1. Interaction of Small Organonitrogen Compounds with Single-Walled Carbon Nanotubes: A Proton NMR Study Donna J. Nelson* and Ravi Kumar Department of Chemistry and Biochemistry, University of Oklahoma, Norman OK E-mail: djnelson@ou.edu Summary: Due to strong van der Waals interactions between SWCNT sidewalls, SWCNTs exist as bundles A key challenge for exploiting SWCNTs widely is overcoming their nonpolar nature, which causes a solvation problem Organic solvents N,N-dimethylformamide (DMF) and 1- methyl-2-pyrrolidone (NMP) increase the solubility of SWCNTs Identifying and comparing such molecular characteristics of NMP and DMF, which facilitate complexing with and solvating SWCNTs, could be applicable to nanomaterials generally Understanding associations between SWCNTs and simple organic functionalities at molecular level will help design SWCNT interactions with larger molecules containing such functional groups Challenges in Applying NMR Spectroscopy to Functionalized SWCNTs NMR is a promising tool but needs further developments for characterizing SWCNTs SWCNTs can be suspended but not really dissolved like traditional organic molecules A surmountable drawback is slight peak broadening which could arise due to two different sources (1) SWCNTs are large polymer-like molecules with slow tumbling rates and short T 2, causing somewhat broadened peaks (2) Another source could be residual paramagnetic metal catalysts used in the SWCNT synthesis Experimental Procedures Steps to make a concentrated solution: Mix SWCNTs + organic compound + solvent Sonicate for 30 minutes Allow to stand at room temperature in order to evaporate organic solvent and some excess organic compound Mix a small aliquot of the resulting mixture + NMR solvent Sonicate for 30 minutes and take the NMR of the solution Steps to make a wet paste: Use the remaining solution from the above procedure Allow to evaporate for 3-5 days at ambient temperature, yielding a wet paste Mix a small amount of the wet paste + NMR solvent, sonicate for 30 minutes, and take the NMR of the solution Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 Changes in 1 H NMR chemical shift values of N,N-dimethylamides upon SWCNT addition The presence of SWCNTs cause the NMR signals of SWCNT:N,N-dimethylamides to move downfield in concentrated solution Conversely, after evaporation to wet paste, NMR data for SWCNT:N,N-dimethylamides generally show larger changes (below) Aldehydic and α proton NMR value changes were generally larger than those for β and phenyl protons SWCNT:N,N-dimethylamides usually show a greater change in NMR values of protons near the carbonyl (aldehydic or α in R) than those in NMe 2, suggesting a greater association with the carbonyl functionality Nelson, D. J.; Perumal, P. T.; Brammer, C. N. Nagarajan, P. S. J. Phys. Chem. C 2009, 113, 17378 Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 a Spectral Database for Organic Compounds, National Institute of Advanced Industrial Science and Technology (AIST). b No signal observed. 1 H NMR chemical shift values for organonitrogens in presence and absence of SWCNTS J. Phys. Chem. C 2010, 114, 10140 (A)1-methyl-2- pyrrolidone (NMP) 1 in CDCl 3 (B) HiPco SWCNT:1 wet paste using toluene and CDCl 3 solvents (C) SWeNT SWCNT:1 wet paste sample using toluene and CDCl 3 solvents Representative 1 H NMR spectra Steric Effect versus Cyclization Greater conformational freedom of 2b and 2c enable them to be more accommodating sterically Cyclic 1 is rigid and has less conformational freedom Heteroatom(s) in compounds 2b and 2c more effectively associate with SWCNTs than those of 1 Cyclic versus Acyclic Increased separation in the carbonyl and amino functionalities in 3 compared to cyclic 1, gives the former greater conformational freedom This causes greater NMR signal changes for all protons of 3 compared to those of 1 Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 Effects of Carbonyl to Amino Proximity Molecule 3 is more linear and flexible which allows it to wrap around a SWCNT and associate via both heteroatoms Molecule 2a is less flexible and can best associate with a SWCNT linearly along one side parallel to its axis This demonstrates again, flexibility enables 3 to interact more effectively with SWCNTs Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 Carbonyl versus Nitrogen Complexation In compound 3 both the carbonyl and nitrogen can complex. However, NMR data show (A) stronger complexation at the carbonyl in 3, and (B) stronger complexation at N in 3 than at N in 2a. Explanations for this are below: (1) Inserting methylene units between the carbonyl group and the amino group in 3 changes the amide to an aminoketone, increasing its flexibility. (2) Compared to 2a, the nitrogen in 3 has more electron density from its three electron donating alkyl substituents. (3) The nitrogen in 2a has just two alkyl substituents. This should cause 3 to interact with the SWCNT more effectively than 2a. Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 Compounds 4: Association with Oxygen and/or Nitrogen Electron Pairs Differences in chemical shift values reveal the relative degrees of association of the two functionalities Greater chemical shift changes of heteroatom-bonded protons, indicates that they are more strongly affected by SWCNTs Protons α to nitrogen experience greater chemical shift change regardless of substituents on nitrogen, indicating stronger SWCNT association with N lone pairs than with O lone pairs Nelson, D. J.; Nagarajan, P. S.; Brammer, C. N.; Perumal, P. T. J. Phys. Chem. C 2010, 114, 10140 1 H NMR Chemical Shift Values for Amines in the Presence and Absence of SWCNTs a Accidental coincidence with H 2 O signal. b Accidental coincidence with 2,3-dimethylbutane signal. c Accidental coincidence with residual CHCl 3 signal. Trends in the NMR of SWCNT:Amines Greater chemical shift differences are observed in wet paste versus concentrated solution samples Greater shift differences are observed in 2,3- dimethylbutane solvent compared to hexane, toluene, and cyclohexane The trend in chemical shift differences in amines is α > β > γ, excluding pyridine Insignificant shift differences are observed for diamines and triamines Aromatic primary amines show smaller chemical shift differences than aliphatic ones Increased branching α to amino increases the chemical shift differences for protons near nitrogen Characteristics with greatest effect upon NMR chemical shifts (1) Having multiple atoms and π bonds, which possess electron pairs for donation to the SWCNT surface (2) Such functional groups separated by a number of methylene units, giving greater molecular flexibility, so the functional groups can associate with SWCNTs independently of each other (3) An acyclic long-chain molecular framework, as opposed to a cyclic or smaller one, in order to enable greater conformational freedom in the molecule and to increase its ability to interact and associate with the SWCNT surface Conclusions Magnitudes of changes in NMR chemical shift values indicate the degree to which SWCNTs associate with compounds such as amino alcohols, amides, and amines Carbonyl associates more strongly than amino in aminoketones Amino associates more strongly than hydroxyl in aminoalcohols These results enable predicting which group interacts more strongly and should better disperse nanostructures as individual molecules ADVANCE LEADERSHIP AWARD Acknowledgements Oklahoma Center for Advancement of Science & Technology