Effect of Single-Walled Carbon Nanotube Association upon

Sep 11, 2009 - Characterizing Covalently Sidewall-Functionalized Single-Walled Carbon Nanotubes by UsingH NMR Spectroscopy. Donna J. Nelson and ...
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J. Phys. Chem. C 2009, 113, 17378–17386

Effect of Single-Walled Carbon Nanotube Association upon Representative Amides Donna J. Nelson,* Paramasivan T. Perumal, Christopher N. Brammer, and Panneer S. Nagarajan Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: August 21, 2009

Select amides have been widely used to suspend nanoarchitectures in organic solvents. In order to determine factors enabling these suspensions, interactions of single-walled carbon nanotubes (SWCNTs) with representative amides 1 were examinedsR(CdO)NMe2, when R ) H, Me, Et, i-Pr, t-Bu, Ph. 1H NMR spectra gave evidence for two types of SWCNT: amide associations, formed after evaporation of the reaction mixture to either a concentrated solution or a wet paste, followed by sonication in NMR solvent. NMR spectra of SWCNTs associated with 1 (SWCNT:1) after evaporation to concentrated solution show broadening and small downfield changes, suggesting weak interactions. Evaporation of SWCNT:1 to a wet paste causes larger spectral changes, predominantly in aldehydic and R proton signals. These are often 10 times those in concentrated solution, especially when R has small steric requirements, which suggests a stronger interaction of 1 with SWCNTs under wet paste conditions. 1H NMR signal changes of 1, which accompany SWCNT:1 association, depend upon (1) degree of evaporation of residual amide and organic solvent, (2) organic and NMR solvent combination, (3) type of proton in R, (4) proximity effects to the carbonyl (R versus NMe2), and (5) steric requirements of R. Introduction There are increasing applications of nanoarchitectures of various shapes, sizes, and compositions, such as graphite sheets,1 nanowires,2 fullerenes,3 nanorods,4 nanohorns,5 multi-walled carbon nanotubes,6 and single-walled carbon nanotubes (SWCNTs).7 Their nonpolar natures foster a common solvation problem, prompting a search for different and improved ways to disperse these nanostructures in order to increase their reactivities. They also share the characteristic that the solubility of each has been reported to increase in the presence of N,Ndimethylformamide (DMF), which has frequently been used as a solvent.1-7 The critical influence of DMF has not been fully investigated in any of the above systems,1-7 so given their similar solubility problems, an explanation of the remarkable effects of DMF in any one system could help understand its effects in them all. Therefore, it was desirable to explore the effects of DMF and other amides upon SWCNT dispersion and the interactions between the two, in order to understand better how to solvate and disperse these nanoarchitectures generally. SWCNTs are appropriate for such an investigation due to their many extraordinary mechanical and electronic properties. SWCNTs possess high tensile strength and thermal stability, ideal for the preparation of high-strength fibers;8 they can carry current densities up to 4000 A/mm2, which are 3 orders of magnitude higher than metals like aluminum and copper.9,10 On the basis of these properties, carbon nanotubes have numerous potential applications, such as molecular electronics, conducting layers in light-emitting and photovoltaic devices, and sensors.11 However, low SWCNT dispersion, solubilization, and separation have hindered fully using these properties.12-14 Improving these capabilities would not only facilitate SWCNT purification but also foster new prospects in aligning and forming molecular * To whom correspondence should be addressed. E-mail: djnelson@ ou.edu.

devices from them, as well as in generating functionalized nanoscale architectures.15,16 Ultrasonic debundling of SWCNTs in amide solvent is an interesting phenomenon that has yet to be explained fully. Reports of SWCNT damage due to extensive ultrasonic debundling17 necessitated alternative debundling methods, such as the use of organic solvents; for example, DMF and N-methyl-2pyrrolidone (NMP) have been investigated as solvents for debundling SWCNTs by adsorption.7 It was suggested that ultrasound sonication and weak charge transfer between the amide and the SWCNT may both be required to drive an efficient debundling process.7a Because improved solubilization techniques by amide association may offer a way to debundle them without damage, the nature and structure of this association merit further study. Adsorption is the process by which an atom of liquid or gas adheres to a solid surface.18-21 Two types of adsorption processes are physical adsorption (physisorption) and chemical adsorption (chemisorption).18-21 Physisorption involves weak interactions, such as van der Waals forces, has low heat of adsorption (e5 kcal/mol), is nonspecific, can have single or multiple layers, is rapid acting, is reversible, and has no electron transfer.18-21 Chemisorption involves a strong interaction such as chemical bonding, has a higher heat of adsorption (g10 kcal/ mol), is highly specific, has only a monolayer, is slow acting, is irreversible, and is accompanied by electron transfer.18-21 There have been many studies of gas-phase SWCNT physisorption and chemisorption of hydrogen,22 oxygen,23 noble gases,24 NO,25 ammonia,26 water,27 COn,28 fluorine,29 CF4,30 organic,31 and combinations of different gases.32 There are fewer reports of physisorption and chemisorption on SWCNTs in solution. One33a of many computational studies33 predicted that the major interaction of SWCNTs with closed-shell compounds was physisorption but that both physisorption and chemisorption were possible with open shells.

