Using the 19F NMR Chemical Shift Anisotropy Tensor To Differentiate

ARTICLE pubs.acs.org/JPCA

Using the 19F NMR Chemical Shift Anisotropy Tensor To Differentiate between the Zigzag and Chiral Forms of Fluorinated Single-Walled Carbon Nanotubes Amrita Kumari and Kavita Dorai* Department of Physics, Indian Institute of Science Education & Research (IISER), Mohali, Chandigarh 160019, India

bS Supporting Information ABSTRACT: The structural characterization of different kinds of zigzag and chiral single-walled carbon nanotubes (SWNTs) has been investigated theoretically using 19 F NMR spectroscopy. The chemical shift anisotropy (CSA) tensor is computed at different levels of theory for the 19F nuclei in different forms of functionalized fluorinated carbon nanotubes (CNT). A set of fluorine CSA parameters comprising the span, skew, and isotropic chemical shift is computed for each form of the fluoronanotubes and multidimensional CSA parameter correlation maps are constructed. We show that these correlations are able to clearly distinguish between the chiral and zigzag forms of fluorinated carbon nanotubes (F-SWNTs). Implications for solid-state and liquid-state NMR experiments are discussed.

’ INTRODUCTION Single-walled carbon nanotubes (SWNTs) have tremendous applications in nanoscience and in biotechnology, and their physicochemical characteristics have hence been extensively studied.1 A SWNT is visualized as a rolled graphene sheet and can be characterized as belonging to the armchair (n,n), zigzag (n,0), or chiral (n,m) class depending on the vector with integer indices (n,m) that connects the two points that meet upon rolling. The electronic properties of SWNTs can be tuned by varying the integers (n,m), leading to metallic nanotubes for n = m or semiconducting nanotubes for n 6¼ m. Among the various physical techniques used to characterize SWNTs, NMR spectroscopy has emerged as a viable method to provide a detailed and accurate depiction of the local electronic structures, through measurement of NMR chemical shifts. Despite the 13C NMR line shapes being considerably broadened due to residual magnetic impurities, there have been a number of experimental NMR studies of SWNTs which found that semiconducting and metallic nanotubes appear in different carbon chemical shift bands.26 Most of these reports focused on solid-state NMR experiments and on studying the electronic properties of SWNTs. Very recently, diffusion ordered NMR spectroscopy has been used to distinguish between isolated and bundled short SWNTs in the solution state.7,8 There has been an explosion of recent work on the theoretical NMR spectroscopy of SWNTs using ab initio and DFT methods.912 It was determined by ab initio and DFT methods that the chemical shifts of the carbons within the carbon tube depend on the tube length, width, and chirality and appear in a r 2011 American Chemical Society

dominant band in the 13C NMR spectrum, with smaller peaks at higher chemical shifts originating from the carbon atoms of the caps.13 Recently, functionalized nanotubes with functional groups attached to the SWNTs have been designed and studied using solution phase NMR.1417 Such studies sought to determine if the functional groups are covalently bonded to the SWNT surface and the spatial proximity of the SWNTs and the functional groups.18,19 Highly purified SWNTs have been fluorinated up to a saturation stoichiometry of C2F to form “fluoronanotubes”, which were then solvated as individual tubes in alcohol solvents.20 Such fluoronanotubes can then be used to prepare covalently functionalized SWNTs via further derivatization.2124 The 13C NMR chemical shifts of fluorinated semiconducting SWNTs were computed using DFT methods, and it was observed that the chemical shifts of the fluorinated carbons were insensitive to the degree of functionalization as well as to the nanotube radius.25 Recent work on fluoronanotubes used NMR to provide a better quantification of the degree of functionalization as compared to Raman spectroscopy.26,27 The effect of curvature on CF bonding has been studied from NMR data on fluorinated carbons including fullerenes, single, double and multiwalled carbon nanotubes.28 The progress made in firstprinciples theoretical studies on the magnetic response of carbon nanotubes using NMR chemical shifts has been recently reviewed.29 Received: April 10, 2011 Revised: May 7, 2011 Published: May 20, 2011 6543

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The Journal of Physical Chemistry A

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CSA tensors are increasingly being exploited as useful structural markers and there have been several computational, liquid-state and solid-state NMR studies that focus on 19F CSA parameters and their applications in structure determination.3133 We performed benchmark computations of the fluorine chemical shift and CSA tensor for different carbon nanotube structures at different levels of theory. Further, we propose the use of multidimensional CSA parameter correlation maps and show that these correlators are able to distinguish between the chiral and zigzag forms of fluorinated carbon nanotubes. We use our multidimensional correlation maps to simulate a solidstate NMR spectrum that clearly differentiates between the chiral and zigzag forms as well as propose a liquidstate NMR relaxation experiment that measures the cross-correlated spin relaxation rate between the CSA of 19F and the 19F13C dipolar interaction.

