals.
thiol functionalization, and the investigations of interactions of small gold clusters with thiol functionalized SWCNTs are underway in our group. Zhang et al. have studied thiol- and thiophene-functionalized SWCNTs and their interactions with gold surface to understand Au−S chemistry. The size and spatial distribution of the sulfur-functionalized side chains along the sidewalls of SWCNTs were determined.30 Chen et al. have demonstrated that a sufficient number of −SH groups in thiolated CNTs serve as anchor centers for achieving high platinum (Pt) dispersion and the Pt/SH−CNTs are good electrocatalysts due to the strong S−Pt interaction.31 Recently, density functional theory study has been reported on the prediction of S−H bond rupture in methanethiol upon interaction with gold atom and small gold clusters.32 In a recent study, hydroxyl (−OH) functional groups were attached to the carbon atoms of nanotube. The OH functionalization on external surfaces makes the nanotube water-soluble.11 Acrylic paints with the addition of OHfunctionalized SWCNTs and aniline-functionalized SWCNTs show improved quality for their applications; in particular, their resistance against degradation by electron beam increased significantly. The incorporation of OH-functionalized SWCNTs in the automotive paint could be chosen as the best additive to improve its physical properties.33 The magnetism of OH-functionalized SWCNT has been examined using density functional theory calculations.34 Recently, Martinez et al. have studied the reactions of OH radical with eight different functionalized SWCNTs of armchair and zigzag types. They have found that the addition is to be sitedependent and the OH radical addition with functionalized SWCNTs is more feasible compared with pristine tube.5 Atomic force microscopy (AFM) images showed the presence of multiple functional groups of −SH on the surface of SWCNTs. Furthermore, the functional groups on the thiolSWCNTs are assembled together.26−30 Another experimental study concluded that the thiol functional groups are likely to combine together and bind to one Au nanoparticle.30 In the present study, we have investigated the covalent functionalization of (5,5) armchair SWCNT by the addition of one and two OH and SH radicals using density functional theory calculations. The addition reactions were carried out only at the outer surface of the middle portion of the tube. The reasons for considering the outer surface and middle portion of SWCNT for functionalization are: (a) the reactions at outer surface are generally more preferred than inner surface of SWCNTs35 and moreover the covalent functionalization (by various functional groups) at outer surface of SWCNTs has been reported experimentally11,14,15,26−28,30,33 and (b) only the middle portion is considered to avoid the edge effect. Scheme 1
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COMPUTATIONAL DETAILS All of the calculations were carried out using the Gaussian 09 suite of programs.40 The (5,5) armchair SWCNT consisting of 150 carbon atoms is considered here. The edges of carbon nanotube were terminated by hydrogen atoms. OH- and SHfunctional groups (only low occupancy, i.e., 1 and 2) were covalently attached on the external surface of the (5,5) SWCNT, and the positions of attachment of these functional groups are shown in Scheme 1. The geometries of the pristine and functionalized SWCNTs were fully optimized using the most popular B3LYP functional and Truhlar’s Minnesota functional, M06-2X.41 The latter functional is widely used and recently popular for its reliability as well as applicability for diversified problems in chemistry. The 6-31G(d) basis set was utilized for calculations with the above-mentioned density functional methods. Reaction energies for the chemisorption (ΔEr) of OH and SH radicals on the external surface of the SWCNTs were calculated using the following equation: ΔEr = ESWCNT + nOH − ESWCNT − nEOH
(1)
ΔEr = ESWCNT + nSH − ESWCNT − nESH
(2)
where n is either one or two; ESWCNT+nOH denotes the total energy of tube with OH functional group(s) attached; ESWCNT+nSH denotes the total energy of tube with SH functional group(s) attached; and ESWCNT, EOH, and ESH correspond to the energies of the SWCNT, OH radical, and SH radical. The chemisorption process is exothermic if the reaction energy ΔEr is negative. The eqs 1 and 2 correspond to the reaction energies for OH and SH functionalization of SWCNT. The deformation energy is calculated as the energy of the functionalized SWCNT but without any functional groups (the single-point energy calculation is done by replacing the functional groups with dummy atoms) minus the energy of 14442
dx.doi.org/10.1021/jp404592u | J. Phys. Chem. C 2013, 117, 14441−14450
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Figure 1. C−O, O−H, C−S, S−H, and hydrogen bond distances (in angstroms) obtained at the B3LYP/6-31G(d) and M06-2X/6-31G(d) levels for the functionalized (5,5) armchair SWCNTs. The values given in parentheses correspond to the M06-2X/6-31G(d) level.
