¹³C NMR Parameters of Disordered Carbons: Atomistic Simulations, DFT Calculations, and Experimental Results

Abstract

The 13 C NMR chemical shifts corresponding to di↵erent sites in atomistic models of disordered carbons were computed at di↵erent H contents by employing DFT calculations. Structural models were generated by molecular dynamics simulations and validated by the pair distribution functions; further bonding analyses were carried out to determine the amount of sp 3 and sp 2 carbons in the structures. Specifically, the obtained results allow the distinction of the chemical shifts associated with di↵erent types of carbon sites, with di↵erent hybridization states and bonded or not to a hydrogen atom. The calculated NMR spectra show excellent agreement with experimental data and are thus useful to identify local structural features of disordered carbons.

Unexpected NMR shieldings of sp- and sp2-hybridized carbon atoms in graphyne systems

“…The situation in γ-graphyne and γ-graphdiyne is quite different, and closer to the NMR of typical organic molecules, which exhibit higher 13 C chemical shifts for sp 2 -hybridized carbons than for sp 2 ones. The chemical shift of sp 2 (B) carbon is pushed to slightly higher values with respect to graphene, while the shifts of sp 2 -hybridized carbons (L) move towards lower chemical shift by about -18 ppm.…”

“…In addition, the NMR shielding constants are quite sensitive to the local molecular structure around the nucleus and thus can reveal both the bonding arrangements as well as interesting local electronic properties. The 13 C NMR has been shown to be a useful tool for studying of different carbon allotropes, such as disordered carbon 13,14 or graphene [14][15][16][17][18] and its different spin states 17,18 .…”

NMR studies of adsorption and diffusion in porous carbonaceous materials

“…This was attributed to partial dehydration of the Fions upon pore entry, which is necessary due to the small pore width in the carbon used. The authors cited a DFT study which has shown that the removal of up to two water molecules from the hydration shell can lead to a 13 ppm chemical shift change, which is consistent with the observed shift change between 0.7 -1.0 V. [97] In the same study, the electrosorption of cations on the same PDC was also tracked by 23 Na NMR. Interestingly, while an even higher gating voltage of 0.6 V was required for entry of the Na cations into the pores, no large shift change was observed, suggesting that the hydration shell is not significantly disrupted upon pore entry.…”

“…[17] In kerogens, the ratio of carbon and hydrogen atoms has a large impact on the carbon coordination and chemical shifts leading to more significant variations in the NMR spectra. [23] The determination of pair distribution functions (PDFs) from X-ray or neutron diffraction experiments constitute a major source of information to characterize the local structure of amorphous carbons, but these curves must be interpreted with care as a range of very different atomic structures can lead to similar average properties in agreement with experimental results. Figure 3 shows the PDF and ring statistics for a TiC-CDC synthesized at 800° C along with two atomic models generated either by Quench Molecular Dynamics, [19] i.e., realising a quench from the liquid to the solid state of the carbon in a molecular dynamics simulation, or by Hybrid Reverse Monte Carlo (HRMC), [18] i.e., by obtaining atomic structures by random displacements of the carbon positions to fit experimental data.…”

“…[22,[72][73][74][75] The linewidth is often broader than the range of chemical environments expected for the samples (up to 200 ppm), suggesting that magnetic susceptibility broadening provides a significant contribution and therefore that linewidths cannot be directly related to the structural disorder present. For samples containing significant amounts of defects such as hydrogen-or oxygen-containing functional groups, distinct chemical environments can be observed; [23,76] however, such systems are chemically quite dissimilar to those typically used for adsorption applications.…”