Figure 1. Visualization of the optimized geometries of the different F-SWNTs considered in this study. A single unit cell containing 64 carbon atoms and a length of 10 Å was used. Please see text for details of 19F substitutions. (a) (8,0) zigzag F-SWNT with 1,2 P-site 19F. (b) (3,5) chiral F-SWNT with 1,2 P-site 19F. (c) (8,0) zigzag F-SWNT with 1,4 zigzag 19F. (d) (3,5) chiral F-SWNT with 1,4 zigzag 19F. (e) (8,0) zigzag F-SWNT with D-site 19F. (f) (3,5) chiral F-SWNT with D-site 19F. (g) (8,0) zigzag F-SWNT with P-site 19F. (h) (3,5) chiral F-SWNT with P-site 19F.

This work focuses on an quantum chemical study of the full CSA tensor of 19F in fluorinated SWNTs. While previous DFT studies of fluorinated SWNTs have computed the 13C NMR chemical shifts, we have chosen to focus in this paper on computing the full CSA tensor of the fluorine nuclei in these systems. To the best of our knowledge, this is the first such study of its kind. Fluorine has a large chemical shift anisotropy and is hence a promising candidate for characterization studies in nanotubes. Fluorine NMR parameters of various crystalline metal fluorides have been recently theoretically calculated.30 Fluorine

’ COMPUTATIONAL METHODS All computations were performed using the Gaussian03 software package.34 Geometry optimization was performed using HartreeFock (HF) methods. Even though HF methods include no description of electron correlation, they were chosen for their low computational cost. After geometry optimization, isotropic 19F chemical shifts as well as the full fluorine chemical shift anisotropy (CSA) tensor were computed using the gauge-including atomic orbitals technique. 3537 Chemical shifts and PAS elements of the CSA tensor are reported relative to the value for CFCl3 computed at the same level of theory. CFCl3 has been used as the reference compound for all computations. Both ab initio HF and DFT methods and a full range of basis sets have been used to determine the fluorine CSA tensor. The computations converged for all basis sets during structural optimization and during calculation of the 19F NMR CSA tensors for all F-SWNTs except for the zigzag 1,4 cis geometry. Four different fluorine substituted forms were considered for this study: 1,2 P-site, 1,4 zigzag, P-site, and D-site substitutions. For 1,2 P-site and 1,4 zigzag nanotubes, two fluorines were added to the carbon at positions parallel and diagonal to the nanotube axis. For the P-site and D-site forms, four fluorines were added to the carbon at positions parallel and diagonal to the nanotube axis.25 The P-site zigzag F-SWNT’s length was increased to 12 Å, for substitution purposes. Optimization of 1,2 P-site form for both zigzag and chiral nanotubes used the HF/6-31 g basis set, while optimization of other forms of zigzag nanotubes was achieved using the HF/sto-3 g basis set. Chiral nanotube optimization was achieved using the HF/3-21 g basis set. Because zigzag nanotubes have a relatively smaller number of atoms per unit cell, they are computationally more tractable systems to handle. Two different forms of SWNTs, namely the zigzag and chiral fluoronanotubes of length 10 Å with 64 carbon atoms in the unit cell were generated using the CONTUBv1.0 software package.38 Fluorine substitutions at different positions in the carbon nanotubes were performed using the Gaussview software package.34 The various (8,0) fluoronanotubes of zigzag form and (3,5) fluoronanotubes of chiral form considered in this study are illustrated in Figure 1. Isolated infinite fluoronanotubes were constructed using a hexagonal unit cell of an appropriate size.12 This work only considers finite band gap zigzag and chiral fluoronanotubes and does not deal with zerobandgap systems such as armchair nanotubes. We followed the convention of Zurek et al.25 in preparing the various forms of 6544

dx.doi.org/10.1021/jp2033388 |J. Phys. Chem. A 2011, 115, 6543–6550


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Table 1. Principal Elements δii (ppm) of 19F CSA Tensor Computed Using HartreeFock Methods with Different Basis Setsa method hf/sto-3 g

hf/sto-3 g*þ hf/sto-3 g**

hf/3-21 g

hf/6-31 g

hf/6-311 g

δ11

CNT form (zigzag)

