Temperature elevation of carbon materials during magic-angle-spinning solid-state NMR measurements (original) (raw)
A J Ambrozio, 2020
The 13 C NMR chemical shifts corresponding to different sites in 6 atomistic models of disordered carbons were computed at different H contents by 7 employing DFT calculations. Structural models were generated by molecular dynamics 8 simulations and validated by the pair distribution functions; further bonding analyses 9 were carried out to determine the amount of sp 3 and sp 2 carbons in the structures. 10 Specifically, the obtained results allow the distinction of the chemical shifts associated 11 with different types of carbon sites, with different hybridization states and bonded or 12 not to a hydrogen atom. The calculated NMR spectra show excellent agreement with 13 experimental data and are thus useful to identify local structural features of disordered 14 carbons. 15 ■ INTRODUCTION 16 Carbon materials have been extensively investigated in the past 17 years because of their unique properties at different allotropies, 18 for instance, graphite-like materialsgraphene, nanographites, 19 pyrocarbons, and disordered carbonscarbon blacks, amor-20 phous carbons, activated carbons, diamond-like materials, 21 fullerenes, carbon nanotubes, kerogens, and others. In many 22 of these studies, different structural models have been 23 employed for the description of the physical properties of 24 carbon materials. 1 Generally, the proposed models are 25 validated by confronting the predicted properties against 26 experimental results derived from diffraction techniques (e.g., 27 X-ray and neutron diffraction) as well as spectroscopic (e.g., 28 Raman and nuclear magnetic resonance spectroscopy) or 29 microscopic (e.g., transmission electron microscopy) meth-30 ods. 2−4 There is a wide range of computational techniques for 31 generating satisfactory structural models for carbon materials, 32 which are mainly based on molecular dynamics (MD), Monte 33 Carlo, or a combination of both. 5,6 In particular, MD has 34 proven to be a useful tool for generating appropriate models of 35 disordered carbon materials. 2−4,7 36 Among the important properties of carbon materials, the 37 nuclear magnetic resonance (NMR) chemical shielding is 38 particularly useful due its sensitivity to the local chemical 39 environment around the probe nuclei. Moreover, the 40 components of the NMR chemical shielding tensor can be 41 obtained from atomistic models and confronted directly with 42 experimental data; consequently, measurements and calcu-43 lations of the components of the chemical shielding tensor are 44 of high interest for both crystalline and disordered 45 materials. 8−10 Especially, the use of NMR to probe the local 46 bonding structure can be an adequate complement to 47 diffraction techniques (e.g, X-ray, electron or neutron 48 diffraction), which are more suited to probe the average 49 local environments or the medium-to long-range order in the 50 material. Recent theoretical reports have used first-principles 51 calculations based on the density functional theory (DFT) to 52 establish correlations between the shielding tensor and 53 structural features of carbon nanotubes, 11 graphite oxide, 12 54 graphene, and graphitic materials. 13,14 55 Amorphous hydrogenated carbons have been extensively 56 studied in the past decades due to their interesting mechanical 57 and tribological properties, which have promising applications. 58 These properties are dependent on chemical and structural 59 features such as the H content in the material and the amount 60 of atoms with sp and sp 3 hybridization. 15−17 Solid-state 13 C 61 NMR methods have been widely used in studies of amorphous 62 carbons, especially regarding the quantitative evaluation of the 63 sp 2 /sp 3 ratio. In fact, this ratio is a particularly important 64 property that can be easily obtained from 13 C NMR spectra, 65 considering the clear distinction in the chemical shift ranges 66 corresponding to sp 2 and sp 3 carbons. 18 As an example, Pan et 67 al. 19 estimated that just ca. 1.5% of the carbon content in an 68 amorphous carbon film could be detected in 13 C NMR spectra 69 obtained with 1 H-13 C cross-polarization (CP), 18 concluding
A solid-state NMR study of C(70): a model molecule for amorphous carbons
Solid State Nuclear Magnetic Resonance
We show that natural abundance, solid-state MAS-NMR (13)C INADEQUATE spectra can be recorded for crystallized C(70), using the through-bond J-coupling for the magnetization transfer. The effect of strong J-coupling can be lessened at high magnetic fields, allowing the observation of cross-peaks between close resonances. DFT calculations of the chemical shifts show an excellent agreement with the experimental values. A correlation is observed between the average CCC bond angles and the (13)C chemical shift, offering a way to understand the dispersion of (13)C chemical shifts in nanoporous activated carbons in terms of local deviations from planarity.