10.1021/jp9072075 CCC: $40.75  2009 American Chemical Society Published on Web 09/11/2009

Effect of SWCNT Association upon Representative Amides

Figure 1. Representative N,N-dimethylamides 1 in SWCNT associations and their conjugate acids 2, in which R ) H (a), Me (b), Et (c), i-Pr (d), t-Bu (e), and Ph (f).

NMR is a particularly versatile method of characterizing chemical compounds, revealing their electronic properties, and studying molecular interactions, but NMR has not achieved its full potential for studying interactions or characterizing reaction products of carbon nanotubes.12 We recently reported using NMR as a primary tool for studying functionalized SWCNT structures and the first use of 2-D NMR to characterize the products of SWCNT covalent sidewall functionalization in liquid phase.12 It is desirable to determine the generality of NMR as an analytical technique for functionalized SWCNTs and their reactions. Magnitudes of IR frequency changes upon SWCNT complexation have been used to measure the strength of the association formed with SWCNTs.34 Changes in NMR values upon amide protonation35a and molecular complexation35b have similarly been used to measure the strength of associations formed with amides. Therefore, changes in NMR values upon SWCNT:1 formation should be similarly useful to measure the strength of SWCNT:amide associations. Consequently, we ascertained and compared the nature and extent of SWCNT interactions with a series of representative amide solvents by using NMR. Results Representative amides, used for the study reported herein, are shown in Figure 1. SWCNTs associated with the amide series 1 (SWCNT:1) will incur a range of steric effects of groups bonded to CdO with simultaneously changing electronic effects. Therefore, this series enables exploring steric versus electronic effects in SWCNT:1. Forming SWCNT:1. SWCNT:1 associations were effected by sonicating the SWCNT and amide in an organic solvent (hexane or toluene), and reaction mixtures were then evaporated to achieve each of two solvent conditions, concentrated solution and wet paste. Each resulting concentrated solution or wet paste sample was sonicated in an NMR solvent (CDCl3 or DMSOd6), and the intermolecular interactions were then measured by using 1H NMR; results are detailed below. Changes (broadening and downfield shifts) in 1H NMR signals of 1, which accompany SWCNT:1 association, are dependent upon (1) degree of residual amide and organic solvent evaporation, (2) solvent combination, (3) type of proton within R (aldehydic, R, etc.), (4) proximity effects to the carbonyl (R versus NMe2), and (5) steric requirements of R. Some potential complications to the 1H NMR study of SWCNT:1(a-f) interactions were (1) accidental coincidence of NMR signals for any of the followingsfree amide 1, reaction solvent, and NMR solvent, (2) the small size of the NMR signals, which result from the small amount of 1 associated with SWCNT, relative to the signals for other species present, and (3) the simultaneous effects of different types of association upon NMR shifts. Most of these were addressed by judicious choice of complexing agent, reaction solvent, and NMR solvent; others were addressed by appropriate reaction design.

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17379 For example, suspending SWCNTs in only the complexing agent 1 would cause the signal for free 1 to swamp the signal for SWCNT:1, which would preclude differentiating between NMR signals of free versus associated 1. This was particularly important, because differentiating between them was essential to appropriate analysis. Consequently, in order to explore the nature of SWCNT:1 associations, two procedures were used to produce the concentrated SWCNT:1 product, which was then suspended in an NMR solvent. Then comparing the NMR of free versus associated 1a-f measured the effects upon NMR shift, which were caused by association. These methods were developed in order to minimize residual amide and organic solvent in the sample, in order to minimize their complicating NMR signals and to expose maximally the SWCNT:1 NMR signals. SWCNT:1 Evaporation to Concentrated Solution. SWCNTs and a small amount of complexing agent 1 were suspended in a small amount of solvent (hexane or toluene) by sonication, in order to achieve SWCNT debundling, dispersion, and association with 1. The SWCNT:1 solution was allowed to stand at room temperature, evaporating organic solvent and some excess 1; a small aliquot of the resulting mixture was sonicated in NMR solvent (CDCl3 or DMSO-d6) for analysis (Figure 2B, C). NMR data for SWCNT:1 concentrated solution samples are compared to those for amides 1 in Table 1. The NMR of each SWCNT: amide (SWCNT:1) showed that proton peaks due to methyls on nitrogen, which are typically sharp and distinct in the free amide 1,36 generally broaden and coalesce slightly. However, association with SWCNTs also has a relatively small effect upon NMR values (∼e0.1 ppm) of all protons, usually a small downfield shift. Evaporating SWCNT:1 to a Wet Paste. The remaining solution of SWCNTs, complexing agent, and organic solvent, from the above procedure, was allowed to evaporate for 3-5 more days at ambient temperature, giving a wet paste. A small amount of the paste was dissolved by sonication in NMR solvent (DMSO-d6 or CDCl3) and analyzed by NMR (Figure 2D, E). NMR data for SWCNT:1 wet paste samples are compared to NMR data for SWCNT:1 concentrated solution samples and for free amides 1 in Table 1. The near-complete removal of reaction solvent and free amide simplifies the SWCNT:amide NMR spectrum, but signals for SWCNT:1 are nevertheless small and somewhat obscure. NMR value changes in wet paste samples are as much as 10 times those observed in concentrated solution samples, which suggests that the wet paste sample preparation method effects a stronger SWCNT:1 association. Downfield changes are usually largest when using the combination of hexane for reaction solvent and DMSO-d6 for NMR solvent. However, the most uniform trends were observed with the hexane/CDCl3 combination. In SWCNT:1 wet paste samples, downfield changes (Table 1) in amide NMR values are greatest for aldehydic protons (1a, R ) H, e0.56 ppm), compared to protons R to CdO (1b, R ) Me, e0.15 ppm), protons β to CdO (1c, R ) Et, e0.21 ppm), and NMe2 protons (1a, R ) H, e0.20 ppm). This interaction in SWCNT:DMF (1a, R ) H) was so strong, that the aldehydic proton signal (∼8.6-8.2 ppm, compared to 7.98 or 7.95 ppm for free DMF) broadened sufficiently to obfuscate the NMR chemical shift value. NMR signals for protons in SWCNT:1b-f (R ) Me, Et, i-Pr, t-Bu, and Ph) were less broadened than those for SWCNT:1a (R ) H); broadening decreased with increasing steric requirements of R and with increasing distance of the proton from CdO. Downfield changes for protons R to CdO generally show a decreasing trend in the order R ) Me (1b) >