δ22

δ33

δiso

1,2 P-Site (F1)

46.67

54.79

86.50

62.66

1,2 P-Site (F2)

61.85

76.08

80.70

72.88

1,4 zigzag (F1)

34.79

47.23

78.03

53.35

1,4 zigzag (F2)

34.78

47.26

78.04

53.36

1,2 P-Site (F1)

30.03

38.15

70.82

46.33

1,2 P-Site (F2)

45.21

59.44

65.02

56.55

1,4 zigzag (F1)

17.67

31.07

61.87

36.87

1,4 zigzag (F2) 1,2 P-Site (F1)

17.66 154.30

31.10 165.69

61.88 135.90

36.88 151.96

1,2 P-Site (F2)

147.55

175.2

128.41

150.39

1,4 zigzag (F1)

82.02

141.28

145.18

122.83

1,4 zigzag (F2)

82.04

141.37

145.19

122.87

1,2 P-Site (F1)

170.22

192.17

119.29

160.56

1,2 P-Site (F2)

159.35

206.58

104.89

156.94

1,4 zigzag (F1)

94.46

162.65

126.59

127.90


94.51 185.08

162.73 218.61

126.60 128.16

127.95 177.28

1,2 P-Site (F2)

168.03

226.31

116.66

170.33

1,4 zigzag (F1)

98.61

175.80

138.20

137.54

1,4 zigzag (F2)

98.68

175.89

138.22

137.60

The CNTs considered are of the zigzag (8,0) form having 64 carbon atoms and a length of 10 Å, with fluorines substituted in two different ways: 1,2 P-site and 1,4 zigzag. a

Table 2. Principal Elements δii (ppm) of 19F CSA Tensor Computed Using DFT Methods with Different Basis Setsa method b3lyp/3-21 g

b3lyp/6-31 g

b3lyp/6-311 g

CNT form (zigzag)

δ11

δ22

δ33

δiso

1,2 P-Site (F1)

194.50

206.32

234.20

211.67

1,2 P-Site (F2)

189.18

209.44

216.08

204.90

1,4 zigzag (F1)

111.13

183.97

236.58

177.22

1,4 zigzag (F2)

111.14

184.08

236.59

177.27

1,2 P-Site (F1)

202.17

225.47

202.88

210.17

1,2 P-Site (F2)

199.11

232.25

178.31

200.55

1,4 zigzag (F1)

120.62

191.95

207.75

173.44


120.67 213.97

192.05 250.16

207.77 215.47

173.50 226.53

1,2 P-Site (F2)

196.51

250.10

193.12

213.24

1,4 zigzag (F1)

122.61

198.67

226.28

182.52

1,4 zigzag (F2)

122.68

198.76

226.31

182.58

The CNTs considered are of the zigzag (8,0) form having 64 carbon atoms and a length of 10 Å and with fluorines substituted at two sites: 1,2 P-site and 1,4 zigzag.

a

fluorine substituted carbon nanotubes. For 1,2 P-site zigzag F-SWNTs, the fluorine at position 97 attached to carbon at position 30 is denoted as F1 while the fluorine at position 98 attached to carbon at position 29 is denoted by F2. For 1,2 P-site chiral F-SWNTs, the fluorine at position 87 attached to the carbon at position 42 is denoted as F1 while the fluorine at position 88 attached to carbon at position 47 is denoted by F2. For 1,4 zigzag Zigzag F-SWNTs, the fluorine at position 97 attached to carbon at position 46 is denoted as F1 while the fluorine at position 98 attached to carbon at position 31 is denoted by F2. For 1,4 zigzag chiral F-SWNTs, the fluorine at position 87 attached to carbon at position 47 is denoted by F1 while the fluorine at position 88 attached to carbon at position 36 is denoted by F2. For D-site zigzag F-SWNTs, the fluorine