the optimized pristine SWCNT. The values of deformation energy are used to gauge the deformation of the SWCNTs by attachment of different functional groups at different sites. Because the functionalization of SWCNTs influences the band gap, we have calculated the HOMO−LUMO energy gap at the
TPSSh/6-31G(d) level using B3LYP/6-31G(d)-optimized geometries for the functionalized SWCNTs and compared the results with that of pristine SWCNT. The TPSSh functional has been reported to be reliable in obtaining the HOMO− LUMO energy gaps for carbon nanotubes.19,42 14443
dx.doi.org/10.1021/jp404592u | J. Phys. Chem. C 2013, 117, 14441−14450
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Figure 2. Selected bond distances (in angstroms) at the B3LYP/6-31G(d) level for the optimized geometries of the functionalized nanotubes. For clarity, only the segments of the nanotubes with functional groups are given here.
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RESULTS AND DISCUSSION In this study, we have considered seven systems for each OH and SH functionalization. The covalent functionalization was considered on the external surface of (5,5) armchair SWCNT. There is only one possibility for the addition of single OH or SH radical at the carbon atom in the middle portion of SWCNT. Six possibilities for the addition of two radicals of OH and similarly for SH are examined (Scheme 1). Two functional groups are attached at carbon atom sites of 1−2 (adjacent), 1− 1′ (adjacent), 1−3 (alternate), 2−6 (alternate), 1−4 (1,4position), and 3−6 (1,4-position). It is fascinating to explore two possibilities for each addition of two functional groups at adjacent (ortho), alternate (meta), and 1,4-positions (para). In the nomenclature, the numbers indicate the carbon atom positions where the functional groups were attached. For example, 1-2-OH means that two OH functional groups were attached at C1 and C2 atoms. Readers should refer to Scheme 1 for atom numbering. We have also compared the results obtained for OH- and SH-functionalized SWCNTs. Structural Characteristics. The bond lengths relevant to functional groups and hydrogen bond distances for the functionalized SWCNTs are provided in Figure 1. In general, the bond lengths at the M06-2X/6-31G(d) level are smaller than that obtained at the B3LYP/6-31G(d) level. In the case of 1-2-OH, the bond length of C−O, in which the oxygen atom is the proton donor of hydrogen bond, is shorter than that in 1OH, whereas the other C−O bond is slightly elongated
compared with single OH-functionalized SWCNT. The same scenario is followed by all other double OH-functionalized SWCNTs except 1-3-OH. However, in case of 1-4-OH, the other C−O bond (the oxygen is a proton acceptor of hydrogen bond) is marginally shorter than the C−O bond length in 1OH. The distances of O−H···O and S−H···O hydrogen bonds provided in Figure 1 have been analyzed. It is known that the bond lengths give some indication of the bond strength. The shorter hydrogen bonds are stronger and vice versa. One can recognize both strong and weak hydrogen bonds depending on the position of attachments of functional groups. Owing to the advantage of having two OH functional groups at adjacent carbon atoms, 1-2-OH and 1-1′-OH have stronger hydrogen bonds than the rest of the OH-functionalized SWCNTs. The length of O−H bond of O−H···O is slightly elongated (except for 1-3-OH and 1-4-OH) compared with that in 1-OH due to the formation of strong intramolecular hydrogen bonds. The length of other O−H bond, which is not involved in hydrogen bond, is either comparable or the same as that of 1-OH. In case of 1−4-OH, the hydrogen bond distance is ∼4 Å, indicating a very weak hydrogen bond. For 1-1′-OH, the O−H···O hydrogen bond distance is 1.84 and 2.21 Å at the B3LYP/631G(d) and M06-2X/6-31G(d) level, respectively. The structure at the latter level is different, and it converged with C2 symmetry consisting of two O−H···O hydrogen bonds. 14444
dx.doi.org/10.1021/jp404592u | J. Phys. Chem. C 2013, 117, 14441−14450
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Figure 3. Selected bond distances (in angstroms) at the M06-2X/6-31G(d) level for the optimized geometries of the functionalized nanotubes. For clarity, only the segments of the nanotubes with functional groups are given here.