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Figure 2. 1H NMR spectrum of (A) N,N-dimethylisobutyramide 1d in CDCl3, (B) the SWCNT:1d concentrated solution sample using hexane and CDCl3 solvents, (C) the SWCNT:1d concentrated solution sample using toluene and CDCl3 solvents, (D) the SWCNT:1d wet paste sample using hexane and CDCl3 solvents, and (E) the SWCNT:1d wet paste sample using toluene and CDCl3 solvents.

Et (1c) > i-Pr (1d). When there are no R protons, R ) t-Bu (1e) or Ph (1f), the β and phenyl protons are changed only slightly upon association with SWCNTs. NMe2 signals of SWCNT:amides with larger NMR value changes (g∼0.10 ppm) coalesced into one broader peak. Relative to the unassociated amide 1, most signals of NMe2 in SWCNT:1 are moved downfield by e∼0.10 ppm. SWCNT:1 Raman Studies. Raman spectra D-bands and G-bands of SWCNT:1 are identical in appearance to those of pristine SWCNT (p-SWCNT). They evidence no covalent bond

formation to SWCNT, but rather that the interactions between SWCNT and complexing agents 1 herein are associations, similar to those predicted previously.33a Discussion NMR Change Determinants. Raman spectra of the products SWCNT:1 show no evidence of covalent bond formation to SWCNTs, so interactions between amides 1a-f and SWCNTs are best regarded as associations, which produce amide proton NMR signal downfield shifts in various magnitudes. Structural

Effect of SWCNT Association upon Representative Amides

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TABLE 1: 1H NMR Chemical Shift Values of N,N-Dimethylamides 1, R(CdO)NMe2, in the Presence and Absence of SWCNT NMR chemical shift values (ppm) concentrated solutiona R protons R H 1a

Me 1b

Et 1c

i-Pr 1d

t-Bu 1e

Phf 1f

SWCNT and organic solvent used

NMR solvent

none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane toluene none hexane none hexane toluene

DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3

β or aromatic

0.97 0.97 0.97 1.03 1.13 1.10 0.98 0.98 0.98 1.08 1.10 1.15 1.20 1.20 1.19 1.28 1.25 1.30 7.45 7.44 7.44 7.46 7.46

NMe2 protons

aldehydic or R 7.95 7.95 7.96 7.98 7.93 7.95 1.96 1.97 1.98 2.02 2.06 2.06 2.28 2.27 2.29 2.24 2.25 2.35 2.84 2.81 2.83 2.79 2.80 2.86

wet pasteb

∆c

averaged

∆e

0 0.01 -0.05 -0.03 0.01 0.02 0.04 0.04 -0.01 0.01 0.01 0.11 -0.03 -0.01 0.01 0.07 0 -0.01 -0.03 0.02 -0.01 0.02 0.02

2.82 2.82 2.82 2.88 2.85 2.85 2.87 2.88 2.88 2.92 2.99 2.96 2.88 2.88 2.88 2.88 2.95 2.99 2.90 2.91 2.89 2.97 2.97 3.04 2.96 2.95 2.93 3.04 3.00 3.05 2.96 2.96 3.08 3.10 3.11

0 0 -0.03 -0.03 0.01 0.01 0.07 0.04 0 0 0.07 0.11 0.01 -0.01 0 0.07 -0.01 -0.03 -0.04 0.01 0 0.02 0.03