at position 100 attached to carbon at position 14 is denoted by F4, the fluorine at position 97 attached to carbon at position 59 is denoted by F1, the fluorine at position 98 attached to carbon at position 44 is denoted by F2 and the fluorine at position 99 attached to carbon at position 29 is denoted by F3. For D-site chiral F-SWNTs, the fluorine at position 90 attached to carbon at position 51 is denoted by F4, the fluorine at position 87 attached to carbon at position 47 is denoted by F1, the fluorine at position 88 attached to carbon at position 32 is denoted by F2, and the fluorine at position 89 attached to carbon at position 36 is denoted by F3. For P-site zigzag F-SWNTs, the fluorine at position 116 attached to carbon at position 73 is denoted by F4, the fluorine at position 113 attached to carbon at position 14 is denoted by F1, the fluorine at position 114 6545



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Table 3. Principal Elements δii (ppm) of 19F CSA Tensor Computed Using HartreeFock Methodology with Different Basis Setsa method hf/sto-3 g

hf/sto-3 g*þ hf/sto-3 g**

hf/3-21 g

hf/6-31 g

hf/6-311 g

δ11

CNT form (chiral)

δ22

δ33

δiso

1,2 P-Site (F1)

34.57

49.85

84.30

56.24

1,2 P-Site (F2)

20.13

66.28

83.63

56.68

1,4 zigzag (F1)

24.89

30.44

78.77

44.70

1,4 zigzag (F2)

32.37

46.29

77.71

52.12

1,2 P-Site (F1)

17.93

33.20

68.62

39.92

1,2 P-Site (F2)

3.48

49.64

67.95

40.35

1,4 zigzag (F1)

7.77

14.28

62.61

28.22


15.26 93.53

30.13 138.74

61.55 118.28

35.65 116.85

1,2 P-Site (F2)

79.85

163.93

120.82

121.53

1,4 zigzag (F1)

61.97

107.41

131.30

100.22

1,4 zigzag (F2)

69.52

124.67

136.03

110.07

1,2 P-Site (F1)

99.15

154.59

101.11

118.28

1,2 P-Site (F2)

91.20

187.46

98.21

125.62

1,4 zigzag (F1)

70.52

117.06

121.57

103.05


81.00 106.48

133.91 171.40

127.68 110.13

114.19 129.34

1,2 P-Site (F2)

102.21

207.74

106.33

138.76

1,4 zigzag (F1)

74.97

124.46

128.90

109.44

1,4 zigzag (F2)

87.23

143.06

137.69

122.66

The CNTs considered are of the chiral (3,5) form having 64 carbon atoms and a length of 10 Å and fluorines substituted at two sites 1,2 P-site and 1,4 zigzag. a

Table 4. Principal Elements δii (ppm) of 19F CSA Tensor Computed Using DFT Methods with Different Basis Setsa method b3lyp/3-21 g

b3lyp/6-31 g

b3lyp/6-311 g

CNT form (chiral)

δ11

δ22

δ33

δiso

1,2 P-Site (F1)

112.84

151.71

204.14

156.23

1,2 P-Site (F2)

105.68

199.40

204.02

169.70

1,4 zigzag (F1)

63.38

150.46

225.96

146.60

1,4 zigzag (F2)

87.91

163.66

229.67

160.42

1,2 P-Site (F1)

111.37

166.44

171.94

149.92

1,2 P-Site (F2)

116.61

215.67

166.71

166.33

1,4 zigzag (F1)

73.88

153.64

206.47

144.66


94.30 109.60

166.58 181.03

209.78 183.61

156.88 158.08

1,2 P-Site (F2)

123.39

232.68

177.70

177.92

1,4 zigzag (F1)

74.18

161.75

219.26

151.73

1,4 zigzag (F2)

94.25

173.92

224.80

164.32

The CNTs considered are of the chiral (3,5) form having 64 carbon atoms and a length of 10 Å, with fluorines substituted at two sites 1,2 P-site and 1,4 zigzag.