H···O hydrogen bond distance than 3-6-OH, despite the fact that two OH functional groups are attached at 1,4-positions (para) in these tubes. The same scenario persists with the analogous SH-functionalized SWCNTs except S−H···S hydrogen bond length of ∼2.4 Å in the structure 3-6-SH. Covalent bonding of OH- or SH-functional group(s) to carbon atom(s) on the outer surface of SWCNT leads to change in hybridization of the functionalized carbon atom(s) from sp2 to sp3. This is evidenced by the increase in C−C bond lengths, which are >1.5 Å, after attaching functional groups (Figures 2 and 3). Functionalization generating two tetrahedral sp3 hybridized carbon atoms in adjacent positions triggers the bond cleavage. Furthermore, it also slightly weakens the neighboring C−C bonds. Recent experimental study has reported that the change in hybridization of some carbon atoms of the carbon nanotube surface can be detected by scanning tunneling microscopy (STM).43 It is known that the covalent functionalization disrupts the conjugation pattern of the SWCNT. Although both OH and SH functionalization lead to elongation of C−C bonds associated with the carbon atom addition sites of SWCNT, OH functionalization results in slightly more elongation than SH functionalization. As shown in Figures 2 and 3, the same trend is followed at both DFT levels. The geometrical data reveal that the changes in the bond lengths in the vicinity of the addition sites vary depending on
Hence, we observe considerable difference in hydrogen bond lengths between the two levels. As shown in Figure 1, in the case of two SH functional groups attached to SWCNTs, the majority of C−S bond lengths are shorter than that of 1-SH. This is in contrast with OH-functionalized SWCNTs. S−H forms a hydrogen bond with another S atom in two SH functional groups attached SWCNTs. Unlike OH-functionalized SWCNTs, the length of S−H bond, which is forming the S−H···S hydrogen bond, does not always increase compared with that of 1-SH. The structure of 1-2-SH is different than other structures because it has a dual S−H···S hydrogen bond. The restrictive geometrical arrangement makes it possible for two weak S−H···S hydrogen bonds. The hydrogen bond distances in 1-2-SH and 1-1′-SH are longer than those in other structures (exception is 1-4-SH). This is in quite contrast with the situation in OH-functionalized SWCNTs. The steric repulsion between the bulky SH groups accommodating at adjacent carbon atom sites could be ascribed for such a behavior in SH-functionalized tubes. Although the OH functional groups are attached at alternate carbon atoms (meta positions) in both 1-3-OH and 2-6-OH, the O−H···O hydrogen bond distance in the former structure is 0.85 Å longer than that of the latter structure. A similar situation is observed for the corresponding SH-functionalized tubes, but the difference in hydrogen bond distance is only 0.4 Å. Again, 14-OH has considerably longer (it has the distance of ∼4 Å) O− 14445
dx.doi.org/10.1021/jp404592u | J. Phys. Chem. C 2013, 117, 14441−14450
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Table 1. Reaction Energies (ΔEr, kcal/mol) Obtained at the B3LYP/6-31G(d) and M06-2X/6-31G(d) Levels for the Addition of OH and SH Radicals at the Specific Sites of the (5,5) Armchair SWCNTa,b OH functionalization structure
ΔEr (kcal/mol)
1-OH 1-2-OH 1-1′-OH 1-3-OH 2-6-OH 1-4-OH 3-6-OH
−37.8 −88.2 −83.9 −59.8 −67.8 −85.8 −78.7
(−42.8) (−103.3) (−98.6) (−66.5) (−73.9) (−102.2) (−92.7)
SH functionalization ΔEdef (kcal/mol)c 27.3 56.9 50.7 48.5 55.8 50.9 51.2
(32.9) (60.4) (54.1) (55.9) (64.6) (54.6) (56.0)
structure
ΔEr (kcal/mol)
1-SH 1-2-SH 1-1′-SH 1-3-SH 2-6-SH 1-4-SH 3-6-SH
−11.2 (−17.7) −23.0 (−43.4) −21.7 (−40.1) −5.8 (−15.1) −1.0 (−11.0) −33.0 (−52.9) −20.7 (−38.2)
ΔEdef (kcal/mol)c 24.0 58.9 49.1 39.8 53.0 45.0 45.5
(30.3) (62.7) (54.0) (50.0) (64.6) (49.5) (50.7)
a Deformation energies of SWCNTs (ΔEdef, kcal/mol) are also given. The values given in parentheses correspond to M06-2X/6-31G(d) level. bSee the structures and nomenclature in Figure 1. cLook at the Computational Details section for calculation of deformation energy.
the functional groups and the positions of attachments of them at the carbon sites. The bond lengths of