R protons β or aromatic

NMe2 protons

aldehydic or R

∆c

averaged

∆e

7.95 8.60-8.41

0.56

2.82 2.85

0.03

7.98 8.61-8.22 8.37-8.19 1.96 2.11

0.44 0.30 0.15

2.88 2.97 3.08 2.87 3.00

0.09 0.20 0.13

0.97 1.19

2.02 2.14 2.15 2.28 2.42

0.12 0.13 0.14

2.92 2.93 3.03 2.88 3.03

0.01 0.11 0.15

1.03 1.29 1.26 0.98 0.98

2.24 2.33 2.34 2.84 2.95

0.09 0.10 0.11

2.88 2.96 2.90 2.90 3.09

0.08 0.02 0.19

1.08 1.29 1.28 1.20 1.21

2.79 2.85 2.85

0.06 0.06 0.01

2.97 3.07 3.07 2.96 2.96

0.10 0.10 0

-

3.04

-

1.28 1.32 7.45 7.44 7.44

0.04 -0.01 -

3.08 2.96 2.96 3.08

0.04 0 -

7.44

0

3.09

0.01

a

After sonication, solvents and unreacted starting materials are evaporated at ambient temperature to give a concentrated solution, which is dissolved by sonication in NMR solvent. b After sonication, solvents and unreacted starting materials are evaporated at ambient temperature (3-5 days) to give a wet paste, which is dissolved by sonication in NMR solvent. c Difference in NMR values of protons closest to CdO in SWCNT:1 in NMR solvent versus 1 in NMR solvent. d Methyl proton signals with large changes coalesced and did not require averaging. e Difference in NMR values of SWCNT:1 in NMR solvent versus 1 in NMR solvent. f Unreacted solid amide is decanted; solvent is evaporated.

effects in SWCNT associations were studied by exploring the change in NMR values upon SWCNT:amide formation by using eight permutations of reaction solvent (hexane versus toluene), degree of evaporation (concentrated solution versus wet paste), and NMR solvent (CDCl3 versus DMSO-d6). SWCNT and 1 were sonicated together in the reaction solvent, followed by evaporating to either a concentrated solution or a wet paste; the resulting product mixture was sonicated in an NMR solvent, and the 1H NMR was taken. Changes in NMR values of aldehydic, R, β, and/or phenyl protons in R of 1, produced upon SWCNT:1 association, are compared in Table 2 by solvent and by extent of evaporation. Regardless of solvent combination, the NMR data for SWCNT: 1, when evaporated to a concentrated solution, reveal rather small changes in NMR values. Conversely, NMR data for SWCNT:1 after evaporation to a wet paste generally show larger changes. Regardless of solvent combination or extent of evaporation, aldehydic and R proton NMR value changes were generally larger than those for β and phenyl protons. Also, regardless of solvent combination or extent of evaporation, SWCNT:1 usually showed a greater change in NMR values of protons near the carbonyl (aldehydic or R in R) than those in NMe2, initially suggesting a greater association with the carbonyl

functionality. However, this casual perusal seemed inconclusive; the protons near the carbonyl were themselves the protons in R and at the site of change in the series, so this could have been the reason for the greater change. In order to probe these trends more thoroughly, changes in NMR values (Table 2) for the series of amides 1a-e were correlated versus several amide physical observables, which are shown in Table 3. These were dissociation constants and steric substituent constants. The two dissociation constants explored were (1) pKBH+ values35c of the amide conjugate acids 2 and (2) pKHB values37a of amides 1 associated by hydrogen bonding to a large proton donor in CCl4 solvent. Five steric substituent constants of the groups R (H, Me, Et, i-Pr, t-Bu, Ph) attached to the carbonyl, were explored. Taft steric parameters,38 Es, van der Waals steric effect constants,39 ν, surface-to-volume ratios,40 G, substituent delimited volumes,40 Va, and axial strain values,41 A-values. Correlations and similar trends could reveal relationships between these constants and proton NMR value changes at pertinent sites in the amide. These can probe characteristics and responses to change of different sites in the amides, while exploring separately the steric and electronic effects in SWCNT: 1. Therefore, differences in R proton NMR values between free

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TABLE 2: Changes in N,N-Dimethylamide 1H NMR Shift Values (ppm) upon SWCNT:1 Formation δH of R, reaction solvent R(CdO)NMe2 amide

R

1a 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f 1a 1b 1c 1d 1e 1f

hexane

δH of NMe2, reaction solvent

toluene

hexane

toluene

solution or paste

NMR solvent

none 1

SWCNT:1

∆

SWCNT:1

∆

none 1

SWCNT:1

∆

SWCNT:1

∆

s s s s s s s s s s s s p p p p p p p p p p p p

CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6 DMSO-d6

7.98 2.02 2.24 2.79 1.28 7.44 7.95 1.96 2.28 2.84 1.20 7.45 7.98 2.02 2.24 2.79 1.28 7.44 7.95 1.96 2.28 2.84 1.20 7.45