a

attached to carbon at position 43 is denoted by F2, and the fluorine at position 115 attached to carbon at position 44 is denoted by F3. For P-site chiral F-SWNTs, the fluorine at position 90 attached to carbon at position 24 is denoted by F4, the fluorine at position 87 attached to carbon at position 37 is denoted by F1, the fluorine at position 88 attached to carbon at position 28 is denoted by F2, and the fluorine at position 89 attached to carbon at position 33 is denoted by F3. The CSA tensor is completely characterized by its principal elements and by three Euler angles that describe tensor orientation in the molecular frame. From the complete 19F CSA tensor we have constructed the anisotropic CSA parameters span (Ω) and skew (k) and used them in conjunction with the isotropic chemical shift δiso to distinguish between different forms of

fluoronanotubes. Ω ¼ σ33 σ11 ðΩ > 0Þ k ¼ ð2σ 22 σ11 σ33 Þ=Ω ð 1 e k e 1Þ i ¼ 13 δii ¼ σii, ref σii 1 δiso ¼ ðδ11 þ δ22 þ δ33 Þ 3

ð1Þ

where the principal components of the CSA tensor are labeled according to the HerzfeldBerger convention σ11 e σ22 e σ33 and σii,ref refers to the principal component of the CSA tensor of the reference compound used. Such multiparameter correlations in the form of multidimensional CSA parameter plots have been used for secondary structure characterization in biomolecules.39,40 6546



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rate ΔFFX between the CSA of 19F with the dipolar interaction between the fluorine and an X (heteronuclear) spin.31 The CSA/ DD relaxation rate is given by

The CSA of the 19F spin can be estimated from liquid-state relaxation experiments by measuring the cross-correlated relaxation

ΔFFX ¼

2 μ0 γF 2 γX pB0 τc ΔσFg 3 5 4π rHX 1 þ ωF 2 τc 2

ð2Þ

where τc is the overall correlation time and ΔσFg is a “geometric” CSA parameter given for an axially symmetric CSA tensor by 1 ð3Þ ΔσFg ¼ Δσ F ð3 cos2 θF, FX 1Þ 2 where θF,FX is the angle subtended by the internuclear FX vector and the symmetric axis of the CSA tensor. Because the ΔσFg

Figure 2. 2D CSA parameter correlation plots between the span (Ω) and skew (k) of the 19F CSA tensor for both zigzag and chiral fluoronanotubes. (a) (Ω,k) plot for 1,2 P-site and 1,4 zigzag fluorine substitutions. and (b) (Ω,k) plot for D-site and P-site fluorine substitutions.

Figure 3. 3D CSA parameter correlation plot, adding isotropic chemical shift δiso along the third dimension: (a) (Ω,k,δiso) triplet for 1,2 P-site and 1,4 zigzag fluorine substitutions in zigzag and chiral fluorinated carbon nanotubes; (b) (Ω,k,δiso) triplet for D-site and P-site fluorine substitutions in zigzag and chiral fluorinated carbon nanotubes.

Table 5. Different 19F CSA Parameters Namely Isotropic Chemical Shift (δiso, ppm), Span (Ω, ppm), and Skew (j), Computed Using DFT Methods with the B3LYP/6-311g Basis Set for Different Forms of Zigzag and Chiral CNTsa ZZ 1,2 P-Site

Chi 1,2 P-Site

ZZ 1,4 zigzag

Chi 1,4 zigzag

ZZ D-Site

Chi D-Site

ZZ P-Site

Chi P-Site

δiso Ω

226.53 112.75

158.08 185.26

182.52 170.83

151.73 212.23

236.75 103.27

271.24 95.84

287.43 89.97

206.82 157.38

k

0.36

0.23

0.11

0.17

0.62

0.47

0.33

0.02

Two fluorine substitutions have been made at the 1,2 P-site and 1,4 zigzag site, respectively, while four fluorine substitutions have been made at the D-site and P-site, respectively. ZZ and Chi refer to the zigzag and chiral forms of F-SWNTs, respectively. a

Table 6. Predicted 19F CSA-DD Cross-Correlated Relaxation Rates from Liquid-State NMR Experiments (ΔCFF, s1) and the Computed Geometric CSA Orientation Parameters (σFg , ppm; Ox, φy, deg) for Various Forms of Fluorinated Zigzag and Chiral SWNTsa ZZ 1,2 P-Site F

Chi 1,2 P-Site

ZZ 1,4 zigzag

Chi 1,4 zigzag

ZZ D-Site

Chi D-Site

ZZ P-Site

Chi P-Site

(σg)

94.51

62.36

111.78

56.90

45.74

40.85

26.52

108.75

ΔCFF

33.098

21.705

39.290

20.012

17.032

15.784

10.381

39.548

φx

87.365

108.784

106.618

82.472

95.175

94.352

106.918

89.877

φy

90.819

125.782

89.352

138.851

127.879

134.910

134.346

106.871

All relaxation rates depicted in the table will need to be scaled by a factor depending on τc the correlation time and ω the Larmor frequency. ZZ and Chi refer to the zigzag and chiral forms of F-SWNTs, respectively. a

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where (Δσi), i = x, y is the anisotropy of the axially symmetric shielding tensor with its symmetry axis along i and the angle φi refers to the orientation of the FX bond vector with respect to the symmetry axis of the CSA tensor oriented along the i axis.

Figure 4. Typical simulated 19F static powder NMR spectra from solidstate NMR experiments simulated using the software package SIMPSON with 1H Larmor frequency ω0/2*π = 100 MHz. (a) Zigzag F-SWNT with fluorine substitution at 1,2 P-site and principal tensor elements δ11 = 213.96, δ22 = 250.16, δ33 = 215.47. (b) Chiral F-SWNT with fluorine substitution at 1,2 P-site and principal tensor elements δ11 = 109.6, δ22 = 181.03, δ33 = 183.61. (c) Zigzag F-SWNT with fluorine substitution at 1,4 and principal tensor elements δ11 = 122.61, δ22 = 198.66, δ33 = 226.28. (d) Chiral F-SWNT with fluorine substitution at 1,4 and principal tensor elements δ11 = 74.19, δ22 = 161.75, δ33 = 219.26. (e) Zigzag F-SWNT with fluorine substitution at D-site and principal tensor elements δ11 = 221.34, δ22 = 241.7, δ33 = 247.84. (f) Chiral F-SWNT with fluorine substitution at D-site and principal tensor elements δ11 = 238.13, δ22 = 308.77, δ33 = 266.81. (g) Zigzag F-SWNT with fluorine substitution at P-site and principal tensor elements δ11 = 263.13, δ22 = 322.84, δ33 = 276.33. (h) Chiral F-SWNT with fluorine substitution at P-site and principal tensor elements δ11 = 151.11, δ22 = 228.02, δ33 = 241.33.

contains both magnitude and orientation information it has been termed as a “geometric” CSA parameter.32 In general, for most molecules it has been observed that the fluorine CSA tensor is not axially symmetric and hence can be written as a sum of two axially symmetric tensors with their symmetry axes oriented along two orthogonal directions (say along x and y).41 The geometric CSA tensor parameter ΔσFg then becomes

ðΔσg ÞF ¼ ðΔσx ÞF

1 1 ð3 cos2 φx 1Þ þ ðΔσ y ÞF ð3 cos2 φy 1Þ 2 2

ð4Þ

’ RESULTS AND DISCUSSION Previous studies on fluoronanotubes found a negligible difference in the 13C chemical shifts between the 1,2 and 1,4 fluorine substitutions and hence concluded that carbon-13 NMR cannot be used to distinguish between these two forms.25 Upon fluorination the CC bonds were found to elongate by around 0.2 Å and the fluorinated carbons were found to be sp3 hybridized; however, the 13C chemical shifts of the fluorinated carbons were quite insensitive to the pattern and degree of functionalization. Differences in isotropic chemical shifts could be due to a combination of changes in the individual CSA tensor components and it is hence necessary to follow the changes in the individual components with local environment, to understand the effects on the isotropic chemical shifts. This study hence focuses on using the 19F CSA tensor to distinguish between the zigzag and chiral forms of such fluorinated carbon nanotubes. Using the CONTUB software, the tubes were generated by rolling from a graphene sheet (Figure 1) and CSA tensor computations were performed using Gaussian03. Tables 14 delineate the results of the CSA tensor computations in terms of the tensor principal elements computed using both HF and DFT methods with different basis sets and for different kinds of fluorine substitutions in both chiral and zigzag forms of fluoronanotubes. Although there is considerable variation in the magnitudes of the principal tensor elements depending on the theoretical method and the size of the basis set used, nevertheless a clear pattern emerges when one compares the values for different forms of zigzag and chiral carbon nanotubes. These two forms (regardless of the kind of fluorine substitution made) are clearly distinguishable, as evidenced by the fairly large differences in the fluorine CSA parameters. Further, an inspection of the δiso isotropic chemical shift values alone from Tables 14 reveals several interesting features of the effect of different 19F substitutions on the fluorine CSA tensor in different F-SWNTs. For instance, for fluorine substitutions at two different sites the maximum value of δiso was observed using DFT methods and a dense b3lyp/6-311 g basis set and is larger for 1, 2 P-site 19F substitutions in both zigzag and chiral F-SWNTs as compared to 1,4 zigzag 19F substitutions. This is indeed to be expected because the closer the two fluorine substitutions are, the greater will be the effect on the CSA tensor. Similar trends are observed for the P-site and D-site 19F substitutions for both chiral and zigzag F-SWNTs. Multidimensional CSA parameter correlation plots are depicted in Figures 2 and 3 for different forms of zigzag and chiral fluorinated carbon nanotubes. The plots clearly distinguish between the chiral and zigzag forms irrespective of the type of fluorine substitution and demonstrate the utility of constructing such correlation maps. Different 19F CSA parameters computed using DFT methods with the B3LYP/6-311g basis set are tabulated for different forms of zigzag and chiral SWNTs in Table 5. Table 6 shows the predicted CSA/DD cross-correlated relaxation rates and the computation of the corresponding 19F parameter that would be obtained if a relaxation experiment were performed in the liquid state on a typical chiral and zigzag fluorinated carbon nanotube. The results are encouraging and 6548