7.93 2.06 2.25 2.80 1.25 7.46 7.95 1.97 2.27 2.81 1.20 7.44 8.42 2.14 2.33 2.85

-0.05 0.04 0.01 0.01 -0.03 0.02 0 0.01 -0.01 -0.03 0 -0.01 0.44 0.12 0.09 0.06

7.95 2.06 2.35 2.86 1.30 7.46 7.96 1.98 2.29 2.83 1.19

-0.03 0.04 0.11 0.07 0.02 0.02 0.01 0.02 0.01 -0.01 -0.01

-0.03 0.07 0.07 0 -0.04 0.02 0 0.01 0 0.01 -0.01 0 0.09 0.01 0.08 0.10

-0.03 0.04 0.11 0.07 0.01 0.03 0 0.01 0 -0.01 -0.03

0.30 0.13 0.10 0.06 0.04 0

2.85 2.99 2.95 2.97 3.00 3.10 2.82 2.88 2.88 2.91 2.95 2.96 2.97 2.93 2.96 3.07

2.85 2.96 2.99 3.04 3.05 3.11 2.82 2.88 2.88 2.89 2.93

8.28 2.15 2.34 2.85 1.32 7.44

3.08 3.03 2.90 3.07 3.08 3.09

0.20 0.11 0.02 0.10 0.04 0.01

8.51 2.11 2.42 2.95 1.21 7.44

0.56 0.15 0.14 0.11 0.01 -0.01

2.88 2.92 2.88 2.97 3.04 3.08 2.82 2.87 2.88 2.90 2.96 2.96 2.88 2.92 2.88 2.97 3.04 3.08 2.82 2.87 2.88 2.90 2.96 2.96

2.85 3.00 3.03 3.09 2.96 2.96

0.03 0.13 0.15 0.19 0 0

H Me Et i-Pr t-Bu Ph H Me Et i-Pr t-Bu Ph H Me Et i-Pr t-Bu Ph H Me Et i-Pr t-Bu Ph

TABLE 3: Dissociation Constants and Steric Constants Pertinent to 1 R(CdO)NMe2

g

dissoc. const

R steric substituent constant e

amide

R

pKBH+a

pKHBb

Esc

νd

Va

1a 1b 1c 1d 1e 1f

H Me Et i-Pr t-Bu Ph

-1.13 -0.21 -0.56 -1.61 -2.03

2.10 2.44 2.36 2.26 2.10 2.23

0 -1.24 -1.31 -1.71 -2.78 -3.79

0 0.52 0.56 0.76 1.24 1.66

0 0.0284 0.0431 0.0574 0.0716 0.0610

a Reference 35c. b Reference 37a. Reference 41b. h Reference 41c.

c

References 38 and 42; values are referenced to H.

amides 1 and SWCNT:1 were correlated versus each physical constant in Table 3. Correlation with pKBH+ or pKHB. Acid dissociation constants (pKa values) measure the tendency of an acid to dissociate, breaking a bond to H and losing H+. The dissociation constants of protonated bases, which are the series of amide conjugate acids 2, are designated pKBH+ values.35c Similarly, dissociation constants of amides 1, which are hydrogen bonded to 4-fluorophenol (a larger proton donor) in CCl4, are designated pKHB values.37a Similarities between a molecular series associating with complexing agents versus a molecular series undergoing protonation have been drawn, by correlating the two corresponding sets of dissociation constants (pK values).42 Another method to identify possible similarities between processes is to correlate measurements of an energy-related characteristic versus a set of published dissociation constants.43 In this study, changes in NMR values were correlated versus protonated base dissociation constants35c (pKBH+ values) and dissociation constants of hydrogen bonded amides37a (pKHB values). Both pKBH+ and pKHB can be used analogously to pKa, and their values are given in Table 3. Herein, 1H NMR changes are produced by amide association with SWCNT, so similarities between this process versus amide protonation or hydrogen bonding can be explored by correlating change in 1H NMR data versus the dissociation constants above: 1H NMR shifts (δ in units of ppm) in the former, versus amide conjugate acid 2

d

Reference 39.

e

Ge

A-valuef

0 16.42 14.83 14.05 13.45 11.83

0 1.74 1.79 2.21 5.4g 3.0h

Reference 40. f Reference 41.

dissociation constants (pKBH+) and hydrogen bonding dissociation constants (pKHB) for the latter. Amide conjugate acids 2 are shown as protonated on the carbonyl oxygen because the weight of current evidence indicates that this is the site of protonation,44 although exceptions and alternate findings are reported.37 A correlation between changes in proton NMR signals versus amide conjugate acid 2 pKBH+ values could reveal a similarity between the process of amides 1 associating with SWCNT and associating with H+. Similarly, a correlation between changes in proton NMR signals versus amide 1 pKHB values could reveal a similarity between the process of amides 1 associating with SWCNT and hydrogen bonding to the large proton donor. A better correlation was obtained by using pKHB, rather than pKBH+ values, and the strongest correlations were observed with the hexane/CDCl3 combination for concentrated solution. Therefore, changes in proton NMR values for samples from that solvent combination are plotted versus the pKHB in Figure 3. Open circles O and squares 0 represent change in NMR signals for protons of R and NMe2 respectively, for the SWCNT:1 concentrated solution samples. Symbols + and × correspond to change in NMR data for R and NMe2, respectively, for the SWCNT:1 wet paste samples. The trendline in Figure 3 represents the best correlation found (r ) 0.96, 0), which was for pKHB values versus NMR data for NMe2 protons from the concentrated solution samples. This is not surprising because

Effect of SWCNT Association upon Representative Amides

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Figure 3. Plot of amide δ 1H NMR value changes upon interaction with SWCNT (using data from the solvent combination of hexane, followed by CDCl3) versus pKHB values. Data are from Tables 2 and 3.

Figure 4. Plot of amide δ 1H NMR value change upon interaction with SWCNT (using data from all solvent combinations of hexane or toluene, followed by CDCl3 or DMSO-d6) versus steric substituent constants Va. Data are from Tables 2 and 3.

pKHB values measure hydrogen bonding of amides 1 to a large proton donor in CCl4, so in two ways they represent a system which is more similar to the SWCNT:1 system used herein than the pKBH+ values do. The pKHB system has (1) greater steric requirements in its complexing agent, which are more similar to the steric requirements of SWCNT used herein, and (2) the nonpolar solvent CCl4, which is more similar to CDCl3 used herein. Correlations of pKHB versus change in NMR values for NMe2 protons (r ) 0.96) are slightly better than those for R protons (r ) 0.89). This suggests that in SWCNT:1 concentrated solution samples, interactions in the NMe2 region of 1 are greater than those near R in concentrated solution samples. There is some support for this suggestion in Tables 1 and 2, where there are some marginally greater changes for NMe2 protons compared to those for R, for concentrated solution samples. Initially, it seems somewhat curious that NMe2 proton data correlate with pKHB values better than that for R protons, for two reasons: (1) R protons are at the point of structural variation in the amide 1 series (R ) H, Me, Et, i-Pr, t-Bu, Ph) and (2) the weight of current evidence indicates that the carbonyl oxygen is the site of protonation in amides.44 However, there are precedents37 for protonation and association in amides to prefer N rather than O. In concentrated solution samples using hexane/CDCl3 solvents, changes in NMe2 proton NMR values correlate more strongly with pKHB values than they do with any other physical characteristic explored. No significant correlation was found between either set of dissociation constants above versus NMR data for wet paste samples. Correlation with Steric Substituent Constants. Steric substituent constants provide different ways to evaluate steric effects in reactions. If effects of substituents observed herein correlate linearly with a set of substituent constants, then observed changes in SWCNT:1 data may be due to effects, which are similar to those described by that set of substituent constants. The most commonly used steric substituent constants, Taft steric parameters, Es, were criticized as not necessarily due to only steric effects but including electronic and polar effects, which do not necessarily evolve in parallel with steric factors.40 Therefore, additional steric substituent constants were developed in order to account for different types of steric effects: steric effect constants based on van der Waals radii,39 ν, surface-tovolume ratios,40 G, substituent delimited volumes,40 Va, and axial strain interactions,41 A-values. Values of these steric substituent

constants for R groups in amides 1 are given in Table 3. Each of these was correlated against the measured change in proton NMR values, upon formation of SWCNT:1, as was done for pKHB above. The strongest correlation was found by using the steric substituent constants Va versus changes in R proton NMR values in SWCNT:1 wet paste samples. Steric constants Va correlated well for each solvent system: hexane/CDCl3 (r ) -0.95), toluene/CDCl3 (r ) -0.95), and hexane/DMSO-d6 (r ) -0.93). The similar behavior displayed by these individual solvent pairs inspired attempting a correlation combining proton data for all wet paste samples (irrespective of solvent systems) versus Va. These NMR data combined over all solvent pairs are correlated versus Va in the Figure 4 plot; this is analogous to the plot in Figure 3, except that in Figure 3, NMR data are differentiated only by extent of evaporation (concentrated solution vs wet paste) and by type of proton (R vs NMe2). The trendline in Figure 4 represents the best correlation found, which was for R protons in wet paste, r ) -0.90. This agrees with general steric trends, which are obvious in Tables 1 and 2. All steric constants in Table 2 correlate at least reasonably well with changes in R proton NMR values of wet paste samples. Combining data for all solvent systems (reaction solvent and NMR solvent) produces the correlation coefficients, r ) 0.82 for Es, -0.81 for ν, -0.90 for Va, -0.82 for G, and -0.75 for A (16 data points each). All of the above indicate a relationship between the changes in NMR data and the steric requirements of R in 1, such that increasing steric effects decreases the change in NMR values. Associations in SWCNT:1 formed by evaporation to a wet paste, generally produce greater changes in NMR values of R protons closest in proximity to CdO in each amide 1 (aldehydic > R > β > phenyl). There was no mathematical reason to expect a good correlation by using all NMR changes in R protons, regardless of solvent combination; indeed, no analogous correlation was found by plotting combined NMR data versus pKHB (above). However, the fact that the correlations of NMR data for R in wet paste samples behave similarly to each other, while those formed in concentrated solution do not, might be expected. Concentrated solution samples could have sufficient solvent to influence trends and correlations, while wet paste samples would not. However, while there is evidence that SWCNT:1 interactions are similar across solvent combinations in wet paste samples, there is no similar evidence for concentrated solution samples.