The Journal of Physical Chemistry A point toward using such relaxation measurements as another easily quantifiable indicator of structural differences between zigzag and chiral fluorinated carbon nanotubes. In Figure 4, a simulation of the 19F MAS powder pattern using the SIMPSON software package42 is depicted for different forms of the carbon nanotubes. The theoretical expected spectrum shows a typical powder line shape with different computed principal values of the CSA tensor and different CSA parameters (span and skew). The powder patterns show distinctive differences between the chiral and zigzag fluoronanotubes for all types of fluorine substitutions, showing that the 19F CSA tensor can be a good probe for structural characterization of nanotubes. During the course of this study we were unable to optimize the geometry for F-SWNTs of the armchair forms, with the computational power available to us, and hence could not draw any conclusions about using our CSA multiparameter correlation technique to distinguish the armchair form of F-SWNTs. However, there have been a number of recent studies both theoretical and experimental, on the armchair SWNTs.4345 We thus would like to comment here that if it were possible to perform either liquidstate or solid-state NMR experiments to estimate the 19F CSA tensor in armchair F-SWNTs, we envisage our technique being a useful structural marker for such fluoronanotubes as well.

’ CONCLUSIONS Carbon nanotubes are extraordinary materials that have possible applications in molecular nanodevices as well as in futuristic biomedical devices. A better understanding of the chemical and physical properties of carbon nanotubes is the key to controlling their structure during synthesis. NMR spectroscopic methods, both theoretical and experimental, are being increasingly seen as a viable characterization technique that could provide a detailed structural characterization. In this study we have used ab initio and DFT methods to compute the full 19F CSA tensor of a range of fluorinated zigzag and chiral nanotubes at different levels of theory and for a wide range of basis sets. Different modes of addition of fluorine (at 1,2 and 1,4 sites) as well as various fluorinated isomers have been considered as model fluoronanotubes. We demonstrate the utility of multidimensional CSA parameter correlation maps as a means of distinguishing between the zigzag and chiral forms of the carbon nanotubes. We also propose different liquid-state and solid-state NMR experiments to characterize the fluorine CSA tensor in fluorinated SWNTs. Functionalized carbon nanotube derivatives are seen as being of prime importance for biomedical research. We believe that the fluorine CSA tensor can be a very useful diagnostic tool to easily and accurately characterize the different forms of carbon nanotubes. It is hoped that the present computational study will serve as a protocol for future experimental work aimed at exploiting the fluorine CSA tensor for accurate structural characterization of different forms of functionalized carbon nanotubes. ’ ASSOCIATED CONTENT

bS

Supporting Information. The principal elements δii, i = 13 (in ppm), of the 19F CSA tensor computed using the HF and DFT methods for different basis sets are tabulated for four fluorine substitutions in both the zigzag and chiral forms, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Using the 19F NMR Chemical Shift Anisotropy Tensor To Differentiate

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