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The unique utility of Va is to measure steric effects, while accounting for the volume and proximity of constituent alkyls to the point of attachment within R. The strong correlation between Va versus NMR value changes (Figure 4) indicates larger alkyl groups produce smaller changes in R proton NMR shifts, upon association with SWCNT (Tables 1 and 2). This suggests that large alkyls prevent close SWCNT:1 association. A-values41 show a much weaker trend with NMR value changes (r ) 0.75), which is not surprising; these SWCNT:1 complexes do not have the torsional flexibility of the cyclohexane system, from which A-values were derived. Nevertheless, at least some correlation between the change in R proton NMR values versus steric effects is strongly suggested because all steric substituent constants in Table 3 produce correlations ranging from good to excellent. No other significant correlations of steric substituent constants were found versus amide 1 proton NMR data changes; none were found for NMe2 protons in wet paste samples and none for R protons or NMe2 protons in concentrated solution samples. The above collectively suggest that the larger changes in R NMR values in wet paste samples, as opposed to concentrated solution samples, indicate a tighter SWCNT:1 association. The tighter association creates greater steric hindrance, which is incurred during a stronger and closer association of 1 with SWCNT upon evaporation to wet paste; this closer association is retained from wet paste after dispersal in NMR solvent. Thus, a larger R manifests itself in steric hindrance for the SWCNT:1 association. This does not necessarily indicate a tighter SWCNT:1 association near R than near NMe2 because protons in R are at the point of variation in the amide 1 series (R ) H, Me, Et, i-Pr, t-Bu, Ph) and so might be expected to show both greater change and better correlation. However, the larger changes in NMR data for wet paste samples generally, combined with results of these correlations, support a closer SWCNT:1 association in wet paste samples than in concentrated solution samples. Effecting Dispersion. It is known that increasing the degree of a compound’s association with SWCNTs causes increased SWCNT dispersion.45 Formation of stronger SWCNT:1 associations are accompanied by greater NMR value changes, caused by an increased SWCNT:amide interaction. Therefore, the magnitude of change in an NMR spectrum of a compound, upon its association with SWCNTs may also help predict its capability to disperse SWCNTs and effect SWCNT debundling. SWCNT association with amides 1a-d produce similar results, but compounds 1e and 1f have much smaller changes in their NMR values (or chemical shifts), predicting lower levels of association and SWCNT dispersing capability, probably due to steric bulkiness, as discussed above. Two of these amides, formamide and DMF, have been reported to be especially effective at SWCNT dispersion, moreso than other common solvents.7c Physisorption versus Chemisorption. The nature and geometrical structures of these amide associations, as well as ways to determine them, are important because they enable these compounds to disperse SWCNTs and effect SWCNT debundling. Therefore, it was desirable to consider whether (a) the magnitudes of NMR changes could provide information to help discern physisorption from chemisorption and (b) the relative magnitudes of changes in NMR values for different protons could reveal the nature and or strength of association at corresponding sites in the amide. The latter information could also reveal predominant point(s) of association of 1 to SWCNT, and this possibility is being explored.

Nelson et al. There are many reports of SWCNTs associating with compounds by physisorption, but very few by chemisorption. DFT calculations33a compared the two, indicating that weak and strong adsorption energies of molecular interactions with SWCNTs are characteristic of physisorption and chemisorption, respectively; molecules such as RNH2, RCH3, and RNO2 (R ) Ph and R ) Me or H) were predicted33a to associate with SWCNT by physisorption, a weak interaction through the π system. Because magnitudes of NMR value changes produced upon SWCNT:1 association indicate the degree of association,34,35 then by analogy (1) smaller NMR value changes indicate lowerenergy associations with SWCNTs, while (2) larger changes correspond to stronger adducts. The changes in NMR values observed upon SWCNT:1 formation in this study are similar in magnitude to those observed in proton NMR studies of amide 1 protonation,35a so the corresponding energy changes herein may also be similar, ∼1-3 kcal/mol. Thus, the SWCNT:1 associations which produce these NMR value changes (e0.56 ppm) are probably too small to be chemisorptions, which are >10 kcal/mol.18-21 Therefore, even the strongest SWCNT:1 associations, which correspond to the largest NMR value changes reported herein, are probably physisorption rather than chemisorption. Conclusions SWCNTs interact with representative N,N-dimethylamides 1 (R ) H, Me, i-Pr, t-Bu, Ph) to form SWCNT:1 associations, which are detected by 1H NMR. SWCNT:1 is formed by sonication in solvent (hexane or toluene), then evaporating solvent and excess amide 1 to produce either a concentrated solution or a wet paste. Each resulting mixture was sonicated in NMR solvent (CDCl3 or DMSO-d6) for analysis. Structural effects in SWCNT:1 associations were studied under eight permutations of reaction conditions: reaction solvent (hexane versus toluene), degree of evaporation (concentrated solution versus wet paste), and NMR solvent (CDCl3 versus DMSOd6). SWCNT:1 concentrated solution samples exhibit rather small changes in NMR values, which vary by solvent combination. Conversely, SWCNT:1 wet paste samples generally show larger changes, which are very similar across all solvent combinations studied. Spectra of SWCNT:1 concentrated solution samples show small downfield changes, suggesting weak interactions. SWCNT:1 wet paste samples show larger spectral changes, predominantly broadening and downfield shifts in aldehydic and R proton signals. NMR shift changes for wet paste samples are often 10 times those of concentrated solution samples, especially when R is small; this suggests stronger SWCNT:1 interactions under wet paste conditions. 1H NMR signal changes, which accompany SWCNT:1 association, depend upon (1) degree of evaporation of residual amide and organic solvent, (2) organic and NMR solvent combination, (3) type of proton in R, (4) proximity effects to the carbonyl (R versus NMe2), and (5) steric requirements of R. Changes in NMR values were correlated versus several amide physical observables (dissociation constants and steric substituent constants) in order to explore similarities between the SWCNT:1 association process and those physical observables. In concentrated solution samples formed using hexane/CDCl3, changes in NMe2 proton NMR values correlate more strongly (r ) 0.96) with pKHB values than they do with any other physical characteristic explored. In wet paste samples formed using all solvent combinations explored, changes in R proton NMR values correlate more strongly (r ) -0.90) with Va steric substituent

Effect of SWCNT Association upon Representative Amides constants than with any other physical characteristic explored. Larger changes in R NMR values in wet paste samples, as opposed to concentrated solution samples, reflect the greater steric hindrance incurred upon a stronger and closer SWCNT:1 association formed during and retained from the evaporation to wet paste; a larger R increases steric hindrance for the SWCNT:1 association. Raman spectra of the products SWCNT:1 show no evidence of covalent bond formation to SWCNTs, so these interactions are best regarded as associations. Magnitudes of NMR changes are sufficiently small to indicate that the SWCNT:1 association is physisorption. Experimental Section Chemicals. Amides 1a-e were purchased from Aldrich Chemical Co., and 1f was from Frinton Laboratories. Purified powder p-SWCNTs46 (PO 279) were donated from Southwest Nanotechnologies, Inc. All were used without further treatment. Evaporation of SWCNT:1 Samples to Concentrated Solution. The procedure for formation of SWCNT:DMF is given as an example. Adapting procedures reported previously,12 DMF association with SWCNTs was explored by horn sonicating (30 min) p-SWCNT (0.5 mg), DMF (20 mg), and hexane or toluene solvent (4 mL). The p-SWCNT/DMF/solvent solution stood at room temperature for about 8 h in order to allow the solvent and much of the DMF to evaporate, producing a concentrated p-SWCNT/DMF solution. An aliquot (1 mL) of this p-SWCNT/ DMF solution was taken for NMR analysis. The NMR sample was prepared by adding the aliquot to chloroform-d or DMSOd6 (10 mL), followed by horn sonication (30 min). Evaporation of SWCNT:1 Samples to Wet Paste. The procedure for formation of SWCNT:DMF is given as an example. The p-SWCNT/DMF solution remaining from above was evaporated to wet paste consistency (∼5 days). Then a solution of the residual wet paste (∼0.1 mg) in chloroform-d or DMSO-d6 (10 mL) was prepared by horn sonication (30 min). NMR Spectra. The resulting solution was added by pipet into a 5 mm NMR tube (Wilmad, 503-PS). Each amide 1 was prepared for NMR spectrum acquisition by dissolving a small amount (0.1 mL) in chloroform-d or DMSO-d6 (10 mL). NMR measurements of SWCNT:1 and of free amides 1 were acquired on a Varian VXR-300 or a Varian VMX-400 spectrometer, with a typical run time of about 30 min (∼1000 transients). The typical error in NMR shifts reported herein is e(0.0017 ppm. COSY spectra (500 MHz Varian VNMR) were occasionally used to assist proton assignment. Raman Spectra. Raman spectra were measured using microscope laser Raman spectroscopy with a Jobin YvonLabRam spectrometer. The laser excitation wavelength was 632 nm with a spectral resolution of 4 cm-1. Acknowledgment. We acknowledge the National Science Foundation, Ford Foundation, and the Oklahoma Center for the Advancement of Science and Technology for support of this research. We are grateful to SouthWest NanoTechnologies, Inc. for a donation of p-SWCNT. We thank H. Rhoads for providing some literature references. We appreciate Susan Nimmo for NMR training. References and Notes (1) (a) Pillay, J.; Ozoemena, K. Chem. Phys. Lett. 2007, 441, 72. (b) Patakfalvi, R.; Diaz, D.; Santiago-Jacinto, P.; Rodriguez-Gattorno, G.; SatoBerru, R. J. Phys. Chem. C 2007, 111, 5331. (c) Cai, D.; Song, M. J. Mater. Chem. 2007, 17, 3678.

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